linux-kernel - Re: [PATCH] pwm-backlight: Avoid backlight flicker when probed from DT

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Date:	Mon, 13 Jul 2015 11:40:04 +0200
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Thierry Reding <thierry.reding@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Jingoo Han <jingoohan1@...il.com>,
	Ajay Kumar <ajaykumar.rs@...sung.com>
Cc:	linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] pwm-backlight: Avoid backlight flicker when probed from
 DT

Hi,

Am Freitag, den 12.06.2015, 18:26 +0200 schrieb Philipp Zabel:
> If the driver is probed from the device tree, and there is a phandle
> property set on it, and the enable GPIO is already configured as output,
> and the backlight is currently disabled, keep it disabled.
> If all these conditions are met, assume there will be some other driver
> that can enable the backlight at the appropriate time.
> 
> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> ---
> I have seen the thread at https://lkml.org/lkml/2014/7/31/259, but I'm not
> sure if it has come to a resolution. This is what I think could be useful
> to keep the current default behaviour of enabling the backlight while at the
> same time allowing to keep it disabled if we are absolutely certain that the
> backlight is currently off, and we think there is some other driver that will
> control it:

do you have any comments on this? Would the conditions mentioned above
be sufficient for your use cases?

best regards
Philipp

> ---
>  drivers/video/backlight/pwm_bl.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 6897f1c..dcb6bfb 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -199,6 +199,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	struct backlight_properties props;
>  	struct backlight_device *bl;
>  	struct pwm_bl_data *pb;
> +	int initial_blank = FB_BLANK_UNBLANK;
>  	int ret;
>  
>  	if (!data) {
> @@ -263,8 +264,21 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  		pb->enable_gpio = gpio_to_desc(data->enable_gpio);
>  	}
>  
> -	if (pb->enable_gpio)
> -		gpiod_direction_output(pb->enable_gpio, 1);
> +	if (pb->enable_gpio) {
> +		/*
> +		 * If the driver is probed from the device tree and there is a
> +		 * phandle link pointing to the backlight node, it is safe to
> +		 * assume that another driver will enable the backlight at the
> +		 * appropriate time. Therefore, if it is disabled, keep it so.
> +		 */
> +		if (of_find_property(pdev->dev.of_node, "phandle", NULL) &&
> +		    gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_OUT &&
> +		    gpiod_get_value(pb->enable_gpio) == 0) {
> +			initial_blank = FB_BLANK_POWERDOWN;
> +		} else {
> +			gpiod_direction_output(pb->enable_gpio, 1);
> +		}
> +	}
>  
>  	pb->power_supply = devm_regulator_get(&pdev->dev, "power");
>  	if (IS_ERR(pb->power_supply)) {
> @@ -323,6 +337,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	}
>  
>  	bl->props.brightness = data->dft_brightness;
> +	bl->props.power = initial_blank;
>  	backlight_update_status(bl);
>  
>  	platform_set_drvdata(pdev, bl);


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