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

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Message-ID: <d9d0947e-66a1-4ed0-b784-53c38bca784f@oss.qualcomm.com>
Date: Thu, 30 Oct 2025 12:27:24 +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 12:07 PM, Yongxing Mou wrote:
> 
> 
> On 10/30/2025 5:41 PM, Konrad Dybcio wrote:
>> 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
> Oh,thanks for sharing that. I saw the phenomenon you mentioned. Previously, my baseline had already merged this patch series: https://lore.kernel.org/all/20250305-leds-qcom-lpg-fix-max-pwm-on-hi-res-v4-0-bfe124a53a9f@linaro.org/, so I didn’t observe the ‘brightness actually going down beyond ~80%.’and just very slight flickering. After I reverted patches, it indeed appeared.  So here i want to confirm,
> pwms = <&pmk8550_pwm 0 5000000>
> can i keep the value 5000000 ?

I think so.. perhaps make sure the period is appropriate for
the backlight device at the other end since you mentioned it's
flickering

Konrad