From 3f53ced44ea112a39eec70216c8c72fa6867726e Mon Sep 17 00:00:00 2001 From: Luke Hoersten Date: Mon, 15 Jun 2026 07:50:23 -0500 Subject: firmware: double-buffer the panel + zero-copy decode → ~22 fps ceiling MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. --- main/display.c | 45 +++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 43 insertions(+), 2 deletions(-) (limited to 'main/display.c') diff --git a/main/display.c b/main/display.c index 1acddba..ca1690e 100644 --- a/main/display.c +++ b/main/display.c @@ -117,8 +117,18 @@ static esp_lcd_dsi_bus_handle_t s_dsi_bus; static esp_lcd_panel_handle_t s_panel; static bool s_up; static uint8_t s_last_pwm; -// RGB888 panel-sized scratch buffer (800*480*3 ≈ 1.15 MB in PSRAM). +// BGR888 panel-sized scratch buffer used by local_screens' CPU +// conversion + rotation path (~1.15 MB in PSRAM). Hot /frame path +// instead writes the JPEG decoder output directly into the panel's +// double-buffered framebuffer (see s_panel_fbs). static uint8_t *s_rot_buf; +// The DPI driver owns the two framebuffers when num_fbs = 2. We grab +// pointers to both after panel_init and alternate which one we hand to +// the JPEG decoder + the panel for each /frame. draw_bitmap with a +// pointer that's inside one of these turns into a cache writeback + +// index swap (microseconds), eliminating the previous ~24 ms memcpy. +static uint8_t *s_panel_fbs[2]; +static int s_back_fb; // index of the fb the next paint will fill // ============================================================================ // ATTINY88 I2C helpers @@ -304,7 +314,10 @@ static esp_err_t dsi_bring_up(void) .pixel_format = LCD_COLOR_PIXEL_FORMAT_RGB888, .in_color_format = LCD_COLOR_FMT_RGB888, .out_color_format = LCD_COLOR_FMT_RGB888, - .num_fbs = 1, + // Double-buffer the panel: the DSI engine streams one fb while + // we fill the other. esp_lcd_panel_draw_bitmap(fb) becomes a + // cache writeback + index swap (~µs) instead of a ~24 ms memcpy. + .num_fbs = 2, .video_timing = { .h_size = PANEL_H_ACTIVE, .v_size = PANEL_V_ACTIVE, @@ -337,6 +350,16 @@ static esp_err_t dsi_bring_up(void) ESP_RETURN_ON_ERROR(esp_lcd_panel_init(s_panel), TAG, "panel_init"); + // Grab pointers to both framebuffers so we can hand them to the + // JPEG decoder as the direct decode destination — that triggers + // the IDF DPI driver's fast path in draw_bitmap (no memcpy). + void *fb0 = NULL, *fb1 = NULL; + ESP_RETURN_ON_ERROR(esp_lcd_dpi_panel_get_frame_buffer(s_panel, 2, &fb0, &fb1), + TAG, "get_frame_buffer"); + s_panel_fbs[0] = fb0; + s_panel_fbs[1] = fb1; + s_back_fb = 1; // fb0 is the one the driver shows first; we fill fb1 next + // Continuous HS clock — TC358762's internal FLL needs a stable // reference, and we still allow LP data-lane blanking so command // packets can be inserted between frames. @@ -437,6 +460,24 @@ esp_err_t display_present_bgr888(const void *bgr888) bgr888); } +void *display_back_buffer(size_t *out_size) +{ + if (!s_up) return NULL; + if (out_size) *out_size = (size_t)PANEL_H_ACTIVE * PANEL_V_ACTIVE * 3; + return s_panel_fbs[s_back_fb]; +} + +esp_err_t display_flip_back_buffer(void) +{ + if (!s_up) return ESP_ERR_INVALID_STATE; + void *fb = s_panel_fbs[s_back_fb]; + esp_err_t err = esp_lcd_panel_draw_bitmap(s_panel, 0, 0, + PANEL_H_ACTIVE, PANEL_V_ACTIVE, + fb); + if (err == ESP_OK) s_back_fb ^= 1; // next /frame fills the other one + return err; +} + esp_err_t display_present_rgb565(const uint16_t *src, uint16_t src_w, uint16_t src_h) -- cgit v1.2.3