Hello, On my Dell Inspiron 14 Plus 7441 (BIOS 2.16.0, aarch64 Linux 7.2.4, NixOS), the AUO B140QAX01.H / AUO0BA4 panel originally exposed an unusable dp_aux_backlight with max_brightness 0. GNOME could not adjust brightness.
Panel DPCD bytes at 0x700-0x72f: 05 9b 91 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 0a 04 0a 00 1b 00 0d 2f 03 ff ff 03 The capability bits indicate PWM brightness setting rather than AUX brightness setting. Inspection of the local panel-edp and DP helper code suggests that this capability combination can leave the AUX backlight maximum at zero. This is a code-path inference, not a separately verified generic DRM driver bug. A privileged GPIO snapshot showed PMK8550 GPIO5 already routed to func3 by firmware, with no Linux PWM provider registered. VREG_EDP_3P3 was enabled. A local DT overlay enables pmk8550_pwm, configures PMK8550 GPIO5 func3, adds a pwm-backlight using channel 0 and vreg_edp_3p3, and connects it to the existing eDP panel. The existing panel enable GPIO is preserved. The chosen period is 4266537 ns, borrowed from the upstream ThinkPad T14s LCD setup; it is not a measured Dell firmware period. The brightness curve is linear with 101 values and default index 80. Validation status: the overlay compiled and applied, and its connection was checked in the compiled DTB. Physical brightness adjustment and suspend/resume have been tested successfully on this laptop. But I have not measured the PWM waveform or tested other panel variants. I can provide the small overlay and further test results. I would also appreciate advice on the correct Dell PWM period and whether the zero-range AUX backlight behavior should be handled separately in the display helpers. Thanks
