Ease DSI PSRAM write bursts: bigger inv buffer + half-rate rain

Two secondary mitigations for the DSI underrun (the primary fix is the
reduced DPI clock in the BSP). Raise LV_INV_BUF_SIZE 32 -> 128 so the
busy rain states stay partial-redraw instead of collapsing into a
full-screen redraw, which dumps a ~1.3 MB PSRAM write burst that
competes with the DSI's continuous framebuffer read. Advance the matrix
rain every other tick (doubled step, visually identical) to halve how
often its ~80 labels invalidate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKPbR6DY2JygHbyLjxm7Uu

firmware/CMakeLists.txt

@@ -1,4 +1,18 @@
 cmake_minimum_required(VERSION 3.16)
 
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+# Grow LVGL's invalidation buffer (default 32) so the ~80 moving rain labels of the
+# busy face states stay as separate partial-redraw rects instead of collapsing into
+# one full-screen redraw. Secondary mitigation for the DSI PSRAM-bandwidth underrun
+# (primary fix is the reduced DPI clock in the BSP): a full-screen redraw dumps a
+# ~1.3 MB PSRAM write burst that competes with the DSI's continuous framebuffer read
+# and can re-trigger the underrun; keeping busy states partial avoids that. Applied
+# globally so LVGL and esp_lvgl_adapter (both size inv_areas[] by LV_INV_BUF_SIZE)
+# agree. The partial-flush path's stack scales with this, so app_main starts the LVGL
+# task with a 16 KB stack (see desklock_main.c) — the 8 KB default overflows at 128.
+# Must sit after the project.cmake include (defines idf_build_set_property) and before
+# project() consumes it.
+idf_build_set_property(COMPILE_OPTIONS "-DLV_INV_BUF_SIZE=128" APPEND)
+
 project(desklock)

firmware/main/face.c

@@ -153,6 +153,15 @@ static void stream_hide(stream_t *s)
 static void rain_tick(void)
 {
     const face_def_t *d = &DEFS[F.state];
+    /* Half-rate: advance the rain every OTHER tick (double the step so the fall
+     * looks identical). Halves how often the 80 rain labels invalidate, cutting the
+     * per-frame PSRAM write traffic that competes with the DSI framebuffer read on
+     * the same bus — a secondary easing of the underrun the reduced DPI clock fixes.
+     * The clock/face still update every tick. */
+    static uint8_t phase;
+    if (phase++ & 1) {
+        return;
+    }
     /* Rain is the reachability signal: it only falls when the gateway is live.
      * A disconnect drains it gracefully (streams finish falling, none respawn). */
     int target = (F.power == 2 || !gw_connected()) ? 0 : d->rain_streams;
@@ -163,7 +172,7 @@ static void rain_tick(void)
             continue;
         }
         alive++;
-        s->y += s->speed;
+        s->y += s->speed * 2;
         lv_obj_set_pos(s->tail, s->x, s->y);
         lv_obj_set_pos(s->head, s->x, s->y + s->len * 26);
         if (F.mutate == 0 && (esp_random() % 3) == 0) {