Re: [PATCH v2] arm64: dts: qcom: x1e80100-medion-sprchrgd-14-s1: Add PWM backlight

["Ettore Chimenti" <ettore.chimenti@linaro.org>]

Hi Sérgio,

On Sun Oct 25, 2026 at 8:51 AM CET, Sérgio Ferreira via B4 Relay wrote:
> From: Sérgio Ferreira <ferreira@se-rg.io>
>
> The eDP panel (CSOT MNE007ZA3-2, IPS LCD) cannot have its brightness
> set over eDP AUX. With no backlight phandle on the panel, panel-edp
> falls back to the DP AUX backlight, which registers dp_aux_backlight
> with a maximum brightness of 0, so brightness control has no effect.
>
> Brightness is driven by PWM instead. UEFI leaves PM8550 LPG channel 3
> running and the PWM output configured on PM8550 GPIO 8, and drives
> PMC8380_3 GPIO 4 high as the backlight enable:
>
>   pm8550 gpio8 : out low func1 vin-0 no pull push-pull medium
>   pmc8380_3 gpio4 : out  high normal  vin-1 pull-down push-pull medium
>
> The panel configuration in the Windows ACPI tables confirmed this.
> (PMIC-controlled PWM, PMIC 1, bank 3, 9-bit, 2 kHz).
>
> Describe this with a pwm-backlight node: enable pm8550_pwm, add pinctrl
> states for the enable GPIO and for gpio8 func1, and attach the
> backlight to the panel.
>
> Tested on a Medion SPRCHRGD 14 S1 (X1E78100): brightness control over
> the full range, brightness keys, suspend/resume and restore after
> reboot.
>
> Assisted-by: LLM
> Signed-off-by: Sérgio Ferreira <ferreira@se-rg.io>
> ---

I tested this on a Medion SPRCHRGD 14 S1, but the controls are inverted.
When increasing the backlight lightness it actually decreases it, and
vice versa.

```
% cat /sys/class/backlight/backlight/max_brightness
4095

% echo 1 | sudo tee /sys/class/backlight/backlight/brightness
1 # BRIGHT

% echo 2000 | sudo tee /sys/class/backlight/backlight/brightness
2000 # LESS BRIGHT

% echo 4095 | sudo tee /sys/class/backlight/backlight/brightness
4095 # OFF

% echo 0 | sudo tee /sys/class/backlight/backlight/brightness
0 # OFF
```

Moreover, I see that from `/sys/kernel/debug/pwm` the period
request differs from the actual:

```
 pwm-3   (backlight           ): requested
    requested configuration:  enabled, 498418/500000 ns, normal polarity
    # with brightness set at 4090:
    actual configuration:     enabled, 425834/425834 ns, normal polarity
    # with brightness set at 2:
                                            0/425834 ns, normal polarity
```

This probably causes the top 250 values to be all at the same
brightness. Not a big deal probably.

Do you have the same behavior? Can you send the EDID of your panel? 

Thanks,
Ettore