linux-kernel - Re: [RFC 1/2] arm64: dts: qcom: x1e78100-t14s: Add LCD variant with backlight support

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Message-ID: <Z81dODU91zDPo/H5@linaro.org>
Date: Sun, 9 Mar 2025 11:19:52 +0200
From: Abel Vesa <abel.vesa@...aro.org>
To: Sebastian Reichel <sre@...nel.org>
Cc: 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>, Johan Hovold <johan@...nel.org>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/2] arm64: dts: qcom: x1e78100-t14s: Add LCD variant with
 backlight support

On 25-03-08 01:01:31, Sebastian Reichel wrote:
> Hi,
> 
> On Thu, Mar 06, 2025 at 11:05:02AM +0200, Abel Vesa wrote:
> > Due to the fact that Lenovo Thinkpad T14s Gen6 is available with both
> > OLED and LCD, the backlight control differs HW-wise. For the LCD variant,
> > the panel's backlight is controlled via one of the PWMs provided by the
> > PMK8550 PMIC. For the OLED variant, the backlight is internal to the
> > panel and therefore it is not described in devicetree.
> > 
> > For this reason, create a generic dtsi for the T14s by renaming the
> > existing dts. While at it, add a node name to panel and drop the enable
> > gpio and pinctrl properties from the panel node. Then add the LCD variant
> > dts file with the old name and describe all backlight related nodes.
> > 
> > So the existing dts will now be used for LCD variant while for OLED new
> > dts will be added.
> > 
> > Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> > ---
> > [...]
> > +	backlight: backlight {
> > +		compatible = "pwm-backlight";
> > +		pwms = <&pmk8550_pwm 0 5000000>;
> 
> I've tried this patch series together with the fix series [0], but
> without the duty cycle calculation change [1]. Instead I changed the
> period from 5000000 to 4266667. With that everything works as
> expected for me.
> 
> [0] https://lore.kernel.org/all/20250305-leds-qcom-lpg-fix-max-pwm-on-hi-res-v4-0-bfe124a53a9f@linaro.org/
> [1] https://lore.kernel.org/all/20250303-leds-qcom-lpg-compute-pwm-value-using-period-v1-1-833e729e3da2@linaro.org/

Yes, I forgot to squash in the correct period.

The period should actually be 4266537. This is because the max PWM value
is actually BIT(resolution) - 1.

Will update in next version.

The [1] patch was basically NACKed by Uwe. It is not needed if we set
the period to 4266537 in DT.

> 
> Greetings,
> 
> -- Sebastian