POWER-FINAL-21 — quiet display power transitions (no colour flash, no white screen at power-off)

Status: INSTALLED; shutdown fixed, startup flash remained → 22 (2026-10-13). Package out/y2linux-platform-v1-power-final-candidate-21/ (fallback = 20), receipts out/deep-sleep-final/build21-receipts/. BOOTIMG ea1da0b7bccf20a846839288d811205431cfbac6f441bc7107bf74a5b542908e, Y2ROOT 9a9b99ce9dbc4768485ac1c7446caef6be2e69cfb29e6e0a1a1271e075a17b77. Identity: build Y2LINUX-PLATFORM-V1-POWER-FINAL-21, kernel 6.18.56-y2linux-platform-v1-power-final-21, root 2026.08-platform-v1.51, release 1.0.0-platform-v1-power-final-candidate.21; Linux 0b3e0e1 on main, Reborn unchanged (217e28d). Flash BOOTIMG + ANDROID only with MT6582_preserve_data_scatter.txt, Download Only. Everything of 20 (sleep) is unchanged. Scope audit: "POWER-FINAL-21 quiet display power transitions scope audit" in the roadmap gap audit.

Owner report

The startup sometimes flashes colours; after the shutdown animation the panel turns white or shows colours.

Causes found (source and log analysis)

• Shutdown: the GC9503V panel driver linked the backlight to DRM (drm_panel_of_backlight). Every pipeline enable

  then called backlight_enable(), which resets bl_power to on. Reborn ends its shutdown on a black frame with the light off and is then stopped; when its DRM file closes, the core restores the stale fbdev buffer and enables the pipeline again — lighting the panel over white or console/colour content. The platform's own bl_power=4 after the kill can land before that close (a race, hence "sometimes").

• Startup: the splash's first drmModeSetCrtc fully re-initialises the DSI link (`modeset: PHY/clocks ready,

  initialize DSI`, ≈0.48 s) while the bootloader's picture is still lit; a panel losing its link with the light on shows colour noise.

What 21 changes

1. panel.c: no drm_panel_of_backlight(). The backlight is owned only by its explicit writers: Reborn (bl_power

   at screen sleep/wake and shutdown), the platform power service, and the offline charger (brightness).

2. backlight.c: .shutdown switches all four ISINKs off (PMIC 0x356[3:0]) during device_shutdown.
3. reborn-splash.c: bl_power=4 immediately before its SetCrtc, bl_power=0 after two vblanks, unconditionally

   (a failed modeset never stays dark). The backlight device exists at 0.27 s, the splash starts at 0.49 s; the initramfs carries the new splash. Expect a very short dark moment between the logo and the splash.

Not changeable: a flash during the bootloader's (LK) own panel initialisation, before Linux starts.

Tests

test_display_quiet_power (no DRM backlight link, sinks off at shutdown, splash modeset dark and always relit) plus the existing splash, backlight and idle suites. Locked-container production suite: 692 cases, exit 0. Package validation, ELF, installed ARM, privacy and inventory PASS.

Hardware result (owner, boot a0c4d60e, archive 118)

The shutdown animation now ends dark (fixed). The colour flash between the logo and the splash remained: the DSI hand-over parks LK's stream at 0.489 s and Linux's first modeset streams at 0.532 s, before the splash runs; see the 22 report. The first USB connection after this flash was again briefly flaky (SSH timeout, one slow ping), recovering by itself within seconds.

What to check after the flash

1. Power on a few times: no colour flash (a brief dark moment after the logo is expected).
2. Turn off a few times: the shutdown animation ends dark and stays dark.
3. Screen off/on and sleep/wake: the screen still comes back lit (Reborn switches the light explicitly).
4. Then the 20 sleep checks on 21: clocks.unused_handoff UART1 never_initialised, a ~35-min sleep (last_lost),

   the overnight drain.