Panel recovery works; live LP brightness remains ineffective

Physical board, 2026-09-27 UTC (September 26 local time). Candidate source 926c7a6105f222665b8929f3ca271c3bc19f1458, system /nix/store/l9bla6rxrr13pq2pfzbqk3094d4wx1s5-nixos-system-nixos-26.11.20260919.20b1ddd, kernel /nix/store/fmhnsgvp375vfwz354l4aawxd83h72xy-linux-riscv64-unknown-linux-gnu-6.6.36-xuantie. The combined boot bundle is identified in the build record.

A one-time U-Boot boot loaded the candidate from the root filesystem, without changing normal /boot files or saving the U-Boot environment. Both /run/current-system and /run/booted-system resolved to the candidate. The actual compositor executable was /nix/store/7zvingic0whhz3z80w2gcpc343vipf7q-sway-unwrapped-riscv64-unknown-linux-gnu-1.12/bin/sway. Shell, UI and keyboard services were active; no units were failed.

Camera result

The operator improved camera framing before these captures. A fullscreen Foot window used colors-dark.background=ffffff. Camera exposure was manual, exposure time 97 (camera readback), gain 170, white balance fixed at 3588, focus fixed at 30, dynamic frame rate disabled. All captures used MJPEG 1280×720 at 30 fps, discarding the first 45 frames. The measurement is mean grayscale in the same interior rectangle (500,350)-(800,425) of the original frame. These are comparative camera values, not calibrated nits.

Live writes at 26, 128, 255, left to right: no demonstrated effect.

Power off then on with requested 128 retained: panel recovers.

The same 26, 128, 255 values, each followed by off/on: three distinct levels. This diagnoses command delivery during re-enable; cycling power on every brightness change is not an implemented or accepted UI solution.

The published contact sheets crop each reviewed camera frame to 1160:460:100:180, scale it to 580×230 and concatenate it horizontally using FFmpeg. Raw frame hashes, capture timestamps and measurements are in result.json; raw full desk images remain local.

Commands and limits

All brightness writes ran as the unprivileged shell user:

runuser -u shell -- sh -c 'echo 128 > /sys/class/backlight/canaan-dsi-backlight/brightness'
runuser -u shell -- env XDG_RUNTIME_DIR=/run/shell SWAYSOCK=/run/shell/sway-ipc.sock swaymsg output DSI-1 power off
runuser -u shell -- env XDG_RUNTIME_DIR=/run/shell SWAYSOCK=/run/shell/sway-ipc.sock swaymsg output DSI-1 power on

Before the first off/on experiment, a separate systemd timer was confirmed active, armed to request power on after 45 seconds using absolute binary paths. The direct on command recovered the panel; the unused timer was stopped afterward. Repeated cycles also recovered. This does not test battery suspend or prove recovery from a kernel hang.

Every live write returned success and the requested sysfs value. The panel log reported a four-byte short packet result. Success and FIFO drain do not establish that active-video LP commands reach the panel. DCS read diagnostics still failed (0x04: -22; 0x0a: -110); successful reads are not claimed. A read-only /dev/mem snapshot was denied by the kernel; no register was written and no snapshot was obtained.

The GPIO power restoration and nondefault re-enable behavior have camera proof. The core live brightness requirement remains UNVERIFIED / negative, so tasks 6.3 and 6.4 remain open and this trial did not install the candidate persistently. No Settings tap or real-finger acceptance is claimed. The next bounded experiment is HS delivery of brightness during video, keeping LP for command-mode initialization and leaving video/PHY/power unchanged.