linux-kernel - Re: [PATCH v1 1/1] arm64: dts: qcom: talos-evk: Add support for dual-channel LVDS panel

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Message-ID: <mit5xey5rorafacljgnayvjpnbkvtpxis72pzhgnuvjmzmc7sg@c2v4bl6dm6wq>
Date: Sat, 1 Nov 2025 10:39:58 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Tessolve Upstream <tessolveupstream@...il.com>
Cc: Konrad Dybcio <konrad.dybcio@....qualcomm.com>, andersson@...nel.org,
        konradybcio@...nel.org, robh@...nel.org, krzk+dt@...nel.org,
        conor+dt@...nel.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] arm64: dts: qcom: talos-evk: Add support for
 dual-channel LVDS panel

On Thu, Oct 30, 2025 at 02:24:15PM +0530, Tessolve Upstream wrote:
> 
> 
> On 29/10/25 18:04, Dmitry Baryshkov wrote:
> > On Wed, Oct 29, 2025 at 05:50:01PM +0530, Tessolve Upstream wrote:
> >>
> >>
> >> On 28/10/25 14:55, Konrad Dybcio wrote:
> >>> On 10/28/25 7:16 AM, Sudarshan Shetty wrote:
> >>>> This patch introduces a new device tree for the QCS615 Talos
> >>>> EVK platform with dual-channel LVDS display support.
> >>>>
> >>>> The new DTS file (`talos-evk-lvds.dts`) is based on the existing
> >>>> `talos-evk.dts` and extends it to enable a dual-channel LVDS display
> >>>> configuration using the TI SN65DSI84 DSI-to-LVDS bridge.
> >>>>
> >>>> where channel-A carries odd pixel and channel-B carries even pixel
> >>>> on the QCS615 talos evk platform.
> >>>>
> >>>> Signed-off-by: Sudarshan Shetty <tessolveupstream@...il.com>
> >>>> ---
> >>>
> >>> [...]
> >>>
> >>>> +	backlight: backlight {
> >>>> +		compatible = "gpio-backlight";
> >>>> +		gpios = <&tlmm 115 GPIO_ACTIVE_HIGH>;
> >>>> +		default-on;
> >>>> +	};
> >>>> +
> >>>> +	lcd0_pwm_en {
> >>>
> >>> Use hyphens instead of underscores in node names
> >>
> >> Okay, will update in next patch.
> >>>
> >>>> +		compatible = "pwm-gpio";
> >>>> +		gpios = <&tlmm 59 GPIO_ACTIVE_HIGH>;
> >>>> +		pinctrl-0 = <&lcd0_bklt_pwm>;
> >>>
> >>> I think your intention was to use pwm-backlight and wire this node
> >>> up to that
> >>
> >> Soc does not support the PWM interface hence we are using gpio 
> >> as "pwm-gpio" by setting the duty cycle to 100%(i.e. "no pwm")
> > 
> > Why do you mention PWM here at all? It's perfectly fine to use
> > gpio-backlight. That looks extremely suspicious since you are not
> > referecing the pwm-gpio device at all.
> 
> Thanks for the feedback.
> GPIO59 (LED_PWM) connected to the panel’s dimming input.
> 
> Since we can only drive the LED_PWM pin high (100% duty), 
> it does not provide true PWM control.
> 
> I’ll remove the non-standard "pwm-gpio" node and model both lines
> using the standard gpio-backlight binding, like this:
> 
> backlight: backlight {
>     compatible = "gpio-backlight";
>     gpios = <&tlmm 115 GPIO_ACTIVE_HIGH>, <&tlmm 59 GPIO_ACTIVE_HIGH>;


You will have to adapt the gpio-backlight bindings and driver to handle
multiple entries here.

>     default-on;
>     pinctrl-0 = <&lcd0_bklt_en>, <&lcd0_bklt_pwm>;
>     pinctrl-names = "default";
> };
> 
> &tlmm {
>        lcd0_bklt_en: lcd0-bklt-en-state {
>                pins = "gpio115";
>                function = "gpio";
>                bias-disable;
>        };
> 
>        lcd0_bklt_pwm: lcd0-bklt-pwm-state {
>                pins = "gpio59";
>                function = "gpio";
>                bias-disable;
>        };
> };
> 
> Let me know your opinion here,
> Thanks in advance.
> > 
> 

-- 
With best wishes
Dmitry