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

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Message-ID: <c33ae377-454e-4f7c-8af3-0738cf0cf701@oss.qualcomm.com>
Date: Thu, 30 Oct 2025 19:07:50 +0800
From: Yongxing Mou <yongxing.mou@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....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 10/30/2025 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 ?