[RFC] Dell Inspiron 7441 AUO0BA4 panel backlight: PWM configuration

[RFC] Dell Inspiron 7441 AUO0BA4 panel backlight: PWM configuration

[Avalon Vargas Centeno]

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

Re: [RFC] Dell Inspiron 7441 AUO0BA4 panel backlight: PWM configuration

[Val Packett]

On 9/11/26 7:11 PM, Avalon Vargas Centeno wrote:
> 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.
> [..]

Weird. I know someone who has a B140QAX01.H in a 7445 (that's how I've 
added it to edp_panels) and they did not have that problem, aux 
backlight worked fine…

panel-edp does not seem to care about DPCD indicating PWM; if the 
brightness property is present in the device tree it *will* be used, and 
AUX is used as a fallback if the external DT-provided brightness was 
absent. If we really need to support this (without adding more 
devicetrees), I guess we could add some opt-in prefer-aux-brightness 
thing..

~val