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: <ae9021dc-e0ca-4631-b45b-3cf9f526bd14@gmail.com>
Date: Thu, 30 Oct 2025 14:24:15 +0530
From: Tessolve Upstream <tessolveupstream@...il.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.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 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>;
    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.
>