Re: [PATCH 1/2] dt-bindings: display: panel: add optional wled-supply

[Arpit Saini <arpit.saini@oss.qualcomm.com>]

On 10/2/2026 1:52 PM, Neil Armstrong wrote:
> On 10/1/26 20:09, Arpit Saini wrote:
>>
>>
>> On 10/1/2026 3:25 PM, Dmitry Baryshkov wrote:
>>> On Thu, Oct 01, 2026 at 08:57:52AM +0200, Krzysztof Kozlowski wrote:
>>>> On 01/10/2026 08:55, Krzysztof Kozlowski wrote:
>>>>> On Tue, Sep 29, 2026 at 06:42:20PM +0530, Arpit Saini wrote:
>>>>>> Some boards drive the ILI7807S panel's backlight from an external
>>>>>> WLED driver whose enable input is wired to a GPIO, typically modeled
>>>>>> as a fixed regulator (e.g. vreg_wled).
>>>>>
>>>>> You describe something else. What's fixed regulator should not matter
>>>>> here. Which pin is it in ILI7807S?
>>>>>
>>>>> It seems you just want to represent GPIO with a regulator. This is just
>>>>> confusing and typical downstream workaround.
>>>>
>>>> What's more, you basically REVERT the review YOU RECEIVED in v1. Really,
>>>> just sneak the same stuff 3 months after like the review never happened.
>>>>
>>>> NAK
>>>
>>> After discussing this offline with Krzysztof. It's not a supply (my
>>> fault), it's an LCD driver. So, the best way to handle your displaycard
>>> seems to add a gpio-backlight, reference it from the panel and then in
>>> the driver check for the backlight's max_brightness level. If it's 1,
>>> then you have to send extra DCS commands to control PWM. If it's
>>> higher, use normal backlight class controls.
>>>
>>
>> Hi Dmitry, Krzysztof
>>
>> I have a few clarifying questions regarding the proposed approach. Please let me know if I've misunderstood anything.
>>      panel_backlight: backlight {
>>        compatible = "gpio-backlight";
>>        gpios = <&tlmm 91 GPIO_ACTIVE_HIGH>;
>>        default-on;
>>    };
>>
>> 1) Adding gpio-backlight and check for max_brightness level if its 1,
>>
>>     If we model LCD_BKLT_EN using gpio-backlight, the backlight device effectively exposes only on/off control (max_brightness = 1),
>>     we can't support the full range of brightness i.e 0 to 16383 (0x3FFF)
>>
>> 2) Adding gpio-backlight and based upon max_brightness level of 1 , are you suggesting to register another
>>     backlight device that can actually drive DCS brightness. In that case we can actually have the MIPI DCS controlled brightness
>>
>>     If so, wouldn't that result in two backlight devices associated with the same panel:
>>
>>      gpio-backlight device for enable/disable
>>      panel backlight device for DCS brightness control
>>       Is that the expected design?
> 
> It's a great question because there's a large variety of how backlight is
> implemented, and some panels can drive a PWM to an actually backlight controller
> which uses external pwm. In this case we should model the backlight IC as
> backlight driver with only 1 or 0 capability and use the DCS to program
> the PWM.
> 
> So it leads exactly to your issue. So perhaps one way would be to either:
> - call into the gpio-backlight from the DCS callback, we may need to fix some locking issues
> - add way to "link" backlight devices so the backlight value can be propagated
> 
> 
> In any case the problem remains that both backlight devices will be exposed
> to userspace, which we don't want. So additional changes will be needed.
> 
>>
>> 3) I previously tried modeling LCD_BKLT_EN using pinctrl states (panel_bl_en / panel_bl_suspend)
>>     for the enable GPIO itself. However, Dmitry suggested modeling it as a regulator instead:
>>
>>     Link : https://lore.kernel.org/all/qkhgg5x67sijiialucvzac275zhpjrtt47a4udjpyzmgvilut5@dcrslq3ai7mc/
>>
>> 4) Modeled optional regulator wled-supply: a regulator that only gates the external backlight driver chip's power/enable,
>>     with DCS remaining the sole brightness path in this current patch , the panel-himax-hx83121a.c does exactly the same.
>>         Would this can be the preferred modeling for such panels?

Hi Neil,

Could you please help me understand why we cannot model the external WLED enable path similarly to panel-himax-hx83121a.c using an optional bl_supply?

In our case, this supply would only enable or disable the external WLED driver through LCD_BKLT_EN/GPIO91,
while the panel’s existing MIPI DCS backlight would continue to control the brightness.

This approach would also avoid exposing a second backlight device to userspace. Would this be an acceptable way to model the ILI7807S panel?

Thanks,
Arpit
>>
>> Please refer to this Hardware diagram I explained earlier ,
>> Link : https://lore.kernel.org/all/bade420c-aeb8-4bdd-b0cf-3ade17b21c18@oss.qualcomm.com/
>>
>>
>> Please let me know your suggestions.
>>
>> Thanks,
>> Arpit
>>
>>
>>    
>>    
>