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

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Message-ID: <4d0f1892-8aa6-4282-8c36-75f1e46cbae1@oss.qualcomm.com>
Date: Wed, 5 Nov 2025 17:58:26 +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 7:27 PM, Konrad Dybcio wrote:
> 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
Compared to ‘brightness actually going down beyond ~80%’, this is just a 
very slight flicker that only occurs when sliding the brightness bar at 
a specific speed.It won’t occur in most cases. Also tried another value 
pwms = <&pmk8550_pwm 0 4266537>, and it looks the same, so can we keep 
it at 5000000 here?