Use single buffer mode for EInkDisplay

Frees up 48kB of staticly allocated RAM in exchange for 48kB just when grayscale rendering is needed

lib/GfxRenderer/GfxRenderer.cpp

@@ -164,7 +164,9 @@ int GfxRenderer::getLineHeight(const int fontId) const {
 
 uint8_t* GfxRenderer::getFrameBuffer() const { return einkDisplay.getFrameBuffer(); }
 
-void GfxRenderer::swapBuffers() const { einkDisplay.swapBuffers(); }
+size_t GfxRenderer::getBufferSize() {
+  return EInkDisplay::BUFFER_SIZE;
+}
 
 void GfxRenderer::grayscaleRevert() const { einkDisplay.grayscaleRevert(); }
 
@@ -174,6 +176,35 @@ void GfxRenderer::copyGrayscaleMsbBuffers() const { einkDisplay.copyGrayscaleMsb
 
 void GfxRenderer::displayGrayBuffer() const { einkDisplay.displayGrayBuffer(); }
 
+/**
+ * This should be called before grayscale buffers are populated.
+ * A `restoreBwBuffer` call should always follow the grayscale render if this method was called.
+ */
+void GfxRenderer::storeBwBuffer() {
+  if (bwBuffer) {
+    Serial.printf("[%lu] [GFX] !! BW buffer already stored - this is likely a bug, freeing it\n", millis());
+    free(bwBuffer);
+  }
+
+  bwBuffer = static_cast<uint8_t *>(malloc(EInkDisplay::BUFFER_SIZE));
+  memcpy(bwBuffer, einkDisplay.getFrameBuffer(), EInkDisplay::BUFFER_SIZE);
+}
+
+/**
+ * This can only be called if `storeBwBuffer` was called prior to the grayscale render.
+ * It should be called to restore the BW buffer state after grayscale rendering is complete.
+ */
+void GfxRenderer::restoreBwBuffer() {
+  if (!bwBuffer) {
+    Serial.printf("[%lu] [GFX] !! BW buffer not stored - this is likely a bug\n", millis());
+    return;
+  }
+
+  einkDisplay.cleanupGrayscaleBuffers(bwBuffer);
+  free(bwBuffer);
+  bwBuffer = nullptr;
+}
+
 void GfxRenderer::renderChar(const EpdFontFamily& fontFamily, const uint32_t cp, int* x, const int* y,
                              const bool pixelState, const EpdFontStyle style) const {
   const EpdGlyph* glyph = fontFamily.getGlyph(cp, style);

lib/GfxRenderer/GfxRenderer.h

@@ -12,6 +12,7 @@ class GfxRenderer {
  private:
   EInkDisplay& einkDisplay;
   RenderMode renderMode;
+  uint8_t* bwBuffer = nullptr;
   std::map<int, EpdFontFamily> fontMap;
   void renderChar(const EpdFontFamily& fontFamily, uint32_t cp, int* x, const int* y, bool pixelState,
                   EpdFontStyle style) const;
@@ -49,9 +50,11 @@ class GfxRenderer {
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;
+  void storeBwBuffer();
+  void restoreBwBuffer();
 
   // Low level functions
   uint8_t* getFrameBuffer() const;
-  void swapBuffers() const;
+  static size_t getBufferSize();
   void grayscaleRevert() const;
 };

open-x4-sdk

@@ -1 +1 @@
-Subproject commit 4d0dcd5ff87fcd86eb2966a123e85b03284a03db
+Subproject commit af965a074b99bdbbf0670d2c284bb34d978f73ed

platformio.ini

@@ -20,6 +20,7 @@ build_flags =
   -DARDUINO_USB_MODE=1
   -DARDUINO_USB_CDC_ON_BOOT=1
   -DMINIZ_NO_ZLIB_COMPATIBLE_NAMES=1
+  -DEINK_DISPLAY_SINGLE_BUFFER_MODE=1
 # https://libexpat.github.io/doc/api/latest/#XML_GE
   -DXML_GE=0
   -DXML_CONTEXT_BYTES=1024

src/screens/EpubReaderScreen.cpp

@@ -217,9 +217,8 @@ void EpubReaderScreen::renderScreen() {
         const int w = textWidth + margin * 2;
         const int h = renderer.getLineHeight(READER_FONT_ID) + margin * 2;
         renderer.grayscaleRevert();
-        uint8_t* fb1 = renderer.getFrameBuffer();
-        renderer.swapBuffers();
-        memcpy(fb1, renderer.getFrameBuffer(), EInkDisplay::BUFFER_SIZE);
+
+        // TODO: This looks like garbage again, need to implement windowed updates
         renderer.fillRect(x, y, w, h, 0);
         renderer.drawText(READER_FONT_ID, x + margin, y + margin, "Indexing...");
         renderer.drawRect(x + 5, y + 5, w - 10, h - 10);
@@ -297,6 +296,9 @@ void EpubReaderScreen::renderContents(std::unique_ptr<Page> page) {
     pagesUntilFullRefresh--;
   }
 
+  // Save bw buffer to reset buffer state after grayscale data sync
+  renderer.storeBwBuffer();
+
   // grayscale rendering
   // TODO: Only do this if font supports it
   {
@@ -315,6 +317,9 @@ void EpubReaderScreen::renderContents(std::unique_ptr<Page> page) {
     renderer.displayGrayBuffer();
     renderer.setRenderMode(GfxRenderer::BW);
   }
+
+  // restore the bw data
+  renderer.restoreBwBuffer();
 }
 
 void EpubReaderScreen::renderStatusBar() const {