[PATCH] backlight: pwm_bl: Avoid backlight flicker if backlight control GPIO is input

[Marek Vasut]

On 7/21/21 9:01 PM, Marek Vasut wrote:

[...]

>>>> @@ -486,18 +500,6 @@ static int pwm_backlight_probe(struct 
>>>> platform_device *pdev)
>>>>            goto err_alloc;
>>>>        }
>>>> -    /*
>>>> -     * If the GPIO is not known to be already configured as output, 
>>>> that
>>>> -     * is, if gpiod_get_direction returns either 1 or -EINVAL, 
>>>> change the
>>>> -     * direction to output and set the GPIO as active.
>>>> -     * Do not force the GPIO to active when it was already output 
>>>> as it
>>>> -     * could cause backlight flickering or we would enable the 
>>>> backlight too
>>>> -     * early. Leave the decision of the initial backlight state for 
>>>> later.
>>>> -     */
>>>> -    if (pb->enable_gpio &&
>>>> -        gpiod_get_direction(pb->enable_gpio) != 0)
>>>> -        gpiod_direction_output(pb->enable_gpio, 1);
>>>
>>> pwm_backlight_initial_power_state() is still called after 
>>> pwm_apply_state()
>>> in pwm_backlight_probe(), so that might still be too late, no ?
>>
>> The initial pwm_apply_state() is essentially a nop or, perhaps, a sanity
>> check if you prefer to think if it that way.
>>
>> It can change the PWM period in some (non-DT) cases but only if the PWM
>> is not already running... and the change of period should not start it
>> running.
> 
> All right, let me give this a try.

ACK, for this case I have here, this works too. Can you submit a proper 
patch, including my AB/TB and I think also the Fixes tag ?