linux-kernel - Re: [PATCH] arm64: dts: qcom: Add backlight support for eDP panel

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Message-ID: <8caa1cec-d881-4e77-8d77-be043f8d13d6@oss.qualcomm.com>
Date: Thu, 30 Oct 2025 10:41:51 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Yongxing Mou <yongxing.mou@....qualcomm.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: Add backlight support for eDP panel

On 10/30/25 10:08 AM, Yongxing Mou wrote:
> 
> 
> On 10/28/2025 5:08 PM, Konrad Dybcio wrote:
>> On 10/28/25 8:04 AM, Yongxing Mou wrote:
>>> Previously, the eDP panel backlight was enabled via UEFI. Added backlight
>>> control node in kernel DTS due to some meta may not enable the backlight.
>>>
>>> Aligned with other x1e80100-based platforms: the PWM signal is controlled
>>> by PMK8550, and the backlight enable signal is handled by PMC8380.
>>>
>>> Signed-off-by: Yongxing Mou <yongxing.mou@....qualcomm.com>
>>> ---
>>
>> The subject must say "hamoa-iot-evk:"
> Thanks and got it , will fix it in next version.>> arch/arm64/boot/dts/qcom/hamoa-iot-evk.dts | 55 ++++++++++++++++++++++++++++++
>>>   1 file changed, 55 insertions(+)
>>>
>>> diff --git a/arch/arm64/boot/dts/qcom/hamoa-iot-evk.dts b/arch/arm64/boot/dts/qcom/hamoa-iot-evk.dts
>>> index 36dd6599402b4650b7f8ad2c0cd22212116a25fe..fda1b3a3c7673be74832c27849231cba4bc1f25f 100644
>>> --- a/arch/arm64/boot/dts/qcom/hamoa-iot-evk.dts
>>> +++ b/arch/arm64/boot/dts/qcom/hamoa-iot-evk.dts
>>> @@ -17,6 +17,16 @@ aliases {
>>>           serial1 = &uart14;
>>>       };
>>>   +    backlight: backlight {
>>> +        compatible = "pwm-backlight";
>>> +        pwms = <&pmk8550_pwm 0 5000000>;
>>
>> Try adjusting the backlight value.. you'll find some funny behavior
>> near the max level.. which reminds me I should send some fixes for
>> some laptop DTs
>>
>  I tried sliding the backlight between the maximum and minimum values. It seems that at a certain sliding speed, when approaching the maximum value, there is some brightness flickering. Is this the phenomenon you’re referring to?> [...]

"maybe" - for me it was the brightness actually going down beyond ~80%

The issue is that the PWM hardware can't really do a period that's
== 5000000 ns

But it seems like this is no longer an issue since the driver now
rounds up to the closest supported value, see last couple commits
to drivers/leds/rgb/leds-qcom-lpg.c

Konrad