[PATCH] video: backlight: Only set maximum brightness for gpio-backlight

[Daniel Thompson]

On Thu, Jul 08, 2021 at 11:10:58AM +0200, Marek Vasut wrote:
> The note in c2adda27d202f ("video: backlight: Add of_find_backlight helper
> in backlight.c") says that gpio-backlight uses brightness as power state.
> Other backlight drivers do not, so limit this workaround to gpio-backlight.
> 
> This fixes the case where e.g. pwm-backlight can perfectly well be set to
> brightness 0 on boot in DT, which without this patch leads to the display
> brightness to be max instead of off.
> 
> Fixes: c2adda27d202f ("video: backlight: Add of_find_backlight helper in backlight.c")
> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Daniel Thompson <daniel.thompson at linaro.org>
> Cc: Meghana Madhyastha <meghana.madhyastha at gmail.com>
> Cc: Noralf Trønnes <noralf at tronnes.org>
> Cc: Sean Paul <seanpaul at chromium.org>
> Cc: Thierry Reding <treding at nvidia.com>

I have to admit that this patch really does makes it clear just how
nasty the hack in of_find_backlight() currently is.

Moreover I think it is also be obsolete. gpio-backlight power mode
handling was pretty broken when this code was introduced. It was fixed
in 2019 by ec665b756e6f ("backlight: gpio backlight: Correct initial
power state handling") by trying to match the behaviour of PWM
backlight.  The new code always sets the brightness to 1 so I think we
can just remove the hack from of_find_backlight() since I think it is
unreachable.


Daniel.



> ---
>  drivers/video/backlight/backlight.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
> index 537fe1b376ad7..dfb66171dec41 100644
> --- a/drivers/video/backlight/backlight.c
> +++ b/drivers/video/backlight/backlight.c
> @@ -676,6 +676,7 @@ EXPORT_SYMBOL(of_find_backlight_by_node);
>  static struct backlight_device *of_find_backlight(struct device *dev)
>  {
>  	struct backlight_device *bd = NULL;
> +	bool is_gpio_backlight = false;
>  	struct device_node *np;
>  
>  	if (!dev)
> @@ -685,6 +686,8 @@ static struct backlight_device *of_find_backlight(struct device *dev)
>  		np = of_parse_phandle(dev->of_node, "backlight", 0);
>  		if (np) {
>  			bd = of_find_backlight_by_node(np);
> +			is_gpio_backlight =
> +				of_device_is_compatible(np, "gpio-backlight");
>  			of_node_put(np);
>  			if (!bd)
>  				return ERR_PTR(-EPROBE_DEFER);
> @@ -692,7 +695,7 @@ static struct backlight_device *of_find_backlight(struct device *dev)
>  			 * Note: gpio_backlight uses brightness as
>  			 * power state during probe
>  			 */
> -			if (!bd->props.brightness)
> +			if (is_gpio_backlight && !bd->props.brightness)
>  				bd->props.brightness = bd->props.max_brightness;
>  		}
>  	}
> -- 
> 2.30.2