Merge branch 'main' into leveldb

46 files changed, 1621 insertions, 822 deletions

diff --git a/src/audio/CMakeLists.txt b/src/audio/CMakeLists.txt
index d4c89214..6361f827 100644
--- a/src/audio/CMakeLists.txt
+++ b/src/audio/CMakeLists.txt
@@ -1,7 +1,7 @@
 idf_component_register(
   SRCS "audio_decoder.cpp" "audio_task.cpp" "chunk.cpp" "fatfs_audio_input.cpp"
   "stream_message.cpp" "i2s_audio_output.cpp" "stream_buffer.cpp"
-  "audio_playback.cpp" "stream_event.cpp" "audio_element.cpp"
+  "audio_playback.cpp" "stream_event.cpp" "pipeline.cpp" "stream_info.cpp"
   INCLUDE_DIRS "include"
   REQUIRES "codecs" "drivers" "cbor" "result" "tasks" "span" "memory")
 
diff --git a/src/audio/audio_decoder.cpp b/src/audio/audio_decoder.cpp
index 9879b042..4b9826a9 100644
--- a/src/audio/audio_decoder.cpp
+++ b/src/audio/audio_decoder.cpp
@@ -2,13 +2,17 @@
 
 #include <string.h>
 
+#include <algorithm>
 #include <cstddef>
 #include <cstdint>
 #include <memory>
+#include <variant>
 
+#include "cbor/tinycbor/src/cborinternal_p.h"
 #include "freertos/FreeRTOS.h"
 
 #include "esp_heap_caps.h"
+#include "esp_log.h"
 #include "freertos/message_buffer.h"
 #include "freertos/portmacro.h"
 
@@ -21,128 +25,113 @@ namespace audio {
 
 static const char* kTag = "DEC";
 
-static const std::size_t kChunkSize = 1024;
-static const std::size_t kReadahead = 8;
-
 AudioDecoder::AudioDecoder()
     : IAudioElement(),
-      arena_(kChunkSize, kReadahead, MALLOC_CAP_SPIRAM),
-      stream_info_({}),
-      has_samples_to_send_(false),
-      needs_more_input_(true) {}
+      current_codec_(),
+      current_input_format_(),
+      current_output_format_(),
+      has_samples_to_send_(false) {}
 
 AudioDecoder::~AudioDecoder() {}
 
-auto AudioDecoder::HasUnprocessedInput() -> bool {
-  return !needs_more_input_ || has_samples_to_send_;
-}
-
-auto AudioDecoder::IsOverBuffered() -> bool {
-  return arena_.BlocksFree() == 0;
-}
-
-auto AudioDecoder::ProcessStreamInfo(const StreamInfo& info) -> void {
-  stream_info_ = info;
-
-  if (info.chunk_size) {
-    chunk_reader_.emplace(info.chunk_size.value());
-  } else {
-    ESP_LOGE(kTag, "no chunk size given");
-    return;
+auto AudioDecoder::ProcessStreamInfo(const StreamInfo& info) -> bool {
+  if (!std::holds_alternative<StreamInfo::Encoded>(info.format)) {
+    return false;
   }
+  ESP_LOGI(kTag, "got new stream");
+  const auto& encoded = std::get<StreamInfo::Encoded>(info.format);
 
   // Reuse the existing codec if we can. This will help with gapless playback,
   // since we can potentially just continue to decode as we were before,
   // without any setup overhead.
   if (current_codec_ != nullptr &&
-      current_codec_->CanHandleFile(info.path.value_or(""))) {
+      current_codec_->CanHandleType(encoded.type)) {
     current_codec_->ResetForNewStream();
-    return;
+    ESP_LOGI(kTag, "reusing existing decoder");
+    return true;
   }
 
-  auto result = codecs::CreateCodecForFile(info.path.value_or(""));
+  // TODO: use audio type from stream
+  auto result = codecs::CreateCodecForType(encoded.type);
   if (result.has_value()) {
+    ESP_LOGI(kTag, "creating new decoder");
     current_codec_ = std::move(result.value());
   } else {
     ESP_LOGE(kTag, "no codec for this file");
-    return;
+    return false;
   }
 
-  stream_info_ = info;
-  has_sent_stream_info_ = false;
+  return true;
 }
 
-auto AudioDecoder::ProcessChunk(const cpp::span<std::byte>& chunk) -> void {
-  if (current_codec_ == nullptr || !chunk_reader_) {
-    // Should never happen, but fail explicitly anyway.
-    ESP_LOGW(kTag, "received chunk without chunk size or codec");
+auto AudioDecoder::Process(const std::vector<InputStream>& inputs,
+                           OutputStream* output) -> void {
+  auto input = inputs.begin();
+  const StreamInfo& info = input->info();
+  if (std::holds_alternative<std::monostate>(info.format) ||
+      info.bytes_in_stream == 0) {
+    // TODO(jacqueline): should we clear the stream format?
+    // output->prepare({});
     return;
   }
 
-  ESP_LOGI(kTag, "received new chunk (size %u)", chunk.size());
-  current_codec_->SetInput(chunk_reader_->HandleNewData(chunk));
-  needs_more_input_ = false;
-}
+  if (!current_input_format_ || *current_input_format_ != info.format) {
+    // The input stream has changed! Immediately throw everything away and
+    // start from scratch.
+    current_input_format_ = info.format;
+    has_samples_to_send_ = false;
 
-auto AudioDecoder::ProcessEndOfStream() -> void {
-  has_samples_to_send_ = false;
-  needs_more_input_ = true;
-  current_codec_.reset();
+    ProcessStreamInfo(info);
+  }
 
-  SendOrBufferEvent(std::unique_ptr<StreamEvent>(
-      StreamEvent::CreateEndOfStream(input_events_)));
-}
+  current_codec_->SetInput(input->data());
 
-auto AudioDecoder::Process() -> void {
-  if (has_samples_to_send_) {
-    // Writing samples is relatively quick (it's just a bunch of memcopy's), so
-    // do them all at once.
-    while (has_samples_to_send_ && !IsOverBuffered()) {
-      if (!has_sent_stream_info_) {
-        has_sent_stream_info_ = true;
+  while (true) {
+    if (has_samples_to_send_) {
+      if (!current_output_format_) {
         auto format = current_codec_->GetOutputFormat();
-        stream_info_->bits_per_sample = format.bits_per_sample;
-        stream_info_->sample_rate = format.sample_rate_hz;
-        stream_info_->channels = format.num_channels;
-        stream_info_->chunk_size = kChunkSize;
-
-        auto event =
-            StreamEvent::CreateStreamInfo(input_events_, *stream_info_);
-        SendOrBufferEvent(std::unique_ptr<StreamEvent>(event));
+        current_output_format_ = StreamInfo::Pcm{
+            .channels = format.num_channels,
+            .bits_per_sample = format.bits_per_sample,
+            .sample_rate = format.sample_rate_hz,
+        };
       }
 
-      auto block = arena_.Acquire();
-      if (!block) {
-        return;
+      if (!output->prepare(*current_output_format_)) {
+        break;
       }
 
-      auto write_res =
-          current_codec_->WriteOutputSamples({block->start, block->size});
-      block->used_size = write_res.first;
+      auto write_res = current_codec_->WriteOutputSamples(output->data());
+      output->add(write_res.first);
       has_samples_to_send_ = !write_res.second;
 
-      auto chunk = std::unique_ptr<StreamEvent>(
-          StreamEvent::CreateArenaChunk(input_events_, *block));
-      if (!SendOrBufferEvent(std::move(chunk))) {
-        return;
+      if (has_samples_to_send_) {
+        // We weren't able to fit all the generated samples into the output
+        // buffer. Stop trying; we'll finish up during the next pass.
+        break;
       }
     }
-    // We will process the next frame during the next call to this method.
-  }
 
-  if (!needs_more_input_) {
     auto res = current_codec_->ProcessNextFrame();
     if (res.has_error()) {
       // TODO(jacqueline): Handle errors.
       return;
     }
-    needs_more_input_ = res.value();
-    has_samples_to_send_ = true;
 
-    if (needs_more_input_) {
-      chunk_reader_->HandleBytesUsed(current_codec_->GetInputPosition());
+    if (res.value()) {
+      // We're out of useable data in this buffer. Finish immediately; there's
+      // nothing to send.
+      input->mark_incomplete();
+      break;
+    } else {
+      has_samples_to_send_ = true;
     }
   }
+
+  std::size_t pos = current_codec_->GetInputPosition();
+  if (pos > 0) {
+    input->consume(pos - 1);
+  }
 }
 
 }  // namespace audio
diff --git a/src/audio/audio_element.cpp b/src/audio/audio_element.cpp
deleted file mode 100644
index cef54631..00000000
--- a/src/audio/audio_element.cpp
+++ /dev/null
@@ -1,56 +0,0 @@
-#include "audio_element.hpp"
-#include <memory>
-
-namespace audio {
-
-IAudioElement::IAudioElement()
-    : input_events_(xQueueCreate(kEventQueueSize, sizeof(void*))),
-      output_events_(nullptr),
-      buffered_output_() {}
-
-IAudioElement::~IAudioElement() {
-  // Ensure we don't leak any memory from events leftover in the queue.
-  while (uxQueueSpacesAvailable(input_events_) < kEventQueueSize) {
-    StreamEvent* event;
-    if (xQueueReceive(input_events_, &event, 0)) {
-      free(event);
-    } else {
-      break;
-    }
-  }
-  // Technically there's a race here if someone is still adding to the queue,
-  // but hopefully the whole pipeline is stopped if an element is being
-  // destroyed.
-  vQueueDelete(input_events_);
-}
-
-auto IAudioElement::SendOrBufferEvent(std::unique_ptr<StreamEvent> event)
-    -> bool {
-  if (!buffered_output_.empty()) {
-    // To ensure we send data in order, don't try to send if we've already
-    // failed to send something.
-    buffered_output_.push_back(std::move(event));
-    return false;
-  }
-  StreamEvent* raw_event = event.release();
-  if (!xQueueSend(output_events_, &raw_event, 0)) {
-    event.reset(raw_event);
-    buffered_output_.push_back(std::move(event));
-    return false;
-  }
-  return true;
-}
-
-auto IAudioElement::FlushBufferedOutput() -> bool {
-  while (!buffered_output_.empty()) {
-    StreamEvent* raw_event = buffered_output_.front().release();
-    buffered_output_.pop_front();
-    if (!xQueueSend(output_events_, &raw_event, 0)) {
-      buffered_output_.emplace_front(raw_event);
-      return false;
-    }
-  }
-  return true;
-}
-
-}  // namespace audio
diff --git a/src/audio/audio_playback.cpp b/src/audio/audio_playback.cpp
index c95a5d63..c51e41fb 100644
--- a/src/audio/audio_playback.cpp
+++ b/src/audio/audio_playback.cpp
@@ -5,71 +5,46 @@
 #include <memory>
 #include <string_view>
 
+#include "driver_cache.hpp"
 #include "freertos/portmacro.h"
 
 #include "audio_decoder.hpp"
+#include "audio_element.hpp"
 #include "audio_task.hpp"
 #include "chunk.hpp"
 #include "fatfs_audio_input.hpp"
 #include "gpio_expander.hpp"
 #include "i2s_audio_output.hpp"
+#include "pipeline.hpp"
 #include "storage.hpp"
 #include "stream_buffer.hpp"
 #include "stream_info.hpp"
 #include "stream_message.hpp"
 
 namespace audio {
-
-auto AudioPlayback::create(drivers::GpioExpander* expander,
-                           std::shared_ptr<drivers::SdStorage> storage)
-    -> cpp::result<std::unique_ptr<AudioPlayback>, Error> {
-  // Create everything
-  auto source = std::make_shared<FatfsAudioInput>(storage);
-  auto codec = std::make_shared<AudioDecoder>();
-
-  auto sink_res = I2SAudioOutput::create(expander);
-  if (sink_res.has_error()) {
-    return cpp::fail(ERR_INIT_ELEMENT);
-  }
-  auto sink = sink_res.value();
-
-  auto playback = std::make_unique<AudioPlayback>();
-
-  // Configure the pipeline
-  playback->ConnectElements(source.get(), codec.get());
-  playback->ConnectElements(codec.get(), sink.get());
-
-  // Launch!
-  StartAudioTask("src", {}, source);
-  StartAudioTask("dec", {}, codec);
-  StartAudioTask("sink", 0, sink);
-
-  playback->input_handle_ = source->InputEventQueue();
-
-  return playback;
+AudioPlayback::AudioPlayback(drivers::DriverCache* drivers)
+    : file_source_(std::make_unique<FatfsAudioInput>()),
+      i2s_output_(std::make_unique<I2SAudioOutput>(drivers->AcquireGpios(),
+                                                   drivers->AcquireDac())) {
+  AudioDecoder* codec = new AudioDecoder();
+  elements_.emplace_back(codec);
+
+  Pipeline* pipeline = new Pipeline(elements_.front().get());
+  pipeline->AddInput(file_source_.get());
+
+  task::StartPipeline(pipeline, i2s_output_.get());
+  // task::StartDrain(i2s_output_.get());
 }
 
-AudioPlayback::AudioPlayback() {}
-
 AudioPlayback::~AudioPlayback() {}
 
 auto AudioPlayback::Play(const std::string& filename) -> void {
-  StreamInfo info;
-  info.path = filename;
-  auto event = StreamEvent::CreateStreamInfo(input_handle_, info);
-  xQueueSend(input_handle_, &event, portMAX_DELAY);
-  event = StreamEvent::CreateEndOfStream(input_handle_);
-  xQueueSend(input_handle_, &event, portMAX_DELAY);
+  // TODO: concurrency, yo!
+  file_source_->OpenFile(filename);
 }
 
 auto AudioPlayback::LogStatus() -> void {
-  auto event = StreamEvent::CreateLogStatus();
-  xQueueSendToFront(input_handle_, &event, portMAX_DELAY);
-}
-
-auto AudioPlayback::ConnectElements(IAudioElement* src, IAudioElement* sink)
-    -> void {
-  src->OutputEventQueue(sink->InputEventQueue());
+  i2s_output_->Log();
 }
 
 }  // namespace audio
diff --git a/src/audio/audio_task.cpp b/src/audio/audio_task.cpp
index ce6d724e..b2a8062e 100644
--- a/src/audio/audio_task.cpp
+++ b/src/audio/audio_task.cpp
@@ -2,16 +2,23 @@
 
 #include <stdlib.h>
 
+#include <algorithm>
+#include <cstddef>
 #include <cstdint>
 #include <deque>
 #include <memory>
+#include <variant>
 
+#include "audio_sink.hpp"
 #include "cbor.h"
+#include "dac.hpp"
+#include "esp_err.h"
 #include "esp_heap_caps.h"
 #include "esp_log.h"
 #include "freertos/portmacro.h"
 #include "freertos/projdefs.h"
 #include "freertos/queue.h"
+#include "pipeline.hpp"
 #include "span.hpp"
 
 #include "arena.hpp"
@@ -25,25 +32,32 @@
 
 namespace audio {
 
+namespace task {
+
 static const char* kTag = "task";
+static const std::size_t kStackSize = 24 * 1024;
+static const std::size_t kDrainStackSize = 1024;
+
+auto StartPipeline(Pipeline* pipeline, IAudioSink* sink) -> void {
+  // Newly created task will free this.
+  AudioTaskArgs* args = new AudioTaskArgs{.pipeline = pipeline, .sink = sink};
 
-auto StartAudioTask(const std::string& name,
-                    std::optional<BaseType_t> core_id,
-                    std::shared_ptr<IAudioElement> element) -> void {
-  auto task_handle = std::make_unique<TaskHandle_t>();
+  ESP_LOGI(kTag, "starting audio pipeline task");
+  xTaskCreatePinnedToCore(&AudioTaskMain, "pipeline", kStackSize, args,
+                          kTaskPriorityAudioPipeline, NULL, 1);
+}
 
+auto StartDrain(IAudioSink* sink) -> void {
+  auto command = new std::atomic<Command>(PLAY);
   // Newly created task will free this.
-  AudioTaskArgs* args = new AudioTaskArgs{.element = element};
-
-  ESP_LOGI(kTag, "starting audio task %s", name.c_str());
-  if (core_id) {
-    xTaskCreatePinnedToCore(&AudioTaskMain, name.c_str(),
-                            element->StackSizeBytes(), args, kTaskPriorityAudio,
-                            task_handle.get(), *core_id);
-  } else {
-    xTaskCreate(&AudioTaskMain, name.c_str(), element->StackSizeBytes(), args,
-                kTaskPriorityAudio, task_handle.get());
-  }
+  AudioDrainArgs* drain_args = new AudioDrainArgs{
+      .sink = sink,
+      .command = command,
+  };
+
+  ESP_LOGI(kTag, "starting audio drain task");
+  xTaskCreate(&AudioDrainMain, "drain", kDrainStackSize, drain_args,
+              kTaskPriorityAudioDrain, NULL);
 }
 
 void AudioTaskMain(void* args) {
@@ -51,118 +65,129 @@ void AudioTaskMain(void* args) {
   // called before the task quits.
   {
     AudioTaskArgs* real_args = reinterpret_cast<AudioTaskArgs*>(args);
-    std::shared_ptr<IAudioElement> element = std::move(real_args->element);
+    std::unique_ptr<Pipeline> pipeline(real_args->pipeline);
+    IAudioSink* sink = real_args->sink;
     delete real_args;
 
-    // Queue of events that we have received on our input queue, but not yet
-    // processed.
-    std::deque<std::unique_ptr<StreamEvent>> pending_events;
-
-    // TODO(jacqueline): quit event
-    while (true) {
-      // First, we pull events from our input queue into pending_events. This
-      // keeps us responsive to any events that need to be handled immediately.
-      // Then we check if there's any events to flush downstream.
-      // Then we pass anything requiring processing to the element.
-
-      bool has_work_to_do =
-          (!pending_events.empty() || element->HasUnflushedOutput() ||
-           element->HasUnprocessedInput()) &&
-          !element->IsOverBuffered();
-
-      if (has_work_to_do) {
-        ESP_LOGD(kTag, "checking for events");
-      } else {
-        ESP_LOGD(kTag, "waiting for events");
-      }
-
-      // If we have no new events to process and the element has nothing left to
-      // do, then just delay forever waiting for a new event.
-      TickType_t ticks_to_wait = has_work_to_do ? 0 : portMAX_DELAY;
-
-      StreamEvent* new_event = nullptr;
-      bool has_event =
-          xQueueReceive(element->InputEventQueue(), &new_event, ticks_to_wait);
-
-      if (has_event) {
-        if (new_event->tag == StreamEvent::UNINITIALISED) {
-          ESP_LOGE(kTag, "discarding invalid event!!");
-        } else if (new_event->tag == StreamEvent::CHUNK_NOTIFICATION) {
-          delete new_event;
-        } else if (new_event->tag == StreamEvent::LOG_STATUS) {
-          element->ProcessLogStatus();
-          if (element->OutputEventQueue() != nullptr) {
-            xQueueSendToFront(element->OutputEventQueue(), &new_event, 0);
-          } else {
-            delete new_event;
-          }
-        } else {
-          // This isn't an event that needs to be actioned immediately. Add it
-          // to our work queue.
-          pending_events.emplace_back(new_event);
-          ESP_LOGD(kTag, "deferring event");
+    std::optional<StreamInfo::Format> output_format;
+
+    std::vector<Pipeline*> elements = pipeline->GetIterationOrder();
+    std::size_t max_inputs =
+        (*std::max_element(elements.begin(), elements.end(),
+                           [](Pipeline const* first, Pipeline const* second) {
+                             return first->NumInputs() < second->NumInputs();
+                           }))
+            ->NumInputs();
+
+    // We need to be able to simultaneously map all of an element's inputs, plus
+    // its output. So preallocate that many ranges.
+    std::vector<MappableRegion<kPipelineBufferSize>> in_regions(max_inputs);
+    MappableRegion<kPipelineBufferSize> out_region;
+    std::for_each(in_regions.begin(), in_regions.end(),
+                  [](const auto& region) { assert(region.is_valid); });
+    assert(out_region.is_valid);
+
+    // Each element has exactly one output buffer.
+    std::vector<HimemAlloc<kPipelineBufferSize>> buffers(elements.size());
+    std::vector<StreamInfo> buffer_infos(buffers.size());
+    std::for_each(buffers.begin(), buffers.end(),
+                  [](const HimemAlloc<kPipelineBufferSize>& alloc) {
+                    assert(alloc.is_valid);
+                  });
+
+    bool playing = true;
+    bool quit = false;
+    while (!quit) {
+      if (playing) {
+        for (int i = 0; i < elements.size(); i++) {
+          std::vector<RawStream> raw_in_streams;
+          elements.at(i)->InStreams(&in_regions, &raw_in_streams);
+          RawStream raw_out_stream = elements.at(i)->OutStream(&out_region);
+
+          // Crop the input and output streams to the ranges that are safe to
+          // touch. For the input streams, this is the region that contains
+          // data. For the output stream, this is the region that does *not*
+          // already contain data.
+          std::vector<InputStream> in_streams;
+          std::for_each(raw_in_streams.begin(), raw_in_streams.end(),
+                        [&](RawStream& s) { in_streams.emplace_back(&s); });
+          OutputStream out_stream(&raw_out_stream);
+
+          elements.at(i)->OutputElement()->Process(in_streams, &out_stream);
+
+          std::for_each(in_regions.begin(), in_regions.end(),
+                        [](auto&& r) { r.Unmap(); });
+          out_region.Unmap();
         }
-        // Loop again, so that we service all incoming events before doing our
-        // possibly expensive processing.
-        continue;
-      }
-
-      if (element->HasUnflushedOutput()) {
-        ESP_LOGD(kTag, "flushing output");
-      }
 
-      // We have no new events. Next, see if there's anything that needs to be
-      // flushed.
-      if (element->HasUnflushedOutput() && !element->FlushBufferedOutput()) {
-        // We had things to flush, but couldn't send it all. This probably
-        // implies that the downstream element is having issues servicing its
-        // input queue, so hold off for a moment before retrying.
-        ESP_LOGW(kTag, "failed to flush buffered output");
-        vTaskDelay(pdMS_TO_TICKS(100));
-        continue;
-      }
+        RawStream raw_sink_stream = elements.front()->OutStream(&out_region);
+        InputStream sink_stream(&raw_sink_stream);
 
-      if (element->HasUnprocessedInput()) {
-        ESP_LOGD(kTag, "processing input events");
-        element->Process();
-        continue;
-      }
-
-      // The element ran out of data, so now it's time to let it process more
-      // input.
-      while (!pending_events.empty()) {
-        std::unique_ptr<StreamEvent> event;
-        pending_events.front().swap(event);
-        pending_events.pop_front();
-        ESP_LOGD(kTag, "processing event, tag %i", event->tag);
+        if (sink_stream.info().bytes_in_stream == 0) {
+          out_region.Unmap();
+          vTaskDelay(pdMS_TO_TICKS(100));
+          continue;
+        }
 
-        if (event->tag == StreamEvent::STREAM_INFO) {
-          ESP_LOGD(kTag, "processing stream info");
+        if (!output_format || output_format != sink_stream.info().format) {
+          // The format of the stream within the sink stream has changed. We
+          // need to reconfigure the sink, but shouldn't do so until we've fully
+          // drained the current buffer.
+          if (xStreamBufferIsEmpty(sink->buffer())) {
+            ESP_LOGI(kTag, "reconfiguring dac");
+            output_format = sink_stream.info().format;
+            sink->Configure(*output_format);
+          }
+        }
 
-          element->ProcessStreamInfo(*event->stream_info);
+        // We've reconfigured the sink, or it was already configured correctly.
+        // Send through some data.
+        if (output_format == sink_stream.info().format &&
+            !std::holds_alternative<std::monostate>(*output_format)) {
+          // TODO: tune the delay on this, as it's currently the only way to
+          // throttle this task's CPU time. Maybe also hold off on the pipeline
+          // if the buffer is already close to full?
+          std::size_t sent = xStreamBufferSend(
+              sink->buffer(), sink_stream.data().data(),
+              sink_stream.data().size_bytes(), pdMS_TO_TICKS(10));
+          if (sent > 0) {
+            ESP_LOGI(kTag, "sunk %u bytes out of %u (%d %%)", sent,
+                     sink_stream.info().bytes_in_stream,
+                     (int)(((float)sent /
+                            (float)sink_stream.info().bytes_in_stream) *
+                           100));
+          }
+          sink_stream.consume(sent);
+        }
 
-        } else if (event->tag == StreamEvent::ARENA_CHUNK) {
-          ESP_LOGD(kTag, "processing arena data");
+        out_region.Unmap();
+      }
+    }
+  }
+  vTaskDelete(NULL);
+}
 
-          memory::ArenaRef ref(event->arena_chunk);
-          auto callback =
-              StreamEvent::CreateChunkNotification(element->InputEventQueue());
-          if (!xQueueSend(event->source, &callback, 0)) {
-            ESP_LOGW(kTag, "failed to send chunk notif");
-            continue;
-          }
+static std::byte sDrainBuf[8 * 1024];
 
-          // TODO(jacqueline): Consider giving the element a full ArenaRef here,
-          // so that it can hang on to it and potentially save an alloc+copy.
-          element->ProcessChunk({ref.ptr.start, ref.ptr.used_size});
+void AudioDrainMain(void* args) {
+  {
+    AudioDrainArgs* real_args = reinterpret_cast<AudioDrainArgs*>(args);
+    IAudioSink* sink = real_args->sink;
+    std::atomic<Command>* command = real_args->command;
+    delete real_args;
 
-          // TODO: think about whether to do the whole queue
-          break;
-        }
+    // TODO(jacqueline): implement PAUSE without busy-waiting.
+    while (*command != QUIT) {
+      std::size_t len = xStreamBufferReceive(sink->buffer(), sDrainBuf,
+                                             sizeof(sDrainBuf), portMAX_DELAY);
+      if (len > 0) {
+        sink->Send({sDrainBuf, len});
       }
     }
   }
   vTaskDelete(NULL);
 }
 
+}  // namespace task
+
 }  // namespace audio
diff --git a/src/audio/fatfs_audio_input.cpp b/src/audio/fatfs_audio_input.cpp
index 5354c5fd..22d707d6 100644
--- a/src/audio/fatfs_audio_input.cpp
+++ b/src/audio/fatfs_audio_input.cpp
@@ -4,9 +4,12 @@
 #include <cstdint>
 #include <memory>
 #include <string>
+#include <variant>
 
 #include "arena.hpp"
 #include "esp_heap_caps.h"
+#include "esp_log.h"
+#include "ff.h"
 #include "freertos/portmacro.h"
 
 #include "audio_element.hpp"
@@ -15,43 +18,23 @@
 #include "stream_event.hpp"
 #include "stream_info.hpp"
 #include "stream_message.hpp"
+#include "types.hpp"
 
 static const char* kTag = "SRC";
 
 namespace audio {
 
-static const std::size_t kChunkSize = 24 * 1024;
-static const std::size_t kChunkReadahead = 2;
-
-FatfsAudioInput::FatfsAudioInput(std::shared_ptr<drivers::SdStorage> storage)
-    : IAudioElement(),
-      arena_(kChunkSize, kChunkReadahead, MALLOC_CAP_SPIRAM),
-      storage_(storage),
-      current_file_(),
-      is_file_open_(false) {}
+FatfsAudioInput::FatfsAudioInput()
+    : IAudioElement(), current_file_(), is_file_open_(false) {}
 
 FatfsAudioInput::~FatfsAudioInput() {}
 
-auto FatfsAudioInput::HasUnprocessedInput() -> bool {
-  return is_file_open_;
-}
-
-auto FatfsAudioInput::IsOverBuffered() -> bool {
-  return arena_.BlocksFree() == 0;
-}
-
-auto FatfsAudioInput::ProcessStreamInfo(const StreamInfo& info) -> void {
+auto FatfsAudioInput::OpenFile(const std::string& path) -> void {
   if (is_file_open_) {
     f_close(&current_file_);
     is_file_open_ = false;
   }
-
-  if (!info.path) {
-    // TODO(jacqueline): Handle errors.
-    return;
-  }
-  ESP_LOGI(kTag, "opening file %s", info.path->c_str());
-  std::string path = *info.path;
+  ESP_LOGI(kTag, "opening file %s", path.c_str());
   FRESULT res = f_open(&current_file_, path.c_str(), FA_READ);
   if (res != FR_OK) {
     ESP_LOGE(kTag, "failed to open file! res: %i", res);
@@ -60,51 +43,36 @@ auto FatfsAudioInput::ProcessStreamInfo(const StreamInfo& info) -> void {
   }
 
   is_file_open_ = true;
-
-  StreamInfo new_info(info);
-  new_info.chunk_size = kChunkSize;
-  ESP_LOGI(kTag, "chunk size: %u bytes", kChunkSize);
-
-  auto event = StreamEvent::CreateStreamInfo(input_events_, new_info);
-  SendOrBufferEvent(std::unique_ptr<StreamEvent>(event));
 }
 
-auto FatfsAudioInput::ProcessChunk(const cpp::span<std::byte>& chunk) -> void {}
-
-auto FatfsAudioInput::ProcessEndOfStream() -> void {
-  if (is_file_open_) {
-    f_close(&current_file_);
-    is_file_open_ = false;
-    SendOrBufferEvent(std::unique_ptr<StreamEvent>(
-        StreamEvent::CreateEndOfStream(input_events_)));
+auto FatfsAudioInput::Process(const std::vector<InputStream>& inputs,
+                              OutputStream* output) -> void {
+  if (!is_file_open_) {
+    // TODO(jacqueline): should we clear the stream format?
+    // output->prepare({});
+    return;
   }
-}
 
-auto FatfsAudioInput::Process() -> void {
-  if (is_file_open_) {
-    auto dest_block = memory::ArenaRef::Acquire(&arena_);
-    if (!dest_block) {
-      return;
-    }
-
-    FRESULT result = f_read(&current_file_, dest_block->ptr.start,
-                            dest_block->ptr.size, &dest_block->ptr.used_size);
-    if (result != FR_OK) {
-      ESP_LOGE(kTag, "file I/O error %d", result);
-      // TODO(jacqueline): Handle errors.
-      return;
-    }
+  StreamInfo::Format format = StreamInfo::Encoded{codecs::STREAM_MP3};
+  if (!output->prepare(format)) {
+    return;
+  }
 
-    if (dest_block->ptr.used_size < dest_block->ptr.size ||
-        f_eof(&current_file_)) {
-      f_close(&current_file_);
-      is_file_open_ = false;
-    }
+  std::size_t max_size = output->data().size_bytes();
+  std::size_t size = 0;
+  FRESULT result =
+      f_read(&current_file_, output->data().data(), max_size, &size);
+  if (result != FR_OK) {
+    ESP_LOGE(kTag, "file I/O error %d", result);
+    // TODO(jacqueline): Handle errors.
+    return;
+  }
 
-    auto dest_event = std::unique_ptr<StreamEvent>(
-        StreamEvent::CreateArenaChunk(input_events_, dest_block->Release()));
+  output->add(size);
 
-    SendOrBufferEvent(std::move(dest_event));
+  if (size < max_size || f_eof(&current_file_)) {
+    f_close(&current_file_);
+    is_file_open_ = false;
   }
 }
 
diff --git a/src/audio/i2s_audio_output.cpp b/src/audio/i2s_audio_output.cpp
index 110227cf..8c374351 100644
--- a/src/audio/i2s_audio_output.cpp
+++ b/src/audio/i2s_audio_output.cpp
@@ -1,6 +1,9 @@
 #include "i2s_audio_output.hpp"
 
 #include <algorithm>
+#include <cstddef>
+#include <memory>
+#include <variant>
 
 #include "esp_err.h"
 #include "freertos/portmacro.h"
@@ -10,68 +13,41 @@
 #include "freertos/projdefs.h"
 #include "gpio_expander.hpp"
 #include "result.hpp"
+#include "stream_info.hpp"
 
-static const TickType_t kIdleTimeBeforeMute = pdMS_TO_TICKS(1000);
 static const char* kTag = "I2SOUT";
 
 namespace audio {
 
-static const std::size_t kDmaQueueLength = 8;
-
-auto I2SAudioOutput::create(drivers::GpioExpander* expander)
-    -> cpp::result<std::shared_ptr<I2SAudioOutput>, Error> {
-  // First, we need to perform initial configuration of the DAC chip.
-  auto dac_result = drivers::AudioDac::create(expander);
-  if (dac_result.has_error()) {
-    ESP_LOGE(kTag, "failed to init dac: %d", dac_result.error());
-    return cpp::fail(DAC_CONFIG);
-  }
-  std::unique_ptr<drivers::AudioDac> dac = std::move(dac_result.value());
-
-  // Soft mute immediately, in order to minimise any clicks and pops caused by
-  // the initial output element and pipeline configuration.
-  // dac->WriteVolume(255);
-  dac->WriteVolume(120);  // for testing
-
-  return std::make_shared<I2SAudioOutput>(expander, std::move(dac));
-}
-
 I2SAudioOutput::I2SAudioOutput(drivers::GpioExpander* expander,
-                               std::unique_ptr<drivers::AudioDac> dac)
-    : expander_(expander),
-      dac_(std::move(dac)),
-      chunk_reader_(),
-      latest_chunk_() {}
-
-I2SAudioOutput::~I2SAudioOutput() {}
-
-auto I2SAudioOutput::HasUnprocessedInput() -> bool {
-  return latest_chunk_.size() > 0;
+                               std::shared_ptr<drivers::AudioDac> dac)
+    : expander_(expander), dac_(std::move(dac)), current_config_() {
+  dac_->WriteVolume(127);  // for testing
+  dac_->SetSource(buffer());
 }
 
-auto I2SAudioOutput::IsOverBuffered() -> bool {
-  return false;
+I2SAudioOutput::~I2SAudioOutput() {
+  dac_->SetSource(nullptr);
 }
 
-auto I2SAudioOutput::ProcessStreamInfo(const StreamInfo& info) -> void {
-  // TODO(jacqueline): probs do something with the channel hey
-
-  if (!info.bits_per_sample || !info.sample_rate) {
-    ESP_LOGE(kTag, "audio stream missing bits or sample rate");
-    return;
+auto I2SAudioOutput::Configure(const StreamInfo::Format& format) -> bool {
+  if (!std::holds_alternative<StreamInfo::Pcm>(format)) {
+    ESP_LOGI(kTag, "ignoring non-pcm stream (%d)", format.index());
+    return false;
   }
 
-  if (!info.chunk_size) {
-    ESP_LOGE(kTag, "audio stream missing chunk size");
-    return;
+  StreamInfo::Pcm pcm = std::get<StreamInfo::Pcm>(format);
+
+  if (current_config_ && pcm == *current_config_) {
+    ESP_LOGI(kTag, "ignoring unchanged format");
+    return true;
   }
-  chunk_reader_.emplace(*info.chunk_size);
 
-  ESP_LOGI(kTag, "incoming audio stream: %u bpp @ %u Hz", *info.bits_per_sample,
-           *info.sample_rate);
+  ESP_LOGI(kTag, "incoming audio stream: %u bpp @ %lu Hz", pcm.bits_per_sample,
+           pcm.sample_rate);
 
   drivers::AudioDac::BitsPerSample bps;
-  switch (*info.bits_per_sample) {
+  switch (pcm.bits_per_sample) {
     case 16:
       bps = drivers::AudioDac::BPS_16;
       break;
@@ -83,11 +59,11 @@ auto I2SAudioOutput::ProcessStreamInfo(const StreamInfo& info) -> void {
       break;
     default:
       ESP_LOGE(kTag, "dropping stream with unknown bps");
-      return;
+      return false;
   }
 
   drivers::AudioDac::SampleRate sample_rate;
-  switch (*info.sample_rate) {
+  switch (pcm.sample_rate) {
     case 44100:
       sample_rate = drivers::AudioDac::SAMPLE_RATE_44_1;
       break;
@@ -96,39 +72,25 @@ auto I2SAudioOutput::ProcessStreamInfo(const StreamInfo& info) -> void {
       break;
     default:
       ESP_LOGE(kTag, "dropping stream with unknown rate");
-      return;
+      return false;
   }
 
+  // TODO(jacqueline): probs do something with the channel hey
+
   dac_->Reconfigure(bps, sample_rate);
-}
+  current_config_ = pcm;
 
-auto I2SAudioOutput::ProcessChunk(const cpp::span<std::byte>& chunk) -> void {
-  latest_chunk_ = chunk_reader_->HandleNewData(chunk);
+  return true;
 }
 
-auto I2SAudioOutput::ProcessEndOfStream() -> void {
-  dac_->Stop();
-  SendOrBufferEvent(std::unique_ptr<StreamEvent>(
-      StreamEvent::CreateEndOfStream(input_events_)));
+auto I2SAudioOutput::Send(const cpp::span<std::byte>& data) -> void {
+  dac_->WriteData(data);
 }
 
-auto I2SAudioOutput::ProcessLogStatus() -> void {
+auto I2SAudioOutput::Log() -> void {
   dac_->LogStatus();
 }
 
-auto I2SAudioOutput::Process() -> void {
-  // Note: avoid logging here! We need to get bytes from the chunk buffer into
-  // the I2S DMA buffer as fast as possible, to avoid running out of samples.
-  std::size_t bytes_written = dac_->WriteData(latest_chunk_);
-  if (bytes_written == latest_chunk_.size_bytes()) {
-    latest_chunk_ = cpp::span<std::byte>();
-    chunk_reader_->HandleBytesLeftOver(0);
-  } else {
-    latest_chunk_ = latest_chunk_.subspan(bytes_written);
-  }
-  return;
-}
-
 auto I2SAudioOutput::SetVolume(uint8_t volume) -> void {
   dac_->WriteVolume(volume);
 }
diff --git a/src/audio/include/audio_decoder.hpp b/src/audio/include/audio_decoder.hpp
index 47642469..6a1b5177 100644
--- a/src/audio/include/audio_decoder.hpp
+++ b/src/audio/include/audio_decoder.hpp
@@ -3,6 +3,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <memory>
+#include <vector>
 
 #include "chunk.hpp"
 #include "ff.h"
@@ -10,6 +11,7 @@
 
 #include "audio_element.hpp"
 #include "codec.hpp"
+#include "stream_info.hpp"
 
 namespace audio {
 
@@ -22,28 +24,19 @@ class AudioDecoder : public IAudioElement {
   AudioDecoder();
   ~AudioDecoder();
 
-  auto StackSizeBytes() const -> std::size_t override { return 10 * 1024; };
-
-  auto HasUnprocessedInput() -> bool override;
-  auto IsOverBuffered() -> bool override;
-
-  auto ProcessStreamInfo(const StreamInfo& info) -> void override;
-  auto ProcessChunk(const cpp::span<std::byte>& chunk) -> void override;
-  auto ProcessEndOfStream() -> void override;
-  auto Process() -> void override;
+  auto Process(const std::vector<InputStream>& inputs, OutputStream* output)
+      -> void override;
 
   AudioDecoder(const AudioDecoder&) = delete;
   AudioDecoder& operator=(const AudioDecoder&) = delete;
 
  private:
-  memory::Arena arena_;
   std::unique_ptr<codecs::ICodec> current_codec_;
-  std::optional<StreamInfo> stream_info_;
-  std::optional<ChunkReader> chunk_reader_;
-
-  bool has_sent_stream_info_;
+  std::optional<StreamInfo::Format> current_input_format_;
+  std::optional<StreamInfo::Format> current_output_format_;
   bool has_samples_to_send_;
-  bool needs_more_input_;
+
+  auto ProcessStreamInfo(const StreamInfo& info) -> bool;
 };
 
 }  // namespace audio
diff --git a/src/audio/include/audio_element.hpp b/src/audio/include/audio_element.hpp
index 91036348..5884f7b2 100644
--- a/src/audio/include/audio_element.hpp
+++ b/src/audio/include/audio_element.hpp
@@ -37,65 +37,11 @@ static const size_t kEventQueueSize = 8;
  */
 class IAudioElement {
  public:
-  IAudioElement();
-  virtual ~IAudioElement();
+  IAudioElement() {}
+  virtual ~IAudioElement() {}
 
-  /*
-   * Returns the stack size in bytes that this element requires. This should
-   * be tuned according to the observed stack size of each element, as different
-   * elements have fairly different stack requirements (particular decoders).
-   */
-  virtual auto StackSizeBytes() const -> std::size_t { return 4096; };
-
-  /* Returns this element's input buffer. */
-  auto InputEventQueue() const -> QueueHandle_t { return input_events_; }
-  /* Returns this element's output buffer. */
-  auto OutputEventQueue() const -> QueueHandle_t { return output_events_; }
-  auto OutputEventQueue(const QueueHandle_t q) -> void { output_events_ = q; }
-
-  virtual auto HasUnprocessedInput() -> bool = 0;
-
-  virtual auto IsOverBuffered() -> bool { return false; }
-
-  auto HasUnflushedOutput() -> bool { return !buffered_output_.empty(); }
-  auto FlushBufferedOutput() -> bool;
-
-  /*
-   * Called when a StreamInfo message is received. Used to configure this
-   * element in preperation for incoming chunks.
-   */
-  virtual auto ProcessStreamInfo(const StreamInfo& info) -> void = 0;
-
-  /*
-   * Called when a ChunkHeader message is received. Includes the data associated
-   * with this chunk of stream data. This method should return the number of
-   * bytes in this chunk that were actually used; leftover bytes will be
-   * prepended to the next call.
-   */
-  virtual auto ProcessChunk(const cpp::span<std::byte>& chunk) -> void = 0;
-
-  virtual auto ProcessEndOfStream() -> void = 0;
-
-  virtual auto ProcessLogStatus() -> void {}
-
-  /*
-   * Called when there has been no data received over the input buffer for some
-   * time. This could be used to synthesize output, or to save memory by
-   * releasing unused resources.
-   */
-  virtual auto Process() -> void = 0;
-
- protected:
-  auto SendOrBufferEvent(std::unique_ptr<StreamEvent> event) -> bool;
-
-  // Queue for events coming into this element. Owned by us.
-  QueueHandle_t input_events_;
-  // Queue for events going into the next element. Not owned by us, may be null
-  // if we're not yet in a pipeline.
-  // FIXME: it would be nicer if this was non-nullable.
-  QueueHandle_t output_events_;
-  // Output events that have been generated, but are yet to be sent downstream.
-  std::deque<std::unique_ptr<StreamEvent>> buffered_output_;
+  virtual auto Process(const std::vector<InputStream>& inputs,
+                       OutputStream* output) -> void = 0;
 };
 
 }  // namespace audio
diff --git a/src/audio/include/audio_playback.hpp b/src/audio/include/audio_playback.hpp
index a1bea64f..cd4be3e7 100644
--- a/src/audio/include/audio_playback.hpp
+++ b/src/audio/include/audio_playback.hpp
@@ -5,7 +5,11 @@
 #include <string>
 #include <vector>
 
+#include "audio_task.hpp"
+#include "driver_cache.hpp"
 #include "esp_err.h"
+#include "fatfs_audio_input.hpp"
+#include "i2s_audio_output.hpp"
 #include "result.hpp"
 #include "span.hpp"
 
@@ -22,13 +26,7 @@ namespace audio {
  */
 class AudioPlayback {
  public:
-  enum Error { ERR_INIT_ELEMENT, ERR_MEM };
-  static auto create(drivers::GpioExpander* expander,
-                     std::shared_ptr<drivers::SdStorage> storage)
-      -> cpp::result<std::unique_ptr<AudioPlayback>, Error>;
-
-  // TODO(jacqueline): configure on the fly once we have things to configure.
-  AudioPlayback();
+  explicit AudioPlayback(drivers::DriverCache* drivers);
   ~AudioPlayback();
 
   /*
@@ -44,9 +42,9 @@ class AudioPlayback {
   AudioPlayback& operator=(const AudioPlayback&) = delete;
 
  private:
-  auto ConnectElements(IAudioElement* src, IAudioElement* sink) -> void;
-
-  QueueHandle_t input_handle_;
+  std::unique_ptr<FatfsAudioInput> file_source_;
+  std::unique_ptr<I2SAudioOutput> i2s_output_;
+  std::vector<std::unique_ptr<IAudioElement>> elements_;
 };
 
 }  // namespace audio
diff --git a/src/audio/include/audio_sink.hpp b/src/audio/include/audio_sink.hpp
new file mode 100644
index 00000000..7a535c35
--- /dev/null
+++ b/src/audio/include/audio_sink.hpp
@@ -0,0 +1,44 @@
+#pragma once
+
+#include <stdint.h>
+#include "audio_element.hpp"
+#include "esp_heap_caps.h"
+#include "freertos/FreeRTOS.h"
+#include "stream_info.hpp"
+namespace audio {
+
+class IAudioSink {
+ private:
+  // TODO: tune. at least about 12KiB seems right for mp3
+  static const std::size_t kDrainBufferSize = 48 * 1024;
+  uint8_t* buffer_;
+  StaticStreamBuffer_t* metadata_;
+  StreamBufferHandle_t handle_;
+
+ public:
+  IAudioSink()
+      : buffer_(reinterpret_cast<uint8_t*>(
+            heap_caps_malloc(kDrainBufferSize,
+                             MALLOC_CAP_INTERNAL | MALLOC_CAP_8BIT))),
+        metadata_(reinterpret_cast<StaticStreamBuffer_t*>(
+            heap_caps_malloc(sizeof(StaticStreamBuffer_t),
+                             MALLOC_CAP_INTERNAL | MALLOC_CAP_8BIT))),
+        handle_(xStreamBufferCreateStatic(kDrainBufferSize,
+                                          1,
+                                          buffer_,
+                                          metadata_)) {}
+
+  virtual ~IAudioSink() {
+    vStreamBufferDelete(handle_);
+    free(buffer_);
+    free(metadata_);
+  }
+
+  virtual auto Configure(const StreamInfo::Format& format) -> bool = 0;
+  virtual auto Send(const cpp::span<std::byte>& data) -> void = 0;
+  virtual auto Log() -> void {}
+
+  auto buffer() -> StreamBufferHandle_t { return handle_; }
+};
+
+}  // namespace audio
diff --git a/src/audio/include/audio_task.hpp b/src/audio/include/audio_task.hpp
index df70ebaa..a7b7a0fa 100644
--- a/src/audio/include/audio_task.hpp
+++ b/src/audio/include/audio_task.hpp
@@ -5,18 +5,33 @@
 #include <string>
 
 #include "audio_element.hpp"
+#include "audio_sink.hpp"
+#include "dac.hpp"
 #include "freertos/portmacro.h"
+#include "pipeline.hpp"
+#include "stream_buffer.hpp"
 
 namespace audio {
 
+namespace task {
+
+enum Command { PLAY, PAUSE, QUIT };
+
 struct AudioTaskArgs {
-  std::shared_ptr<IAudioElement>& element;
+  Pipeline* pipeline;
+  IAudioSink* sink;
 };
+struct AudioDrainArgs {
+  IAudioSink* sink;
+  std::atomic<Command>* command;
+};
+
+extern "C" void AudioTaskMain(void* args);
+extern "C" void AudioDrainMain(void* args);
 
-auto StartAudioTask(const std::string& name,
-                    std::optional<BaseType_t> core_id,
-                    std::shared_ptr<IAudioElement> element) -> void;
+auto StartPipeline(Pipeline* pipeline, IAudioSink* sink) -> void;
+auto StartDrain(IAudioSink* sink) -> void;
 
-void AudioTaskMain(void* args);
+}  // namespace task
 
 }  // namespace audio
diff --git a/src/audio/include/fatfs_audio_input.hpp b/src/audio/include/fatfs_audio_input.hpp
index 9f2d676c..24f62e3c 100644
--- a/src/audio/include/fatfs_audio_input.hpp
+++ b/src/audio/include/fatfs_audio_input.hpp
@@ -3,41 +3,37 @@
 #include <cstdint>
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "arena.hpp"
 #include "chunk.hpp"
 #include "freertos/FreeRTOS.h"
 
+#include "ff.h"
 #include "freertos/message_buffer.h"
 #include "freertos/queue.h"
 #include "span.hpp"
 
 #include "audio_element.hpp"
-#include "storage.hpp"
 #include "stream_buffer.hpp"
+#include "stream_info.hpp"
 
 namespace audio {
 
 class FatfsAudioInput : public IAudioElement {
  public:
-  explicit FatfsAudioInput(std::shared_ptr<drivers::SdStorage> storage);
+  explicit FatfsAudioInput();
   ~FatfsAudioInput();
 
-  auto HasUnprocessedInput() -> bool override;
-  auto IsOverBuffered() -> bool override;
+  auto OpenFile(const std::string& path) -> void;
 
-  auto ProcessStreamInfo(const StreamInfo& info) -> void override;
-  auto ProcessChunk(const cpp::span<std::byte>& chunk) -> void override;
-  auto ProcessEndOfStream() -> void override;
-  auto Process() -> void override;
+  auto Process(const std::vector<InputStream>& inputs, OutputStream* output)
+      -> void override;
 
   FatfsAudioInput(const FatfsAudioInput&) = delete;
   FatfsAudioInput& operator=(const FatfsAudioInput&) = delete;
 
  private:
-  memory::Arena arena_;
-  std::shared_ptr<drivers::SdStorage> storage_;
-
   FIL current_file_;
   bool is_file_open_;
 };
diff --git a/src/audio/include/i2s_audio_output.hpp b/src/audio/include/i2s_audio_output.hpp
index 2bea091b..07430777 100644
--- a/src/audio/include/i2s_audio_output.hpp
+++ b/src/audio/include/i2s_audio_output.hpp
@@ -2,34 +2,28 @@
 
 #include <cstdint>
 #include <memory>
+#include <vector>
 
 #include "audio_element.hpp"
+#include "audio_sink.hpp"
 #include "chunk.hpp"
 #include "result.hpp"
 
 #include "dac.hpp"
 #include "gpio_expander.hpp"
+#include "stream_info.hpp"
 
 namespace audio {
 
-class I2SAudioOutput : public IAudioElement {
+class I2SAudioOutput : public IAudioSink {
  public:
-  enum Error { DAC_CONFIG, I2S_CONFIG, STREAM_INIT };
-  static auto create(drivers::GpioExpander* expander)
-      -> cpp::result<std::shared_ptr<I2SAudioOutput>, Error>;
-
   I2SAudioOutput(drivers::GpioExpander* expander,
-                 std::unique_ptr<drivers::AudioDac> dac);
+                 std::shared_ptr<drivers::AudioDac> dac);
   ~I2SAudioOutput();
 
-  auto HasUnprocessedInput() -> bool override;
-  auto IsOverBuffered() -> bool override;
-
-  auto ProcessStreamInfo(const StreamInfo& info) -> void override;
-  auto ProcessChunk(const cpp::span<std::byte>& chunk) -> void override;
-  auto ProcessEndOfStream() -> void override;
-  auto ProcessLogStatus() -> void override;
-  auto Process() -> void override;
+  auto Configure(const StreamInfo::Format& format) -> bool override;
+  auto Send(const cpp::span<std::byte>& data) -> void override;
+  auto Log() -> void override;
 
   I2SAudioOutput(const I2SAudioOutput&) = delete;
   I2SAudioOutput& operator=(const I2SAudioOutput&) = delete;
@@ -38,10 +32,9 @@ class I2SAudioOutput : public IAudioElement {
   auto SetVolume(uint8_t volume) -> void;
 
   drivers::GpioExpander* expander_;
-  std::unique_ptr<drivers::AudioDac> dac_;
+  std::shared_ptr<drivers::AudioDac> dac_;
 
-  std::optional<ChunkReader> chunk_reader_;
-  cpp::span<std::byte> latest_chunk_;
+  std::optional<StreamInfo::Pcm> current_config_;
 };
 
 }  // namespace audio
diff --git a/src/audio/include/pipeline.hpp b/src/audio/include/pipeline.hpp
new file mode 100644
index 00000000..7a658a38
--- /dev/null
+++ b/src/audio/include/pipeline.hpp
@@ -0,0 +1,43 @@
+#pragma once
+
+#include <memory>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "freertos/portmacro.h"
+
+#include "audio_element.hpp"
+#include "himem.hpp"
+#include "stream_info.hpp"
+
+namespace audio {
+
+static const std::size_t kPipelineBufferSize = 64 * 1024;
+
+class Pipeline {
+ public:
+  explicit Pipeline(IAudioElement* output);
+  ~Pipeline();
+  auto AddInput(IAudioElement* input) -> Pipeline*;
+
+  auto OutputElement() const -> IAudioElement*;
+
+  auto NumInputs() const -> std::size_t;
+
+  auto InStreams(std::vector<MappableRegion<kPipelineBufferSize>>*,
+                 std::vector<RawStream>*) -> void;
+
+  auto OutStream(MappableRegion<kPipelineBufferSize>*) -> RawStream;
+
+  auto GetIterationOrder() -> std::vector<Pipeline*>;
+
+ private:
+  IAudioElement* root_;
+  std::vector<std::unique_ptr<Pipeline>> subtrees_;
+
+  HimemAlloc<kPipelineBufferSize> output_buffer_;
+  StreamInfo output_info_;
+};
+
+}  // namespace audio
diff --git a/src/audio/include/stream_info.hpp b/src/audio/include/stream_info.hpp
index bf67364f..28095935 100644
--- a/src/audio/include/stream_info.hpp
+++ b/src/audio/include/stream_info.hpp
@@ -4,19 +4,96 @@
 #include <optional>
 #include <string>
 #include <string_view>
+#include <type_traits>
+#include <utility>
+#include <variant>
 
-#include "cbor.h"
 #include "result.hpp"
-#include "sys/_stdint.h"
+#include "span.hpp"
+#include "types.hpp"
 
 namespace audio {
 
 struct StreamInfo {
-  std::optional<std::string> path;
-  std::optional<uint8_t> channels;
-  std::optional<uint8_t> bits_per_sample;
-  std::optional<uint16_t> sample_rate;
-  std::optional<size_t> chunk_size;
+  // The number of bytes that are available for consumption within this
+  // stream's buffer.
+  std::size_t bytes_in_stream{0};
+
+  // The total length of this stream, in case its source is finite (e.g. a
+  // file on disk). May be absent for endless streams (internet streams,
+  // generated audio, etc.)
+  std::optional<std::size_t> length_bytes{};
+
+  struct Encoded {
+    // The codec that this stream is associated with.
+    codecs::StreamType type;
+
+    bool operator==(const Encoded&) const = default;
+  };
+
+  struct Pcm {
+    // Number of channels in this stream.
+    uint8_t channels;
+    // Number of bits per sample.
+    uint8_t bits_per_sample;
+    // The sample rate.
+    uint32_t sample_rate;
+
+    bool operator==(const Pcm&) const = default;
+  };
+
+  typedef std::variant<std::monostate, Encoded, Pcm> Format;
+  Format format{};
+
+  bool operator==(const StreamInfo&) const = default;
+};
+
+class RawStream {
+ public:
+  StreamInfo* info;
+  cpp::span<std::byte> data;
+  bool is_incomplete;
+
+  RawStream(StreamInfo* i, cpp::span<std::byte> d)
+      : info(i), data(d), is_incomplete(false) {}
+};
+
+/*
+ * A byte buffer + associated metadata, which is not allowed to modify any of
+ * the underlying data.
+ */
+class InputStream {
+ public:
+  explicit InputStream(RawStream* s) : raw_(s) {}
+
+  void consume(std::size_t bytes) const;
+
+  void mark_incomplete() const;
+
+  const StreamInfo& info() const;
+
+  cpp::span<const std::byte> data() const;
+
+ private:
+  RawStream* raw_;
+};
+
+class OutputStream {
+ public:
+  explicit OutputStream(RawStream* s) : raw_(s) {}
+
+  void add(std::size_t bytes) const;
+
+  bool prepare(const StreamInfo::Format& new_format);
+
+  const StreamInfo& info() const;
+
+  cpp::span<std::byte> data() const;
+
+  bool is_incomplete() const;
+
+ private:
+  RawStream* raw_;
 };
 
 }  // namespace audio
diff --git a/src/audio/pipeline.cpp b/src/audio/pipeline.cpp
new file mode 100644
index 00000000..bab2f3ff
--- /dev/null
+++ b/src/audio/pipeline.cpp
@@ -0,0 +1,57 @@
+#include "pipeline.hpp"
+#include <memory>
+#include "stream_info.hpp"
+
+namespace audio {
+
+Pipeline::Pipeline(IAudioElement* output) : root_(output), subtrees_() {
+  assert(output != nullptr);
+}
+
+Pipeline::~Pipeline() {}
+
+auto Pipeline::AddInput(IAudioElement* input) -> Pipeline* {
+  subtrees_.push_back(std::make_unique<Pipeline>(input));
+  return subtrees_.back().get();
+}
+
+auto Pipeline::OutputElement() const -> IAudioElement* {
+  return root_;
+}
+
+auto Pipeline::NumInputs() const -> std::size_t {
+  return subtrees_.size();
+}
+
+auto Pipeline::InStreams(
+    std::vector<MappableRegion<kPipelineBufferSize>>* regions,
+    std::vector<RawStream>* out) -> void {
+  for (int i = 0; i < subtrees_.size(); i++) {
+    RawStream s = subtrees_[i]->OutStream(&regions->at(i));
+    out->push_back(s);
+  }
+}
+
+auto Pipeline::OutStream(MappableRegion<kPipelineBufferSize>* region)
+    -> RawStream {
+  return {&output_info_, region->Map(output_buffer_)};
+}
+
+auto Pipeline::GetIterationOrder() -> std::vector<Pipeline*> {
+  std::vector<Pipeline*> to_search{this};
+  std::vector<Pipeline*> found;
+
+  while (!to_search.empty()) {
+    Pipeline* current = to_search.back();
+    to_search.pop_back();
+    found.push_back(current);
+
+    for (const auto& i : current->subtrees_) {
+      to_search.push_back(i.get());
+    }
+  }
+
+  return found;
+}
+
+}  // namespace audio
diff --git a/src/audio/stream_info.cpp b/src/audio/stream_info.cpp
new file mode 100644
index 00000000..7d833d25
--- /dev/null
+++ b/src/audio/stream_info.cpp
@@ -0,0 +1,70 @@
+#include "stream_info.hpp"
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+#include "result.hpp"
+#include "span.hpp"
+#include "types.hpp"
+
+namespace audio {
+
+void InputStream::consume(std::size_t bytes) const {
+  assert(raw_->info->bytes_in_stream >= bytes);
+  auto new_data = raw_->data.subspan(bytes);
+  std::move(new_data.begin(), new_data.end(), raw_->data.begin());
+  raw_->info->bytes_in_stream = new_data.size_bytes();
+}
+
+void InputStream::mark_incomplete() const {
+  raw_->is_incomplete = true;
+}
+
+const StreamInfo& InputStream::info() const {
+  return *raw_->info;
+}
+
+cpp::span<const std::byte> InputStream::data() const {
+  return raw_->data.first(raw_->info->bytes_in_stream);
+}
+
+void OutputStream::add(std::size_t bytes) const {
+  assert(raw_->info->bytes_in_stream + bytes <= raw_->data.size_bytes());
+  raw_->info->bytes_in_stream += bytes;
+}
+
+bool OutputStream::prepare(const StreamInfo::Format& new_format) {
+  if (std::holds_alternative<std::monostate>(raw_->info->format)) {
+    raw_->info->format = new_format;
+    raw_->info->bytes_in_stream = 0;
+    return true;
+  }
+  if (new_format == raw_->info->format) {
+    return true;
+  }
+  if (raw_->is_incomplete) {
+    raw_->info->format = new_format;
+    raw_->info->bytes_in_stream = 0;
+    return true;
+  }
+  return false;
+}
+
+const StreamInfo& OutputStream::info() const {
+  return *raw_->info;
+}
+
+cpp::span<std::byte> OutputStream::data() const {
+  return raw_->data.subspan(raw_->info->bytes_in_stream);
+}
+
+bool OutputStream::is_incomplete() const {
+  return raw_->is_incomplete;
+}
+
+}  // namespace audio
diff --git a/src/codecs/codec.cpp b/src/codecs/codec.cpp
index 4e9a6a47..bdc8f51e 100644
--- a/src/codecs/codec.cpp
+++ b/src/codecs/codec.cpp
@@ -5,7 +5,7 @@
 
 namespace codecs {
 
-auto CreateCodecForFile(const std::string& file)
+auto CreateCodecForType(StreamType type)
     -> cpp::result<std::unique_ptr<ICodec>, CreateCodecError> {
   return std::make_unique<MadMp3Decoder>();  // TODO.
 }
diff --git a/src/codecs/include/codec.hpp b/src/codecs/include/codec.hpp
index 6897acf2..4595f877 100644
--- a/src/codecs/include/codec.hpp
+++ b/src/codecs/include/codec.hpp
@@ -10,6 +10,7 @@
 
 #include "result.hpp"
 #include "span.hpp"
+#include "types.hpp"
 
 namespace codecs {
 
@@ -17,7 +18,7 @@ class ICodec {
  public:
   virtual ~ICodec() {}
 
-  virtual auto CanHandleFile(const std::string& path) -> bool = 0;
+  virtual auto CanHandleType(StreamType type) -> bool = 0;
 
   struct OutputFormat {
     uint8_t num_channels;
@@ -31,7 +32,7 @@ class ICodec {
 
   virtual auto ResetForNewStream() -> void = 0;
 
-  virtual auto SetInput(cpp::span<std::byte> input) -> void = 0;
+  virtual auto SetInput(cpp::span<const std::byte> input) -> void = 0;
 
   /*
    * Returns the codec's next read position within the input buffer. If the
@@ -63,7 +64,7 @@ class ICodec {
 
 enum CreateCodecError { UNKNOWN_EXTENSION };
 
-auto CreateCodecForFile(const std::string& file)
+auto CreateCodecForType(StreamType type)
     -> cpp::result<std::unique_ptr<ICodec>, CreateCodecError>;
 
 }  // namespace codecs
diff --git a/src/codecs/include/mad.hpp b/src/codecs/include/mad.hpp
index 1f5791b9..074784fb 100644
--- a/src/codecs/include/mad.hpp
+++ b/src/codecs/include/mad.hpp
@@ -17,10 +17,10 @@ class MadMp3Decoder : public ICodec {
   MadMp3Decoder();
   ~MadMp3Decoder();
 
-  auto CanHandleFile(const std::string& path) -> bool override;
+  auto CanHandleType(StreamType type) -> bool override;
   auto GetOutputFormat() -> OutputFormat override;
   auto ResetForNewStream() -> void override;
-  auto SetInput(cpp::span<std::byte> input) -> void override;
+  auto SetInput(cpp::span<const std::byte> input) -> void override;
   auto GetInputPosition() -> std::size_t override;
   auto ProcessNextFrame() -> cpp::result<bool, ProcessingError> override;
   auto WriteOutputSamples(cpp::span<std::byte> output)
diff --git a/src/codecs/mad.cpp b/src/codecs/mad.cpp
index 1112bd62..dd839537 100644
--- a/src/codecs/mad.cpp
+++ b/src/codecs/mad.cpp
@@ -1,16 +1,18 @@
 #include "mad.hpp"
+#include <stdint.h>
 
 #include <cstdint>
 
 #include "mad.h"
 
 #include "codec.hpp"
+#include "types.hpp"
 
 namespace codecs {
 
-static int scaleTo24Bits(mad_fixed_t sample) {
+static uint32_t scaleToBits(mad_fixed_t sample, uint8_t bits) {
   // Round the bottom bits.
-  sample += (1L << (MAD_F_FRACBITS - 16));
+  sample += (1L << (MAD_F_FRACBITS - bits));
 
   // Clip the leftover bits to within range.
   if (sample >= MAD_F_ONE)
@@ -18,8 +20,8 @@ static int scaleTo24Bits(mad_fixed_t sample) {
   else if (sample < -MAD_F_ONE)
     sample = -MAD_F_ONE;
 
-  /* quantize */
-  return sample >> (MAD_F_FRACBITS + 1 - 16);
+  // Quantize.
+  return sample >> (MAD_F_FRACBITS + 1 - bits);
 }
 
 MadMp3Decoder::MadMp3Decoder() {
@@ -35,8 +37,8 @@ MadMp3Decoder::~MadMp3Decoder() {
   mad_header_finish(&header_);
 }
 
-auto MadMp3Decoder::CanHandleFile(const std::string& path) -> bool {
-  return true;  // TODO.
+auto MadMp3Decoder::CanHandleType(StreamType type) -> bool {
+  return type == STREAM_MP3;
 }
 
 auto MadMp3Decoder::GetOutputFormat() -> OutputFormat {
@@ -52,7 +54,7 @@ auto MadMp3Decoder::ResetForNewStream() -> void {
   has_decoded_header_ = false;
 }
 
-auto MadMp3Decoder::SetInput(cpp::span<std::byte> input) -> void {
+auto MadMp3Decoder::SetInput(cpp::span<const std::byte> input) -> void {
   mad_stream_buffer(&stream_,
                     reinterpret_cast<const unsigned char*>(input.data()),
                     input.size());
@@ -113,15 +115,26 @@ auto MadMp3Decoder::WriteOutputSamples(cpp::span<std::byte> output)
   }
 
   while (current_sample_ < synth_.pcm.length) {
-    if (output_byte + (3 * synth_.pcm.channels) >= output.size()) {
+    if (output_byte + (2 * synth_.pcm.channels) >= output.size()) {
       return std::make_pair(output_byte, false);
     }
 
     for (int channel = 0; channel < synth_.pcm.channels; channel++) {
+      // TODO(jacqueline): output 24 bit samples when (if?) we have a downmix
+      // step in the pipeline.
+      /*
       uint32_t sample_24 =
-          scaleTo24Bits(synth_.pcm.samples[channel][current_sample_]);
+          scaleToBits(synth_.pcm.samples[channel][current_sample_], 24);
+      output[output_byte++] = static_cast<std::byte>((sample_24 >> 16) & 0xFF);
       output[output_byte++] = static_cast<std::byte>((sample_24 >> 8) & 0xFF);
       output[output_byte++] = static_cast<std::byte>((sample_24)&0xFF);
+      // 24 bit samples must still be aligned to 32 bits. The LSB is ignored.
+      output[output_byte++] = static_cast<std::byte>(0);
+      */
+      uint16_t sample_16 =
+          scaleToBits(synth_.pcm.samples[channel][current_sample_], 16);
+      output[output_byte++] = static_cast<std::byte>((sample_16 >> 8) & 0xFF);
+      output[output_byte++] = static_cast<std::byte>((sample_16)&0xFF);
     }
     current_sample_++;
   }
diff --git a/src/drivers/CMakeLists.txt b/src/drivers/CMakeLists.txt
index c4e4c172..072a8b68 100644
--- a/src/drivers/CMakeLists.txt
+++ b/src/drivers/CMakeLists.txt
@@ -1,6 +1,6 @@
 idf_component_register(
-  SRCS "dac.cpp" "gpio_expander.cpp" "battery.cpp" "storage.cpp" "i2c.cpp"
-  "spi.cpp" "display.cpp" "display_init.cpp"
+  SRCS "touchwheel.cpp" "dac.cpp" "gpio_expander.cpp" "battery.cpp" "storage.cpp" "i2c.cpp"
+  "spi.cpp" "display.cpp" "display_init.cpp" "driver_cache.cpp"
   INCLUDE_DIRS "include"
   REQUIRES "esp_adc" "fatfs" "result" "lvgl" "span")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/drivers/battery.cpp b/src/drivers/battery.cpp
index 00e7796a..8d747c07 100644
--- a/src/drivers/battery.cpp
+++ b/src/drivers/battery.cpp
@@ -13,8 +13,8 @@ static const adc_unit_t kAdcUnit = ADC_UNIT_1;
 // Max battery voltage should be a little over 2V due to our divider, so we need
 // the max attenuation to properly handle the full range.
 static const adc_atten_t kAdcAttenuation = ADC_ATTEN_DB_11;
-// Corresponds to GPIO 34.
-static const adc_channel_t kAdcChannel = ADC_CHANNEL_6;
+// Corresponds to SENSOR_VP.
+static const adc_channel_t kAdcChannel = ADC_CHANNEL_0;
 
 Battery::Battery() {
   adc_oneshot_unit_init_cfg_t unit_config = {
diff --git a/src/drivers/dac.cpp b/src/drivers/dac.cpp
index 1ab562f9..e82f0d27 100644
--- a/src/drivers/dac.cpp
+++ b/src/drivers/dac.cpp
@@ -7,36 +7,38 @@
 #include "driver/i2c.h"
 #include "driver/i2s_common.h"
 #include "driver/i2s_std.h"
-#include "driver/i2s_types.h"
 #include "esp_attr.h"
 #include "esp_err.h"
 #include "esp_log.h"
 #include "freertos/portmacro.h"
 #include "freertos/projdefs.h"
+#include "hal/gpio_types.h"
 #include "hal/i2c_types.h"
 
 #include "gpio_expander.hpp"
 #include "hal/i2s_types.h"
 #include "i2c.hpp"
+#include "soc/clk_tree_defs.h"
 #include "sys/_stdint.h"
 
 namespace drivers {
 
 static const char* kTag = "AUDIODAC";
 static const uint8_t kPcm5122Address = 0x4C;
-static const uint8_t kPcm5122Timeout = pdMS_TO_TICKS(100);
 static const i2s_port_t kI2SPort = I2S_NUM_0;
 
-static const AudioDac::SampleRate kDefaultSampleRate =
-    AudioDac::SAMPLE_RATE_44_1;
-static const AudioDac::BitsPerSample kDefaultBps = AudioDac::BPS_16;
-
-auto AudioDac::create(GpioExpander* expander)
-    -> cpp::result<std::unique_ptr<AudioDac>, Error> {
+auto AudioDac::create(GpioExpander* expander) -> cpp::result<AudioDac*, Error> {
   // TODO: tune.
   i2s_chan_handle_t i2s_handle;
   i2s_chan_config_t channel_config =
       I2S_CHANNEL_DEFAULT_CONFIG(kI2SPort, I2S_ROLE_MASTER);
+  // Use the maximum possible DMA buffer size, since a smaller number of large
+  // copies is faster than a large number of small copies.
+  channel_config.dma_frame_num = 1024;
+  // Triple buffering should be enough to keep samples flowing smoothly.
+  // TODO(jacqueline): verify this with 192kHz 32bps.
+  channel_config.dma_desc_num = 4;
+  // channel_config.auto_clear = true;
 
   ESP_ERROR_CHECK(i2s_new_channel(&channel_config, &i2s_handle, NULL));
   //
@@ -50,21 +52,22 @@ auto AudioDac::create(GpioExpander* expander)
   i2s_std_config_t i2s_config = {
       .clk_cfg = dac->clock_config_,
       .slot_cfg = dac->slot_config_,
-      .gpio_cfg =
-          {// TODO: investigate running in three wire mode for less noise
-           .mclk = GPIO_NUM_0,
-           .bclk = GPIO_NUM_26,
-           .ws = GPIO_NUM_27,
-           .dout = GPIO_NUM_5,
-           .din = I2S_GPIO_UNUSED,
-           .invert_flags =
-               {
-                   .mclk_inv = false,
-                   .bclk_inv = false,
-                   .ws_inv = false,
-               }},
+      .gpio_cfg = {.mclk = GPIO_NUM_0,
+                   .bclk = GPIO_NUM_26,
+                   .ws = GPIO_NUM_27,
+                   .dout = GPIO_NUM_5,
+                   .din = I2S_GPIO_UNUSED,
+                   .invert_flags =
+                       {
+                           .mclk_inv = false,
+                           .bclk_inv = false,
+                           .ws_inv = true,
+                       }},
   };
 
+  // gpio_set_direction(GPIO_NUM_0, GPIO_MODE_OUTPUT);
+  // gpio_set_level(GPIO_NUM_0, 0);
+
   if (esp_err_t err =
           i2s_channel_init_std_mode(i2s_handle, &i2s_config) != ESP_OK) {
     ESP_LOGE(kTag, "failed to initialise i2s channel %x", err);
@@ -81,65 +84,64 @@ auto AudioDac::create(GpioExpander* expander)
 
   // The DAC should be booted but in power down mode, but it might not be if we
   // didn't shut down cleanly. Reset it to ensure it is in a consistent state.
-  dac->WriteRegister(Register::POWER_MODE, 0b10001);
-  dac->WriteRegister(Register::POWER_MODE, 1 << 4);
-  dac->WriteRegister(Register::RESET, 0b10001);
+  dac->WriteRegister(pcm512x::POWER, 1 << 4);
+  dac->WriteRegister(pcm512x::RESET, 0b10001);
+
+  // Use BCK for the internal PLL.
+  // dac->WriteRegister(Register::PLL_CLOCK_SOURCE, 1 << 4);
+  // dac->WriteRegister(Register::DAC_CLOCK_SOURCE, 0b11 << 5);
+
+  // dac->WriteRegister(Register::PLL_ENABLE, 0);
+  // dac->WriteRegister(Register::DAC_CLOCK_SOURCE, 0b0110000);
+  // dac->WriteRegister(Register::CLOCK_ERRORS, 0b01000001);
+  // dac->WriteRegister(Register::I2S_FORMAT, 0b110000);
+  //  dac->WriteRegister(Register::INTERPOLATION, 1 << 4);
+
+  dac->Reconfigure(BPS_16, SAMPLE_RATE_44_1);
 
   // Now configure the DAC for standard auto-clock SCK mode.
-  dac->WriteRegister(Register::DAC_CLOCK_SOURCE, 0b11 << 5);
 
   // Enable auto clocking, and do your best to carry on despite errors.
   // dac->WriteRegister(Register::CLOCK_ERRORS, 0b1111101);
 
-  i2s_channel_enable(dac->i2s_handle_);
+  // i2s_channel_enable(dac->i2s_handle_);
 
-  dac->WaitForPowerState(
-      [](bool booted, PowerState state) { return state == STANDBY; });
+  dac->WaitForPowerState([](bool booted, PowerState state) {
+    return state == RUN || state == STANDBY;
+  });
 
-  return dac;
+  return dac.release();
 }
 
 AudioDac::AudioDac(GpioExpander* gpio, i2s_chan_handle_t i2s_handle)
     : gpio_(gpio),
       i2s_handle_(i2s_handle),
+      i2s_active_(false),
+      active_page_(),
       clock_config_(I2S_STD_CLK_DEFAULT_CONFIG(44100)),
       slot_config_(I2S_STD_MSB_SLOT_DEFAULT_CONFIG(I2S_DATA_BIT_WIDTH_16BIT,
                                                    I2S_SLOT_MODE_STEREO)) {
-  gpio_->set_pin(GpioExpander::AUDIO_POWER_ENABLE, true);
+  clock_config_.clk_src = I2S_CLK_SRC_PLL_160M;
+  gpio_->set_pin(GpioExpander::AMP_EN, true);
   gpio_->Write();
 }
 
 AudioDac::~AudioDac() {
   i2s_channel_disable(i2s_handle_);
   i2s_del_channel(i2s_handle_);
-  gpio_->set_pin(GpioExpander::AUDIO_POWER_ENABLE, false);
+  gpio_->set_pin(GpioExpander::AMP_EN, false);
   gpio_->Write();
 }
 
 void AudioDac::WriteVolume(uint8_t volume) {
-  WriteRegister(Register::DIGITAL_VOLUME_L, volume);
-  WriteRegister(Register::DIGITAL_VOLUME_R, volume);
+  // Left channel.
+  WriteRegister(pcm512x::DIGITAL_VOLUME_2, volume);
+  // Right channel.
+  WriteRegister(pcm512x::DIGITAL_VOLUME_3, volume);
 }
 
 std::pair<bool, AudioDac::PowerState> AudioDac::ReadPowerState() {
-  uint8_t result = 0;
-
-  I2CTransaction transaction;
-  transaction.start()
-      .write_addr(kPcm5122Address, I2C_MASTER_WRITE)
-      .write_ack(DSP_BOOT_POWER_STATE)
-      .start()
-      .write_addr(kPcm5122Address, I2C_MASTER_READ)
-      .read(&result, I2C_MASTER_NACK)
-      .stop();
-
-  esp_err_t err = transaction.Execute();
-  if (err == ESP_ERR_TIMEOUT) {
-    return std::pair(false, POWERDOWN);
-  } else {
-  }
-  ESP_ERROR_CHECK(err);
-
+  uint8_t result = ReadRegister(pcm512x::POWER_STATE);
   bool is_booted = result >> 7;
   PowerState detail = (PowerState)(result & 0b1111);
   return std::pair(is_booted, detail);
@@ -163,13 +165,29 @@ bool AudioDac::WaitForPowerState(
 }
 
 auto AudioDac::Reconfigure(BitsPerSample bps, SampleRate rate) -> void {
-  // Disable the current output, if it isn't already stopped.
-  WriteRegister(Register::POWER_MODE, 1 << 4);
-  i2s_channel_disable(i2s_handle_);
+  if (i2s_active_) {
+    WriteRegister(pcm512x::MUTE, 0b10001);
+    vTaskDelay(1);
+    WriteRegister(pcm512x::POWER, 1 << 4);
+    i2s_channel_disable(i2s_handle_);
+  }
 
   // I2S reconfiguration.
-
-  slot_config_.slot_bit_width = (i2s_slot_bit_width_t)bps;
+  uint8_t bps_bits = 0;
+  switch (bps) {
+    case BPS_16:
+      slot_config_.data_bit_width = I2S_DATA_BIT_WIDTH_16BIT;
+      bps_bits = 0;
+      break;
+    case BPS_24:
+      slot_config_.data_bit_width = I2S_DATA_BIT_WIDTH_24BIT;
+      bps_bits = 0b10;
+      break;
+    case BPS_32:
+      slot_config_.data_bit_width = I2S_DATA_BIT_WIDTH_32BIT;
+      bps_bits = 0b11;
+      break;
+  }
   ESP_ERROR_CHECK(i2s_channel_reconfig_std_slot(i2s_handle_, &slot_config_));
 
   clock_config_.sample_rate_hz = rate;
@@ -181,30 +199,185 @@ auto AudioDac::Reconfigure(BitsPerSample bps, SampleRate rate) -> void {
   ESP_ERROR_CHECK(i2s_channel_reconfig_std_clock(i2s_handle_, &clock_config_));
 
   // DAC reconfiguration.
+  // Inspired heavily by https://github.com/tommag/PCM51xx_Arduino (MIT).
+
+  // Check that the bit clock (PLL input) is between 1MHz and 50MHz. It always
+  // should be.
+  uint32_t bckFreq = rate * bps * 2;
+  if (bckFreq < 1000000 || bckFreq > 50000000) {
+    ESP_LOGE(kTag, "bck freq out of range");
+    return;
+  }
+
+  // 24 bits is not supported for 44.1kHz and 48kHz.
+  if ((rate == SAMPLE_RATE_44_1 || rate == SAMPLE_RATE_48) && bps == BPS_24) {
+    // TODO(jacqueline): I think this *can* be implemented, but requires a bunch
+    // of maths.
+    ESP_LOGE(kTag, "sample rate and bps mismatch");
+    return;
+  }
 
-  // TODO: base on BPS
-  WriteRegister(Register::I2S_FORMAT, 0);
+  // Initialize system clock from the I2S BCK input
+  //  Disable clock autoset and ignore SCK detection
+  WriteRegister(pcm512x::ERROR_DETECT, 0x1A);
+  // Set PLL clock source to BCK
+  WriteRegister(pcm512x::PLL_REF, 0x10);
+  // Set DAC clock source to PLL output
+  WriteRegister(pcm512x::DAC_REF, 0x10);
+
+  // PLL configuration
+  int p, j, d, r;
+
+  // Clock dividers
+  int nmac, ndac, ncp, dosr, idac;
+
+  if (rate == SAMPLE_RATE_11_025 || rate == SAMPLE_RATE_22_05 ||
+      rate == SAMPLE_RATE_44_1) {
+    // 44.1kHz and derivatives.
+    // P = 1, R = 2, D = 0 for all supported combinations.
+    // Set J to have PLL clk = 90.3168 MHz
+    p = 1;
+    r = 2;
+    j = 90316800 / bckFreq / r;
+    d = 0;
+
+    // Derive clocks from the 90.3168MHz PLL
+    nmac = 2;
+    ndac = 16;
+    ncp = 4;
+    dosr = 8;
+    idac = 1024;  // DSP clock / sample rate
+  } else {
+    // 8kHz and multiples.
+    // PLL config for a 98.304 MHz PLL clk
+    if (bps == BPS_24 && bckFreq > 1536000) {
+      p = 3;
+    } else if (bckFreq > 12288000) {
+      p = 2;
+    } else {
+      p = 1;
+    }
+
+    r = 2;
+    j = 98304000 / (bckFreq / p) / r;
+    d = 0;
+
+    // Derive clocks from the 98.304MHz PLL
+    switch (rate) {
+      case SAMPLE_RATE_16:
+        nmac = 6;
+        break;
+      case SAMPLE_RATE_32:
+        nmac = 3;
+        break;
+      default:
+        nmac = 2;
+        break;
+    }
+
+    ndac = 16;
+    ncp = 4;
+    dosr = 384000 / rate;
+    idac = 98304000 / nmac / rate;  // DSP clock / sample rate
+  }
+
+  // Configure PLL
+  WriteRegister(pcm512x::PLL_COEFF_0, p - 1);
+  WriteRegister(pcm512x::PLL_COEFF_1, j);
+  WriteRegister(pcm512x::PLL_COEFF_2, (d >> 8) & 0x3F);
+  WriteRegister(pcm512x::PLL_COEFF_3, d & 0xFF);
+  WriteRegister(pcm512x::PLL_COEFF_4, r - 1);
+
+  // Clock dividers
+  WriteRegister(pcm512x::DSP_CLKDIV, nmac - 1);
+  WriteRegister(pcm512x::DAC_CLKDIV, ndac - 1);
+  WriteRegister(pcm512x::NCP_CLKDIV, ncp - 1);
+  WriteRegister(pcm512x::OSR_CLKDIV, dosr - 1);
+
+  // IDAC (nb of DSP clock cycles per sample)
+  WriteRegister(pcm512x::IDAC_1, (idac >> 8) & 0xFF);
+  WriteRegister(pcm512x::IDAC_2, idac & 0xFF);
+
+  // FS speed mode
+  int speedMode;
+  if (rate <= SAMPLE_RATE_48) {
+    speedMode = 0;
+  } else if (rate <= SAMPLE_RATE_96) {
+    speedMode = 1;
+  } else if (rate <= SAMPLE_RATE_192) {
+    speedMode = 2;
+  } else {
+    speedMode = 3;
+  }
+  WriteRegister(pcm512x::FS_SPEED_MODE, speedMode);
+
+  WriteRegister(pcm512x::I2S_1, (0b11 << 4) | bps_bits);
+  WriteRegister(pcm512x::I2S_2, 0);
 
   // Configuration is all done, so we can now bring the DAC and I2S stream back
   // up. I2S first, since otherwise the DAC will see that there's no clocks and
   // shut itself down.
   ESP_ERROR_CHECK(i2s_channel_enable(i2s_handle_));
-  WriteRegister(Register::POWER_MODE, 0);
+  WriteRegister(pcm512x::POWER, 0);
+
+  if (i2s_active_) {
+    vTaskDelay(1);
+    WriteRegister(pcm512x::MUTE, 0);
+  }
+  i2s_active_ = true;
 }
 
-auto AudioDac::WriteData(cpp::span<std::byte> data) -> std::size_t {
+auto AudioDac::WriteData(const cpp::span<const std::byte>& data) -> void {
   std::size_t bytes_written = 0;
   esp_err_t err = i2s_channel_write(i2s_handle_, data.data(), data.size_bytes(),
-                                    &bytes_written, 0);
+                                    &bytes_written, portMAX_DELAY);
   if (err != ESP_ERR_TIMEOUT) {
     ESP_ERROR_CHECK(err);
   }
-  return bytes_written;
+}
+
+extern "C" IRAM_ATTR auto callback(i2s_chan_handle_t handle,
+                                   i2s_event_data_t* event,
+                                   void* user_ctx) -> bool {
+  if (event == nullptr || user_ctx == nullptr) {
+    return false;
+  }
+  if (event->data == nullptr || event->size == 0) {
+    return false;
+  }
+  uint8_t** buf = reinterpret_cast<uint8_t**>(event->data);
+  StreamBufferHandle_t src = reinterpret_cast<StreamBufferHandle_t>(user_ctx);
+  BaseType_t ret = false;
+  std::size_t bytes_received =
+      xStreamBufferReceiveFromISR(src, *buf, event->size, &ret);
+  if (bytes_received < event->size) {
+    memset(*buf + bytes_received, 0, event->size - bytes_received);
+  }
+  return ret;
+}
+
+auto AudioDac::SetSource(StreamBufferHandle_t buffer) -> void {
+  if (i2s_active_) {
+    ESP_ERROR_CHECK(i2s_channel_disable(i2s_handle_));
+  }
+  i2s_event_callbacks_t callbacks{
+      .on_recv = NULL,
+      .on_recv_q_ovf = NULL,
+      .on_sent = NULL,
+      .on_send_q_ovf = NULL,
+  };
+  if (buffer != nullptr) {
+    callbacks.on_sent = &callback;
+  }
+  i2s_channel_register_event_callback(i2s_handle_, &callbacks, buffer);
+  if (i2s_active_) {
+    ESP_ERROR_CHECK(i2s_channel_enable(i2s_handle_));
+  }
 }
 
 auto AudioDac::Stop() -> void {
   LogStatus();
-  WriteRegister(Register::POWER_MODE, 1 << 4);
+  WriteRegister(pcm512x::POWER, 1 << 4);
   i2s_channel_disable(i2s_handle_);
 }
 
@@ -218,35 +391,47 @@ auto AudioDac::Stop() -> void {
 auto AudioDac::LogStatus() -> void {
   uint8_t res;
 
-  res = ReadRegister(Register::SAMPLE_RATE_DETECTION);
-  ESP_LOGI(kTag, "detected sample rate (want 3): %u", (res >> 4) && 0b111);
+  res = ReadRegister(pcm512x::RATE_DET_1);
+  ESP_LOGI(kTag, "detected sample rate (want 3): %u", (res & 0b01110000) >> 4);
   ESP_LOGI(kTag, "detected SCK ratio (want 6): %u", res && 0b1111);
 
-  res = ReadRegister(Register::BCK_DETECTION);
+  res = ReadRegister(pcm512x::RATE_DET_3);
   ESP_LOGI(kTag, "detected BCK (want... 16? 32?): %u", res);
 
-  res = ReadRegister(Register::CLOCK_ERROR_STATE);
+  res = ReadRegister(pcm512x::RATE_DET_4);
   ESP_LOGI(kTag, "clock errors (want zeroes): ");
   ESP_LOGI(kTag, BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(res & 0b1111111));
 
-  res = ReadRegister(Register::CLOCK_STATUS);
+  res = ReadRegister(pcm512x::CLOCK_STATUS);
   ESP_LOGI(kTag, "clock status (want zeroes): ");
   ESP_LOGI(kTag, BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(res & 0b10111));
 
-  res = ReadRegister(Register::AUTO_MUTE_STATE);
-  ESP_LOGI(kTag, "automute status (want 3): %u", res & 0b11);
-
-  res = ReadRegister(Register::SOFT_MUTE_STATE);
-  ESP_LOGI(kTag, "soft mute pin status (want 3): %u", res & 0b11);
-
-  res = ReadRegister(Register::SAMPLE_RATE_STATE);
-  ESP_LOGI(kTag, "detected sample speed mode (want 0): %u", res & 0b11);
+  res = ReadRegister(pcm512x::DIGITAL_MUTE_DET);
+  ESP_LOGI(kTag, "automute status (want 0): %u", res & 0b10001);
 
   auto power = ReadPowerState();
   ESP_LOGI(kTag, "current power state (want 5): %u", power.second);
 }
 
-void AudioDac::WriteRegister(Register reg, uint8_t val) {
+void AudioDac::WriteRegister(pcm512x::Register r, uint8_t val) {
+  SelectPage(r.page);
+  WriteRegisterRaw(r.reg, val);
+}
+
+uint8_t AudioDac::ReadRegister(pcm512x::Register r) {
+  SelectPage(r.page);
+  return ReadRegisterRaw(r.reg);
+}
+
+void AudioDac::SelectPage(uint8_t page) {
+  if (active_page_ && active_page_ == page) {
+    return;
+  }
+  WriteRegisterRaw(0, page);
+  active_page_ = page;
+}
+
+void AudioDac::WriteRegisterRaw(uint8_t reg, uint8_t val) {
   I2CTransaction transaction;
   transaction.start()
       .write_addr(kPcm5122Address, I2C_MASTER_WRITE)
@@ -256,7 +441,7 @@ void AudioDac::WriteRegister(Register reg, uint8_t val) {
   transaction.Execute();
 }
 
-uint8_t AudioDac::ReadRegister(Register reg) {
+uint8_t AudioDac::ReadRegisterRaw(uint8_t reg) {
   uint8_t result = 0;
   I2CTransaction transaction;
   transaction.start()
diff --git a/src/drivers/display.cpp b/src/drivers/display.cpp
index 6ec82787..f8594a5a 100644
--- a/src/drivers/display.cpp
+++ b/src/drivers/display.cpp
@@ -20,6 +20,7 @@
 
 #include "display_init.hpp"
 #include "gpio_expander.hpp"
+#include "soc/soc.h"
 
 static const char* kTag = "DISPLAY";
 
@@ -28,6 +29,10 @@ static const uint8_t kDisplayWidth = 128 + 2;
 static const uint8_t kDisplayHeight = 160 + 1;
 static const uint8_t kTransactionQueueSize = 10;
 
+static const gpio_num_t kDisplayDr = GPIO_NUM_33;
+static const gpio_num_t kDisplayLedEn = GPIO_NUM_32;
+static const gpio_num_t kDisplayCs = GPIO_NUM_22;
+
 /*
  * The size of each of our two display buffers. This is fundamentally a balance
  * between performance and memory usage. LVGL docs recommend a buffer 1/10th the
@@ -58,11 +63,28 @@ extern "C" void FlushDataCallback(lv_disp_drv_t* disp_drv,
 
 auto Display::create(GpioExpander* expander,
                      const displays::InitialisationData& init_data)
-    -> std::unique_ptr<Display> {
-  // First, turn on the LED backlight.
-  expander->set_pin(GpioExpander::DISPLAY_LED, 0);
-  expander->set_pin(GpioExpander::DISPLAY_POWER_ENABLE, 1);
-  expander->Write();
+    -> Display* {
+  ESP_LOGI(kTag, "Init I/O pins");
+  gpio_config_t dr_config{
+      .pin_bit_mask = 1ULL << kDisplayDr,
+      .mode = GPIO_MODE_OUTPUT,
+      .pull_up_en = GPIO_PULLUP_ENABLE,
+      .pull_down_en = GPIO_PULLDOWN_DISABLE,
+      .intr_type = GPIO_INTR_DISABLE,
+  };
+  gpio_config(&dr_config);
+  gpio_set_level(kDisplayDr, 0);
+
+  // TODO: use pwm for the backlight.
+  gpio_config_t led_config{
+      .pin_bit_mask = 1ULL << kDisplayLedEn,
+      .mode = GPIO_MODE_OUTPUT,
+      .pull_up_en = GPIO_PULLUP_ENABLE,
+      .pull_down_en = GPIO_PULLDOWN_DISABLE,
+      .intr_type = GPIO_INTR_DISABLE,
+  };
+  gpio_config(&led_config);
+  gpio_set_level(kDisplayLedEn, 1);
 
   // Next, init the SPI device
   spi_device_interface_config_t spi_cfg = {
@@ -76,7 +98,7 @@ auto Display::create(GpioExpander* expander,
       .cs_ena_posttrans = 0,
       .clock_speed_hz = SPI_MASTER_FREQ_40M,
       .input_delay_ns = 0,
-      .spics_io_num = GPIO_NUM_22,
+      .spics_io_num = kDisplayCs,
       .flags = 0,
       .queue_size = kTransactionQueueSize,
       .pre_cb = NULL,
@@ -103,13 +125,16 @@ auto Display::create(GpioExpander* expander,
   display->driver_.draw_buf = &display->buffers_;
   display->driver_.hor_res = kDisplayWidth;
   display->driver_.ver_res = kDisplayHeight;
+  display->driver_.sw_rotate = 1;
+  display->driver_.rotated = LV_DISP_ROT_270;
+  display->driver_.antialiasing = 0;
   display->driver_.flush_cb = &FlushDataCallback;
   display->driver_.user_data = display.get();
 
   ESP_LOGI(kTag, "Registering driver");
   display->display_ = lv_disp_drv_register(&display->driver_);
 
-  return display;
+  return display.release();
 }
 
 Display::Display(GpioExpander* gpio, spi_device_handle_t handle)
@@ -190,9 +215,7 @@ void Display::SendTransaction(TransactionType type,
     transaction.tx_buffer = data;
   }
 
-  // TODO(jacqueline): Move this to an on-board GPIO for speed.
-  gpio_->set_pin(GpioExpander::DISPLAY_DR, type);
-  gpio_->Write();
+  gpio_set_level(kDisplayDr, type);
 
   // TODO(jacqueline): Handle these errors.
   esp_err_t ret = spi_device_polling_transmit(handle_, &transaction);
diff --git a/src/drivers/driver_cache.cpp b/src/drivers/driver_cache.cpp
new file mode 100644
index 00000000..650e6f16
--- /dev/null
+++ b/src/drivers/driver_cache.cpp
@@ -0,0 +1,43 @@
+#include "driver_cache.hpp"
+
+#include <memory>
+#include <mutex>
+
+#include "display.hpp"
+#include "display_init.hpp"
+#include "storage.hpp"
+#include "touchwheel.hpp"
+
+namespace drivers {
+
+DriverCache::DriverCache() : gpios_(std::make_unique<GpioExpander>()) {}
+DriverCache::~DriverCache() {}
+
+auto DriverCache::AcquireGpios() -> GpioExpander* {
+  return gpios_.get();
+}
+
+auto DriverCache::AcquireDac() -> std::shared_ptr<AudioDac> {
+  return Acquire(dac_, [&]() -> AudioDac* {
+    return AudioDac::create(AcquireGpios()).value_or(nullptr);
+  });
+}
+
+auto DriverCache::AcquireDisplay() -> std::shared_ptr<Display> {
+  return Acquire(display_, [&]() -> Display* {
+    return Display::create(AcquireGpios(), displays::kST7735R);
+  });
+}
+
+auto DriverCache::AcquireStorage() -> std::shared_ptr<SdStorage> {
+  return Acquire(storage_, [&]() -> SdStorage* {
+    return SdStorage::create(AcquireGpios()).value_or(nullptr);
+  });
+}
+
+auto DriverCache::AcquireTouchWheel() -> std::shared_ptr<TouchWheel> {
+  return Acquire(touchwheel_,
+                 [&]() -> TouchWheel* { return new TouchWheel(); });
+}
+
+}  // namespace drivers
diff --git a/src/drivers/i2c.cpp b/src/drivers/i2c.cpp
index 04a6d7d1..6c6fc407 100644
--- a/src/drivers/i2c.cpp
+++ b/src/drivers/i2c.cpp
@@ -9,8 +9,8 @@
 namespace drivers {
 
 static const i2c_port_t kI2CPort = I2C_NUM_0;
-static const gpio_num_t kI2CSdaPin = GPIO_NUM_2;
-static const gpio_num_t kI2CSclPin = GPIO_NUM_4;
+static const gpio_num_t kI2CSdaPin = GPIO_NUM_4;
+static const gpio_num_t kI2CSclPin = GPIO_NUM_2;
 static const uint32_t kI2CClkSpeed = 400'000;
 
 static constexpr int kCmdLinkSize = I2C_LINK_RECOMMENDED_SIZE(12);
@@ -36,6 +36,9 @@ esp_err_t init_i2c(void) {
   if (esp_err_t err = i2c_driver_install(kI2CPort, config.mode, 0, 0, 0)) {
     return err;
   }
+  if (esp_err_t err = i2c_set_timeout(kI2CPort, 400000)) {
+    return err;
+  }
 
   // TODO: INT line
 
@@ -57,8 +60,8 @@ I2CTransaction::~I2CTransaction() {
   free(buffer_);
 }
 
-esp_err_t I2CTransaction::Execute() {
-  return i2c_master_cmd_begin(I2C_NUM_0, handle_, kI2CTimeout);
+esp_err_t I2CTransaction::Execute(uint8_t port) {
+  return i2c_master_cmd_begin(port, handle_, kI2CTimeout);
 }
 
 I2CTransaction& I2CTransaction::start() {
diff --git a/src/drivers/include/dac.hpp b/src/drivers/include/dac.hpp
index 06808a78..4952c992 100644
--- a/src/drivers/include/dac.hpp
+++ b/src/drivers/include/dac.hpp
@@ -12,6 +12,7 @@
 #include "esp_err.h"
 #include "freertos/FreeRTOS.h"
 #include "freertos/portmacro.h"
+#include "freertos/stream_buffer.h"
 #include "result.hpp"
 #include "span.hpp"
 
@@ -20,6 +21,91 @@
 
 namespace drivers {
 
+namespace pcm512x {
+class Register {
+ public:
+  uint8_t page;
+  uint8_t reg;
+
+  constexpr Register(uint8_t p, uint8_t r) : page(p), reg(r) {}
+};
+
+constexpr Register RESET(0, 1);
+constexpr Register POWER(0, 2);
+constexpr Register MUTE(0, 3);
+constexpr Register PLL_EN(0, 4);
+constexpr Register SPI_MISO_FUNCTION(0, 6);
+constexpr Register DSP(0, 7);
+constexpr Register GPIO_EN(0, 8);
+constexpr Register BCLK_LRCLK_CFG(0, 9);
+constexpr Register DSP_GPIO_INPUT(0, 10);
+constexpr Register MASTER_MODE(0, 12);
+constexpr Register PLL_REF(0, 13);
+constexpr Register DAC_REF(0, 14);
+constexpr Register GPIO_DACIN(0, 16);
+constexpr Register GPIO_PLLIN(0, 18);
+constexpr Register SYNCHRONIZE(0, 19);
+constexpr Register PLL_COEFF_0(0, 20);
+constexpr Register PLL_COEFF_1(0, 21);
+constexpr Register PLL_COEFF_2(0, 22);
+constexpr Register PLL_COEFF_3(0, 23);
+constexpr Register PLL_COEFF_4(0, 24);
+constexpr Register DSP_CLKDIV(0, 27);
+constexpr Register DAC_CLKDIV(0, 28);
+constexpr Register NCP_CLKDIV(0, 29);
+constexpr Register OSR_CLKDIV(0, 30);
+constexpr Register MASTER_CLKDIV_1(0, 32);
+constexpr Register MASTER_CLKDIV_2(0, 33);
+constexpr Register FS_SPEED_MODE(0, 34);
+constexpr Register IDAC_1(0, 35);
+constexpr Register IDAC_2(0, 36);
+constexpr Register ERROR_DETECT(0, 37);
+constexpr Register I2S_1(0, 40);
+constexpr Register I2S_2(0, 41);
+constexpr Register DAC_ROUTING(0, 42);
+constexpr Register DSP_PROGRAM(0, 43);
+constexpr Register CLKDET(0, 44);
+constexpr Register AUTO_MUTE(0, 59);
+constexpr Register DIGITAL_VOLUME_1(0, 60);
+constexpr Register DIGITAL_VOLUME_2(0, 61);
+constexpr Register DIGITAL_VOLUME_3(0, 62);
+constexpr Register DIGITAL_MUTE_1(0, 63);
+constexpr Register DIGITAL_MUTE_2(0, 64);
+constexpr Register DIGITAL_MUTE_3(0, 65);
+constexpr Register GPIO_OUTPUT_1(0, 80);
+constexpr Register GPIO_OUTPUT_2(0, 81);
+constexpr Register GPIO_OUTPUT_3(0, 82);
+constexpr Register GPIO_OUTPUT_4(0, 83);
+constexpr Register GPIO_OUTPUT_5(0, 84);
+constexpr Register GPIO_OUTPUT_6(0, 85);
+constexpr Register GPIO_CONTROL_1(0, 86);
+constexpr Register GPIO_CONTROL_2(0, 87);
+constexpr Register OVERFLOW(0, 90);
+constexpr Register RATE_DET_1(0, 91);
+constexpr Register RATE_DET_2(0, 92);
+constexpr Register RATE_DET_3(0, 93);
+constexpr Register RATE_DET_4(0, 94);
+constexpr Register CLOCK_STATUS(0, 95);
+constexpr Register ANALOG_MUTE_DET(0, 108);
+constexpr Register POWER_STATE(0, 118);
+constexpr Register GPIN(0, 119);
+constexpr Register DIGITAL_MUTE_DET(0, 120);
+
+constexpr Register OUTPUT_AMPLITUDE(1, 1);
+constexpr Register ANALOG_GAIN_CTRL(1, 2);
+constexpr Register UNDERVOLTAGE_PROT(1, 5);
+constexpr Register ANALOG_MUTE_CTRL(1, 6);
+constexpr Register ANALOG_GAIN_BOOST(1, 7);
+constexpr Register VCOM_CTRL_1(1, 8);
+constexpr Register VCOM_CTRL_2(1, 9);
+
+constexpr Register CRAM_CTRL(44, 1);
+
+constexpr Register FLEX_A(253, 63);
+constexpr Register FLEX_B(253, 64);
+
+}  // namespace pcm512x
+
 /**
  * Interface for a PCM5122PWR DAC, configured over I2C.
  */
@@ -31,8 +117,7 @@ class AudioDac {
     FAILED_TO_INSTALL_I2S,
   };
 
-  static auto create(GpioExpander* expander)
-      -> cpp::result<std::unique_ptr<AudioDac>, Error>;
+  static auto create(GpioExpander* expander) -> cpp::result<AudioDac*, Error>;
 
   AudioDac(GpioExpander* gpio, i2s_chan_handle_t i2s_handle);
   ~AudioDac();
@@ -64,14 +149,21 @@ class AudioDac {
     BPS_32 = I2S_DATA_BIT_WIDTH_32BIT,
   };
   enum SampleRate {
+    SAMPLE_RATE_11_025 = 11025,
+    SAMPLE_RATE_16 = 16000,
+    SAMPLE_RATE_22_05 = 22050,
+    SAMPLE_RATE_32 = 32000,
     SAMPLE_RATE_44_1 = 44100,
     SAMPLE_RATE_48 = 48000,
+    SAMPLE_RATE_96 = 96000,
+    SAMPLE_RATE_192 = 192000,
   };
 
   // TODO(jacqueline): worth supporting channels here as well?
   auto Reconfigure(BitsPerSample bps, SampleRate rate) -> void;
 
-  auto WriteData(cpp::span<std::byte> data) -> std::size_t;
+  auto WriteData(const cpp::span<const std::byte>& data) -> void;
+  auto SetSource(StreamBufferHandle_t buffer) -> void;
 
   auto Stop() -> void;
   auto LogStatus() -> void;
@@ -83,6 +175,8 @@ class AudioDac {
  private:
   GpioExpander* gpio_;
   i2s_chan_handle_t i2s_handle_;
+  bool i2s_active_;
+  std::optional<uint8_t> active_page_;
 
   i2s_std_clk_config_t clock_config_;
   i2s_std_slot_config_t slot_config_;
@@ -93,29 +187,12 @@ class AudioDac {
    */
   bool WaitForPowerState(std::function<bool(bool, PowerState)> predicate);
 
-  enum Register {
-    PAGE_SELECT = 0,
-    RESET = 1,
-    POWER_MODE = 2,
-    DE_EMPHASIS = 7,
-    DAC_CLOCK_SOURCE = 14,
-    CLOCK_ERRORS = 37,
-    I2S_FORMAT = 40,
-    DIGITAL_VOLUME_L = 61,
-    DIGITAL_VOLUME_R = 62,
-
-    SAMPLE_RATE_DETECTION = 91,
-    BCK_DETECTION = 93,
-    CLOCK_ERROR_STATE = 94,
-    CLOCK_STATUS = 95,
-    AUTO_MUTE_STATE = 108,
-    SOFT_MUTE_STATE = 114,
-    SAMPLE_RATE_STATE = 115,
-    DSP_BOOT_POWER_STATE = 118,
-  };
+  void WriteRegister(pcm512x::Register r, uint8_t val);
+  uint8_t ReadRegister(pcm512x::Register r);
 
-  void WriteRegister(Register reg, uint8_t val);
-  uint8_t ReadRegister(Register reg);
+  void SelectPage(uint8_t page);
+  void WriteRegisterRaw(uint8_t reg, uint8_t val);
+  uint8_t ReadRegisterRaw(uint8_t reg);
 };
 
 }  // namespace drivers
diff --git a/src/drivers/include/display.hpp b/src/drivers/include/display.hpp
index 8157c3a5..9e4a0224 100644
--- a/src/drivers/include/display.hpp
+++ b/src/drivers/include/display.hpp
@@ -23,8 +23,7 @@ class Display {
    * us back any kind of signal to tell us we're actually using them correctly.
    */
   static auto create(GpioExpander* expander,
-                     const displays::InitialisationData& init_data)
-      -> std::unique_ptr<Display>;
+                     const displays::InitialisationData& init_data) -> Display*;
 
   Display(GpioExpander* gpio, spi_device_handle_t handle);
   ~Display();
diff --git a/src/drivers/include/driver_cache.hpp b/src/drivers/include/driver_cache.hpp
new file mode 100644
index 00000000..c56ebc3f
--- /dev/null
+++ b/src/drivers/include/driver_cache.hpp
@@ -0,0 +1,54 @@
+#pragma once
+
+#include <memory>
+#include <mutex>
+
+#include "dac.hpp"
+#include "display.hpp"
+#include "gpio_expander.hpp"
+#include "storage.hpp"
+#include "touchwheel.hpp"
+
+namespace drivers {
+
+class DriverCache {
+ private:
+  std::unique_ptr<GpioExpander> gpios_;
+  std::weak_ptr<AudioDac> dac_;
+  std::weak_ptr<Display> display_;
+  std::weak_ptr<SdStorage> storage_;
+  std::weak_ptr<TouchWheel> touchwheel_;
+  // TODO(jacqueline): Haptics, samd
+
+  std::mutex mutex_;
+
+  template <typename T, typename F>
+  auto Acquire(std::weak_ptr<T> ptr, F factory) -> std::shared_ptr<T> {
+    std::shared_ptr<T> acquired = ptr.lock();
+    if (acquired) {
+      return acquired;
+    }
+
+    std::lock_guard<std::mutex> lock(mutex_);
+
+    acquired = ptr.lock();
+    if (acquired) {
+      return acquired;
+    }
+    acquired.reset(factory());
+    ptr = acquired;
+    return acquired;
+  }
+
+ public:
+  DriverCache();
+  ~DriverCache();
+
+  auto AcquireGpios() -> GpioExpander*;
+  auto AcquireDac() -> std::shared_ptr<AudioDac>;
+  auto AcquireDisplay() -> std::shared_ptr<Display>;
+  auto AcquireStorage() -> std::shared_ptr<SdStorage>;
+  auto AcquireTouchWheel() -> std::shared_ptr<TouchWheel>;
+};
+
+}  // namespace drivers
diff --git a/src/drivers/include/gpio_expander.hpp b/src/drivers/include/gpio_expander.hpp
index a6e96d87..cd3719a0 100644
--- a/src/drivers/include/gpio_expander.hpp
+++ b/src/drivers/include/gpio_expander.hpp
@@ -35,28 +35,28 @@ class GpioExpander {
   static const uint8_t kPca8575Timeout = pdMS_TO_TICKS(100);
 
   // Port A:
-  // 0 - audio power enable
-  // 1 - usb interface power enable (active low)
-  // 2 - display power enable
-  // 3 - touchpad power enable
-  // 4 - sd card power enable
-  // 5 - sd mux switch
-  // 6 - LDO enable
-  // 7 - charge power ok (active low)
-  // All power switches low, sd mux pointing away from us, inputs high.
-  static const uint8_t kPortADefault = 0b10000010;
+  // 0 - sd card mux switch
+  // 1 - sd card mux enable (active low)
+  // 2 - key up
+  // 3 - key down
+  // 4 - key lock
+  // 5 - display reset
+  // 6 - NC
+  // 7 - sd card power (active low)
+  // Default to SD card off, inputs high.
+  static const uint8_t kPortADefault = 0b10111110;
 
   // Port B:
-  // 0 - 3.5mm jack detect (active low)
-  // 1 - unused
-  // 2 - volume up
-  // 3 - volume down
-  // 4 - lock switch
-  // 5 - touchpad interupt
-  // 6 - display DR
-  // 7 - display LED
-  // Inputs all high, all others low.
-  static const uint8_t kPortBDefault = 0b00111101;
+  // 0 - trs output enable
+  // 1 - 3.5mm jack detect (active low)
+  // 2 - NC
+  // 3 - NC
+  // 4 - NC
+  // 5 - NC
+  // 6 - NC
+  // 7 - NC
+  // Default input high, trs output low
+  static const uint8_t kPortBDefault = 0b00000010;
 
   /*
    * Convenience mehod for packing the port a and b bytes into a single 16 bit
@@ -99,30 +99,30 @@ class GpioExpander {
   /* Maps each pin of the expander to its number in a `pack`ed uint16. */
   enum Pin {
     // Port A
-    AUDIO_POWER_ENABLE = 0,
-    USB_INTERFACE_POWER_ENABLE = 1,
-    DISPLAY_POWER_ENABLE = 2,
-    TOUCHPAD_POWER_ENABLE = 3,
-    SD_CARD_POWER_ENABLE = 4,
-    SD_MUX_SWITCH = 5,
-    LDO_ENABLE = 6,
-    CHARGE_POWER_OK = 7,  // Active-low input
+    SD_MUX_SWITCH = 0,
+    SD_MUX_EN_ACTIVE_LOW = 1,
+    KEY_UP = 2,
+    KEY_DOWN = 3,
+    KEY_LOCK = 4,
+    DISPLAY_RESET = 5,
+    // UNUSED = 6,
+    SD_CARD_POWER_ENABLE = 7,
 
     // Port B
-    PHONE_DETECT = 8,  // Active-high input
-    // UNUSED = 9,
-    VOL_UP = 10,
-    VOL_DOWN = 11,
-    LOCK = 12,
-    TOUCHPAD_INT = 13,
-    DISPLAY_DR = 14,
-    DISPLAY_LED = 15,
+    AMP_EN = 8,
+    PHONE_DETECT = 9,
+    // UNUSED = 10,
+    // UNUSED = 11,
+    // UNUSED = 12,
+    // UNUSED = 13,
+    // UNUSED = 14,
+    // UNUSED = 15,
   };
 
   /* Nicer value names for use with the SD_MUX_SWITCH pin. */
   enum SdController {
-    SD_MUX_ESP = 1,
-    SD_MUX_USB = 0,
+    SD_MUX_ESP = 0,
+    SD_MUX_SAMD = 1,
   };
 
   /**
diff --git a/src/drivers/include/i2c.hpp b/src/drivers/include/i2c.hpp
index dbdd8a11..811c9333 100644
--- a/src/drivers/include/i2c.hpp
+++ b/src/drivers/include/i2c.hpp
@@ -35,7 +35,7 @@ class I2CTransaction {
    * ESP_ERR_INVALID_STATE I2C driver not installed or not in master mode.
    * ESP_ERR_TIMEOUT Operation timeout because the bus is busy.
    */
-  esp_err_t Execute();
+  esp_err_t Execute(uint8_t port = I2C_NUM_0);
 
   /*
    * Enqueues a start condition. May also be used for repeated start
diff --git a/src/drivers/include/storage.hpp b/src/drivers/include/storage.hpp
index 64ce4782..c19ec935 100644
--- a/src/drivers/include/storage.hpp
+++ b/src/drivers/include/storage.hpp
@@ -25,14 +25,13 @@ class SdStorage {
     FAILED_TO_MOUNT,
   };
 
-  static auto create(GpioExpander* gpio)
-      -> cpp::result<std::shared_ptr<SdStorage>, Error>;
+  static auto create(GpioExpander* gpio) -> cpp::result<SdStorage*, Error>;
 
   SdStorage(GpioExpander* gpio,
             esp_err_t (*do_transaction)(sdspi_dev_handle_t, sdmmc_command_t*),
             sdspi_dev_handle_t handle_,
-            std::unique_ptr<sdmmc_host_t>& host_,
-            std::unique_ptr<sdmmc_card_t>& card_,
+            std::unique_ptr<sdmmc_host_t> host_,
+            std::unique_ptr<sdmmc_card_t> card_,
             FATFS* fs_);
   ~SdStorage();
 
diff --git a/src/drivers/include/touchwheel.hpp b/src/drivers/include/touchwheel.hpp
new file mode 100644
index 00000000..3dfa182b
--- /dev/null
+++ b/src/drivers/include/touchwheel.hpp
@@ -0,0 +1,49 @@
+#pragma once
+
+#include <stdint.h>
+#include <functional>
+
+#include "esp_err.h"
+#include "result.hpp"
+
+#include "gpio_expander.hpp"
+
+namespace drivers {
+
+struct TouchWheelData {
+  bool is_touched = false;
+  uint8_t wheel_position = -1;
+};
+
+class TouchWheel {
+ public:
+  TouchWheel();
+  ~TouchWheel();
+
+  // Not copyable or movable.
+  TouchWheel(const TouchWheel&) = delete;
+  TouchWheel& operator=(const TouchWheel&) = delete;
+
+  auto Update() -> void;
+  auto GetTouchWheelData() const -> TouchWheelData;
+
+ private:
+  TouchWheelData data_;
+
+  enum Register {
+    FIRMWARE_VERSION = 0x1,
+    DETECTION_STATUS = 0x2,
+    KEY_STATUS_A = 0x3,
+    KEY_STATUS_B = 0x4,
+    SLIDER_POSITION = 0x5,
+    CALIBRATE = 0x6,
+    RESET = 0x7,
+    LOW_POWER = 0x8,
+    SLIDER_OPTIONS = 0x14,
+  };
+
+  void WriteRegister(uint8_t reg, uint8_t val);
+  uint8_t ReadRegister(uint8_t reg);
+};
+
+}  // namespace drivers
diff --git a/src/drivers/storage.cpp b/src/drivers/storage.cpp
index 88159744..d90bd811 100644
--- a/src/drivers/storage.cpp
+++ b/src/drivers/storage.cpp
@@ -49,8 +49,9 @@ static esp_err_t do_transaction(sdspi_dev_handle_t handle,
 }
 }  // namespace callback
 
-auto SdStorage::create(GpioExpander* gpio)
-    -> cpp::result<std::shared_ptr<SdStorage>, Error> {
+auto SdStorage::create(GpioExpander* gpio) -> cpp::result<SdStorage*, Error> {
+  gpio->set_pin(GpioExpander::SD_CARD_POWER_ENABLE, 0);
+  gpio->set_pin(GpioExpander::SD_MUX_EN_ACTIVE_LOW, 0);
   gpio->set_pin(GpioExpander::SD_MUX_SWITCH, GpioExpander::SD_MUX_ESP);
   gpio->Write();
 
@@ -103,16 +104,16 @@ auto SdStorage::create(GpioExpander* gpio)
     return cpp::fail(Error::FAILED_TO_MOUNT);
   }
 
-  return std::make_unique<SdStorage>(gpio, do_transaction, handle, host, card,
-                                     fs);
+  return new SdStorage(gpio, do_transaction, handle, std::move(host),
+                       std::move(card), fs);
 }
 
 SdStorage::SdStorage(GpioExpander* gpio,
                      esp_err_t (*do_transaction)(sdspi_dev_handle_t,
                                                  sdmmc_command_t*),
                      sdspi_dev_handle_t handle,
-                     std::unique_ptr<sdmmc_host_t>& host,
-                     std::unique_ptr<sdmmc_card_t>& card,
+                     std::unique_ptr<sdmmc_host_t> host,
+                     std::unique_ptr<sdmmc_card_t> card,
                      FATFS* fs)
     : gpio_(gpio),
       do_transaction_(do_transaction),
@@ -136,6 +137,10 @@ SdStorage::~SdStorage() {
   // Uninstall the SPI driver
   sdspi_host_remove_device(this->handle_);
   sdspi_host_deinit();
+
+  gpio_->set_pin(GpioExpander::SD_CARD_POWER_ENABLE, 0);
+  gpio_->set_pin(GpioExpander::SD_MUX_EN_ACTIVE_LOW, 1);
+  gpio_->Write();
 }
 
 auto SdStorage::HandleTransaction(sdspi_dev_handle_t handle,
diff --git a/src/drivers/touchwheel.cpp b/src/drivers/touchwheel.cpp
new file mode 100644
index 00000000..51a67187
--- /dev/null
+++ b/src/drivers/touchwheel.cpp
@@ -0,0 +1,95 @@
+#include "touchwheel.hpp"
+#include <stdint.h>
+
+#include <cstdint>
+
+#include "assert.h"
+#include "driver/gpio.h"
+#include "driver/i2c.h"
+#include "esp_err.h"
+#include "esp_log.h"
+#include "freertos/projdefs.h"
+#include "hal/gpio_types.h"
+#include "hal/i2c_types.h"
+
+#include "i2c.hpp"
+
+namespace drivers {
+
+static const char* kTag = "TOUCHWHEEL";
+static const uint8_t kTouchWheelAddress = 0x1C;
+static const gpio_num_t kIntPin = GPIO_NUM_25;
+
+TouchWheel::TouchWheel() {
+  gpio_config_t int_config{
+      .pin_bit_mask = 1ULL << kIntPin,
+      .mode = GPIO_MODE_INPUT,
+      .pull_up_en = GPIO_PULLUP_ENABLE,
+      .pull_down_en = GPIO_PULLDOWN_DISABLE,
+      .intr_type = GPIO_INTR_DISABLE,
+  };
+  gpio_config(&int_config);
+
+  WriteRegister(Register::RESET, 1);
+  // TODO(daniel): do we need this? how long does reset take?
+  vTaskDelay(pdMS_TO_TICKS(1));
+  WriteRegister(Register::SLIDER_OPTIONS, 0b11000000);
+  WriteRegister(Register::CALIBRATE, 1);
+}
+
+TouchWheel::~TouchWheel() {}
+
+void TouchWheel::WriteRegister(uint8_t reg, uint8_t val) {
+  // uint8_t maskedReg = reg | kWriteMask;
+  uint8_t maskedReg = reg;
+  I2CTransaction transaction;
+  transaction.start()
+      .write_addr(kTouchWheelAddress, I2C_MASTER_WRITE)
+      .write_ack(maskedReg, val)
+      .stop();
+  transaction.Execute();
+  // TODO(jacqueline): check for errors again when i find where all the ffc
+  // cables went q.q
+  // ESP_ERROR_CHECK(transaction.Execute());
+}
+
+uint8_t TouchWheel::ReadRegister(uint8_t reg) {
+  uint8_t res;
+  I2CTransaction transaction;
+  transaction.start()
+      .write_addr(kTouchWheelAddress, I2C_MASTER_WRITE)
+      .write_ack(reg)
+      .start()
+      .write_addr(kTouchWheelAddress, I2C_MASTER_READ)
+      .read(&res, I2C_MASTER_NACK)
+      .stop();
+  ESP_ERROR_CHECK(transaction.Execute());
+  return res;
+}
+
+void TouchWheel::Update() {
+  // Read data from device into member struct
+  bool has_data = !gpio_get_level(GPIO_NUM_25);
+  if (!has_data) {
+    return;
+  }
+  uint8_t status = ReadRegister(Register::DETECTION_STATUS);
+  if (status & 0b10000000) {
+    // Still calibrating.
+    return;
+  }
+  if (status & 0b10) {
+    // Slider detect.
+    data_.wheel_position = ReadRegister(Register::SLIDER_POSITION);
+  }
+  if (status & 0b1) {
+    // Key detect.
+    // TODO(daniel): implement me
+  }
+}
+
+TouchWheelData TouchWheel::GetTouchWheelData() const {
+  return data_;
+}
+
+}  // namespace drivers
diff --git a/src/main/CMakeLists.txt b/src/main/CMakeLists.txt
index d5ab0429..524a7d30 100644
--- a/src/main/CMakeLists.txt
+++ b/src/main/CMakeLists.txt
@@ -1,5 +1,5 @@
 idf_component_register(
   SRCS "main.cpp" "app_console.cpp"
   INCLUDE_DIRS "."
-  REQUIRES "audio" "drivers" "dev_console" "drivers" "database")
+  REQUIRES "audio" "drivers" "dev_console" "drivers" "database" "ui")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/main/main.cpp b/src/main/main.cpp
index 88943f21..4fb355e2 100644
--- a/src/main/main.cpp
+++ b/src/main/main.cpp
@@ -1,30 +1,25 @@
 #include <dirent.h>
+#include <stdint.h>
 #include <stdio.h>
 
 #include <cstddef>
 #include <cstdint>
 #include <memory>
 
-#include "core/lv_disp.h"
-#include "core/lv_obj_pos.h"
 #include "driver/gpio.h"
 #include "driver/i2c.h"
 #include "driver/sdspi_host.h"
 #include "driver/spi_common.h"
 #include "driver/spi_master.h"
+#include "driver_cache.hpp"
 #include "esp_freertos_hooks.h"
 #include "esp_heap_caps.h"
 #include "esp_intr_alloc.h"
 #include "esp_log.h"
-#include "font/lv_font.h"
 #include "freertos/portmacro.h"
+#include "freertos/projdefs.h"
 #include "hal/gpio_types.h"
 #include "hal/spi_types.h"
-#include "lvgl/lvgl.h"
-#include "misc/lv_color.h"
-#include "misc/lv_style.h"
-#include "misc/lv_timer.h"
-#include "widgets/lv_label.h"
 
 #include "app_console.hpp"
 #include "audio_playback.hpp"
@@ -35,64 +30,14 @@
 #include "display_init.hpp"
 #include "gpio_expander.hpp"
 #include "i2c.hpp"
+#include "lvgl_task.hpp"
 #include "spi.hpp"
 #include "storage.hpp"
+#include "touchwheel.hpp"
 
 static const char* TAG = "MAIN";
 
-void IRAM_ATTR tick_hook(void) {
-  lv_tick_inc(1);
-}
-
-static const size_t kLvglStackSize = 8 * 1024;
-static StaticTask_t sLvglTaskBuffer = {};
-static StackType_t sLvglStack[kLvglStackSize] = {0};
-
-struct LvglArgs {
-  drivers::GpioExpander* gpio_expander;
-};
-
-extern "C" void lvgl_main(void* voidArgs) {
-  ESP_LOGI(TAG, "starting LVGL task");
-  LvglArgs* args = (LvglArgs*)voidArgs;
-  drivers::GpioExpander* gpio_expander = args->gpio_expander;
-
-  // Dispose of the args now that we've gotten everything out of them.
-  delete args;
-
-  ESP_LOGI(TAG, "init lvgl");
-  lv_init();
-
-  // LVGL has been initialised, so we can now start reporting ticks to it.
-  esp_register_freertos_tick_hook(&tick_hook);
-
-  ESP_LOGI(TAG, "init display");
-  std::unique_ptr<drivers::Display> display =
-      drivers::Display::create(gpio_expander, drivers::displays::kST7735R);
-
-  lv_style_t style;
-  lv_style_init(&style);
-  lv_style_set_text_color(&style, LV_COLOR_MAKE(0xFF, 0, 0));
-  // TODO: find a nice bitmap font for this display size and density.
-  // lv_style_set_text_font(&style, &lv_font_montserrat_24);
-
-  auto label = lv_label_create(NULL);
-  lv_label_set_text(label, "COLOURS!!");
-  lv_obj_add_style(label, &style, 0);
-
-  lv_obj_center(label);
-  lv_scr_load(label);
-
-  while (1) {
-    lv_timer_handler();
-    vTaskDelay(pdMS_TO_TICKS(10));
-  }
-
-  // TODO: break from the loop to kill this task, so that we can do our RAII
-  // cleanup, unregister our tick callback and so on.
-}
-
-extern "C" void db_main(void *whatever) {
+void db_main(void *whatever) {
   ESP_LOGI(TAG, "Init database");
   auto db_res = database::Database::Open();
   if (db_res.has_error()) {
@@ -111,30 +56,24 @@ extern "C" void app_main(void) {
 
   ESP_ERROR_CHECK(drivers::init_i2c());
   ESP_ERROR_CHECK(drivers::init_spi());
+  std::unique_ptr<drivers::DriverCache> drivers =
+      std::make_unique<drivers::DriverCache>();
 
   ESP_LOGI(TAG, "Init GPIOs");
-  drivers::GpioExpander* expander = new drivers::GpioExpander();
+  drivers::GpioExpander* expander = drivers->AcquireGpios();
 
   ESP_LOGI(TAG, "Enable power rails for development");
-  expander->with([&](auto& gpio) {
-    gpio.set_pin(drivers::GpioExpander::AUDIO_POWER_ENABLE, 1);
-    gpio.set_pin(drivers::GpioExpander::USB_INTERFACE_POWER_ENABLE, 0);
-    gpio.set_pin(drivers::GpioExpander::SD_CARD_POWER_ENABLE, 1);
-    gpio.set_pin(drivers::GpioExpander::SD_MUX_SWITCH,
-                 drivers::GpioExpander::SD_MUX_ESP);
-  });
+  expander->with(
+      [&](auto& gpio) { gpio.set_pin(drivers::GpioExpander::AMP_EN, 1); });
 
   ESP_LOGI(TAG, "Init battery measurement");
   drivers::Battery* battery = new drivers::Battery();
   ESP_LOGI(TAG, "it's reading %d mV!", (int)battery->Millivolts());
 
   ESP_LOGI(TAG, "Init SD card");
-  auto storage_res = drivers::SdStorage::create(expander);
-  std::shared_ptr<drivers::SdStorage> storage;
-  if (storage_res.has_error()) {
+  auto storage = drivers->AcquireStorage();
+  if (!storage) {
     ESP_LOGE(TAG, "Failed! Do you have an SD card?");
-  } else {
-    storage = std::move(storage_res.value());
   }
 
   ESP_LOGI(TAG, "Launch database task");
@@ -144,22 +83,18 @@ extern "C" void app_main(void) {
     reinterpret_cast<StackType_t*>(heap_caps_malloc(db_stack_size, MALLOC_CAP_SPIRAM));
   xTaskCreateStatic(&db_main, "LEVELDB", db_stack_size, NULL, 1, database_stack, &database_task_buffer);
 
-  ESP_LOGI(TAG, "Launch LVGL task");
-  LvglArgs* lvglArgs = (LvglArgs*)calloc(1, sizeof(LvglArgs));
-  lvglArgs->gpio_expander = expander;
-  xTaskCreateStaticPinnedToCore(&lvgl_main, "LVGL", kLvglStackSize,
-                                (void*)lvglArgs, 1, sLvglStack,
-                                &sLvglTaskBuffer, 1);
+  ESP_LOGI(TAG, "Init touch wheel");
+  std::shared_ptr<drivers::TouchWheel> touchwheel =
+      drivers->AcquireTouchWheel();
+
+  std::atomic<bool> lvgl_quit;
+  TaskHandle_t lvgl_task_handle;
+  ui::StartLvgl(drivers.get(), &lvgl_quit, &lvgl_task_handle);
 
-  std::shared_ptr<audio::AudioPlayback> playback;
+  std::unique_ptr<audio::AudioPlayback> playback;
   if (storage) {
     ESP_LOGI(TAG, "Init audio pipeline");
-    auto playback_res = audio::AudioPlayback::create(expander, storage);
-    if (playback_res.has_error()) {
-      ESP_LOGE(TAG, "Failed! Playback will not work.");
-    } else {
-      playback = std::move(playback_res.value());
-    }
+    playback = std::make_unique<audio::AudioPlayback>(drivers.get());
   }
 
   ESP_LOGI(TAG, "Waiting for background tasks before launching console...");
@@ -169,7 +104,15 @@ extern "C" void app_main(void) {
   console::AppConsole console(playback.get());
   console.Launch();
 
+  uint8_t prev_position = 0;
   while (1) {
+    touchwheel->Update();
+    auto wheel_data = touchwheel->GetTouchWheelData();
+    if (wheel_data.wheel_position != prev_position) {
+      prev_position = wheel_data.wheel_position;
+      ESP_LOGI(TAG, "Touch wheel pos: %u", prev_position);
+    }
     vTaskDelay(pdMS_TO_TICKS(100));
   }
 }
+
diff --git a/src/memory/CMakeLists.txt b/src/memory/CMakeLists.txt
index 67e64267..69afc454 100644
--- a/src/memory/CMakeLists.txt
+++ b/src/memory/CMakeLists.txt
@@ -1,2 +1,2 @@
-idf_component_register(SRCS "arena.cpp" INCLUDE_DIRS "include" REQUIRES "span")
+idf_component_register(SRCS "arena.cpp" INCLUDE_DIRS "include" REQUIRES "span" "esp_psram")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/memory/include/himem.hpp b/src/memory/include/himem.hpp
new file mode 100644
index 00000000..517ebfdf
--- /dev/null
+++ b/src/memory/include/himem.hpp
@@ -0,0 +1,83 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+
+#include "esp32/himem.h"
+#include "span.hpp"
+
+/*
+ * Wrapper around an ESP-IDF himem allocation, which uses RAII to clean up after
+ * itself.
+ */
+template <std::size_t size>
+class HimemAlloc {
+ public:
+  esp_himem_handle_t handle;
+  const bool is_valid;
+
+  HimemAlloc() : is_valid(esp_himem_alloc(size, &handle) == ESP_OK) {}
+
+  ~HimemAlloc() {
+    if (is_valid) {
+      esp_himem_free(handle);
+    }
+  }
+
+  // Not copyable or movable.
+  HimemAlloc(const HimemAlloc&) = delete;
+  HimemAlloc& operator=(const HimemAlloc&) = delete;
+};
+
+/*
+ * Wrapper around an ESP-IDF himem allocation, which maps a HimemAlloc into the
+ * usable address space. Instances always contain the last memory region that
+ * was mapped within them.
+ */
+template <std::size_t size>
+class MappableRegion {
+ private:
+  std::byte* bytes_;
+
+ public:
+  esp_himem_rangehandle_t range_handle;
+  const bool is_valid;
+
+  MappableRegion()
+      : bytes_(nullptr),
+        is_valid(esp_himem_alloc_map_range(size, &range_handle) == ESP_OK) {}
+
+  ~MappableRegion() {
+    if (bytes_ != nullptr) {
+      esp_himem_unmap(range_handle, bytes_, size);
+    }
+    if (is_valid) {
+      esp_himem_free_map_range(range_handle);
+    }
+  }
+
+  auto Get() -> cpp::span<std::byte> {
+    if (bytes_ == nullptr) {
+      return {};
+    }
+    return {bytes_, size};
+  }
+
+  auto Map(const HimemAlloc<size>& alloc) -> cpp::span<std::byte> {
+    assert(bytes_ == nullptr);
+    ESP_ERROR_CHECK(esp_himem_map(alloc.handle, range_handle, 0, 0, size, 0,
+                                  reinterpret_cast<void**>(&bytes_)));
+    return Get();
+  }
+
+  auto Unmap() -> void {
+    if (bytes_ != nullptr) {
+      ESP_ERROR_CHECK(esp_himem_unmap(range_handle, bytes_, size));
+      bytes_ = nullptr;
+    }
+  }
+
+  // Not copyable or movable.
+  MappableRegion(const MappableRegion&) = delete;
+  MappableRegion& operator=(const MappableRegion&) = delete;
+};
diff --git a/src/tasks/tasks.cpp b/src/tasks/tasks.cpp
index 32de431a..b9fce7ec 100644
--- a/src/tasks/tasks.cpp
+++ b/src/tasks/tasks.cpp
@@ -1,4 +1,5 @@
 #include "tasks.hpp"
 
 const UBaseType_t kTaskPriorityLvgl = 4;
-const UBaseType_t kTaskPriorityAudio = 5;
+const UBaseType_t kTaskPriorityAudioPipeline = 5;
+const UBaseType_t kTaskPriorityAudioDrain = 6;
diff --git a/src/tasks/tasks.hpp b/src/tasks/tasks.hpp
index 24f8509a..47668aea 100644
--- a/src/tasks/tasks.hpp
+++ b/src/tasks/tasks.hpp
@@ -3,4 +3,5 @@
 #include "freertos/portmacro.h"
 
 extern const UBaseType_t kTaskPriorityLvgl;
-extern const UBaseType_t kTaskPriorityAudio;
+extern const UBaseType_t kTaskPriorityAudioPipeline;
+extern const UBaseType_t kTaskPriorityAudioDrain;
diff --git a/src/ui/CMakeLists.txt b/src/ui/CMakeLists.txt
new file mode 100644
index 00000000..bdccbbdb
--- /dev/null
+++ b/src/ui/CMakeLists.txt
@@ -0,0 +1,5 @@
+idf_component_register(
+  SRCS "lvgl_task.cpp"
+  INCLUDE_DIRS "include"
+  REQUIRES "drivers")
+target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/ui/include/lvgl_task.hpp b/src/ui/include/lvgl_task.hpp
new file mode 100644
index 00000000..ca3fc771
--- /dev/null
+++ b/src/ui/include/lvgl_task.hpp
@@ -0,0 +1,17 @@
+#pragma once
+
+#include <atomic>
+#include <cstdbool>
+
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+
+#include "driver_cache.hpp"
+
+namespace ui {
+
+auto StartLvgl(drivers::DriverCache* drivers,
+               std::atomic<bool>* quit,
+               TaskHandle_t* handle) -> bool;
+
+}  // namespace ui
diff --git a/src/ui/lvgl_task.cpp b/src/ui/lvgl_task.cpp
new file mode 100644
index 00000000..12dfd34e
--- /dev/null
+++ b/src/ui/lvgl_task.cpp
@@ -0,0 +1,108 @@
+#include "lvgl_task.hpp"
+
+#include <dirent.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+
+#include "core/lv_disp.h"
+#include "core/lv_obj.h"
+#include "core/lv_obj_pos.h"
+#include "core/lv_obj_tree.h"
+#include "esp_log.h"
+#include "font/lv_font.h"
+#include "freertos/portmacro.h"
+#include "freertos/projdefs.h"
+#include "freertos/timers.h"
+#include "hal/gpio_types.h"
+#include "hal/spi_types.h"
+#include "lvgl/lvgl.h"
+#include "misc/lv_color.h"
+#include "misc/lv_style.h"
+#include "misc/lv_timer.h"
+#include "widgets/lv_label.h"
+
+#include "display.hpp"
+#include "driver_cache.hpp"
+#include "gpio_expander.hpp"
+
+namespace ui {
+
+static const char* kTag = "lv_task";
+
+auto tick_hook(TimerHandle_t xTimer) -> void {
+  lv_tick_inc(1);
+}
+
+struct LvglArgs {
+  drivers::DriverCache* drivers;
+  std::atomic<bool>* quit;
+};
+
+void LvglMain(void* voidArgs) {
+  LvglArgs* args = reinterpret_cast<LvglArgs*>(voidArgs);
+  drivers::DriverCache* drivers = args->drivers;
+  std::atomic<bool>* quit = args->quit;
+  delete args;
+
+  {
+    ESP_LOGI(kTag, "init lvgl");
+    lv_init();
+
+    // LVGL has been initialised, so we can now start reporting ticks to it.
+    TimerHandle_t tick_timer =
+        xTimerCreate("lv_tick", pdMS_TO_TICKS(1), pdTRUE, NULL, &tick_hook);
+
+    ESP_LOGI(kTag, "init display");
+    std::shared_ptr<drivers::Display> display = drivers->AcquireDisplay();
+
+    lv_style_t style;
+    lv_style_init(&style);
+    lv_style_set_text_color(&style, LV_COLOR_MAKE(0xFF, 0, 0));
+    // TODO: find a nice bitmap font for this display size and density.
+    // lv_style_set_text_font(&style, &lv_font_montserrat_24);
+
+    auto label = lv_label_create(NULL);
+    lv_label_set_text(label, "COLOURS!!");
+    lv_obj_add_style(label, &style, 0);
+
+    lv_obj_center(label);
+    lv_scr_load(label);
+
+    while (!quit->load()) {
+      lv_timer_handler();
+      vTaskDelay(pdMS_TO_TICKS(10));
+    }
+
+    // TODO(robin? daniel?): De-init the UI stack here.
+    lv_obj_del(label);
+    lv_style_reset(&style);
+
+    xTimerDelete(tick_timer, portMAX_DELAY);
+
+    lv_deinit();
+  }
+
+  vTaskDelete(NULL);
+}
+
+static const size_t kLvglStackSize = 8 * 1024;
+static StaticTask_t sLvglTaskBuffer = {};
+static StackType_t sLvglStack[kLvglStackSize] = {0};
+
+auto StartLvgl(drivers::DriverCache* drivers,
+               std::atomic<bool>* quit,
+               TaskHandle_t* handle) -> bool {
+  LvglArgs* args = new LvglArgs();
+  args->drivers = drivers;
+  args->quit = quit;
+
+  return xTaskCreateStaticPinnedToCore(&LvglMain, "LVGL", kLvglStackSize,
+                                       reinterpret_cast<void*>(args), 1,
+                                       sLvglStack, &sLvglTaskBuffer, 1);
+}
+
+}  // namespace ui