firmware: double-buffer the panel + zero-copy decode → ~22 fps ceiling

Per-frame paint cost drops from ~24 ms to ~45 µs (≈500× faster) by
enabling num_fbs=2 on the DPI panel and decoding straight into the
back framebuffer. Measured on the bench:

  before: lock=7us ttfb=370us body=40ms dec=6ms paint=24ms post=35us = ~70ms / ~14fps
  after : lock=7us ttfb=330us body=38ms dec=6ms paint=42us  post=25us = ~45ms / ~22fps

How the win actually lands:
- num_fbs=2 in the esp_lcd_dpi_panel_config_t makes the IDF driver
  allocate two framebuffers and stream from one while we fill the
  other.
- display_back_buffer() returns the inactive fb pointer + its size.
- jpeg_decoder_decode() now accepts a caller-provided destination
  buffer instead of owning its own scratch. http_api passes the panel
  back-fb so the hardware JPEG decoder writes BGR888 pixels straight
  into where the DSI will eventually scan from. Zero memcpy in the
  hot path.
- display_flip_back_buffer() calls esp_lcd_panel_draw_bitmap with the
  fb pointer. Because the buffer is inside the panel's own fb range,
  the IDF driver skips its memcpy and just does a cache writeback +
  swaps cur_fb_index. The actual flip happens on the next vsync,
  asynchronously — the call returns in microseconds.

The remaining ceiling is network body time (~38 ms for ~210 KB JPEGs)
and the hardware decoder (~6 ms). Per-viewport JPEG quality (smaller
files = shorter body) is the next lever; everything firmware-side is
already at or near floor.

Also drop the old static jpeg output scratch + JPEG_DECODER_MAX_OUTPUT_BYTES
constant — nothing references them anymore.

1 files changed, 14 insertions, 0 deletions

diff --git a/main/display.h b/main/display.h
index 5f939b9..0ecff4d 100644
--- a/main/display.h
+++ b/main/display.h
@@ -43,4 +43,18 @@ esp_err_t display_present_rgb565(const uint16_t *src,
 // hand it straight to esp_lcd_panel_draw_bitmap. No CPU pixel work, no
 // format conversion, no rotation — Scrypted is responsible for sending
 // the buffer pre-rotated and pre-scaled to panel-native dimensions.
+// If the buffer happens to be one of the panel's own framebuffers
+// (see display_back_buffer / display_flip_back_buffer below), the IDF
+// driver skips the memcpy entirely; otherwise it copies via CPU.
 esp_err_t display_present_bgr888(const void *bgr888);
+
+// Double-buffer accessors for the zero-memcpy /frame path. The DPI
+// panel owns two BGR888 framebuffers; `display_back_buffer` hands back
+// the one that is NOT currently being streamed by the DSI engine so
+// callers (the JPEG decoder) can fill it in place. When the buffer is
+// ready, `display_flip_back_buffer` swaps it in — turns into a cache
+// writeback + index swap inside the IDF driver (no memcpy).
+//   `out_size` (if non-null) is set to the buffer's byte size, useful
+//   for passing to jpeg_decoder_process as its output capacity.
+void      *display_back_buffer(size_t *out_size);
+esp_err_t  display_flip_back_buffer(void);