linux-kernel - Re: [PATCH v3 1/2] backlight: pwm_bl: Move the checks for initial power state to a separate function

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Date:   Mon, 21 Nov 2016 09:10:05 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     lee.jones@...aro.org, tomi.valkeinen@...com,
        linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, p.zabel@...gutronix.de
Subject: Re: [PATCH v3 1/2] backlight: pwm_bl: Move the checks for initial
 power state to a separate function

On Tue, Nov 01, 2016 at 02:59:32PM +0200, Peter Ujfalusi wrote:
> Move the checks to select the initial state for the backlight to a new
> function and document the checks we are doing.
> 
> With the separate function it is going to be easier to fix or improve the
> initial power state configuration later and it is easier to read the code.
> 
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
> ---
>  drivers/video/backlight/pwm_bl.c | 53 ++++++++++++++++++++++++++--------------
>  1 file changed, 34 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 12614006211e..4b07da278b4f 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -192,6 +192,32 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  }
>  #endif
>  
> +static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb)
> +{
> +	struct device_node *node = pb->dev->of_node;
> +
> +	/* Not booted with device tree or no phandle link to the node */
> +	if (!node || !node->phandle)
> +		return FB_BLANK_UNBLANK;
> +
> +	/*
> +	 * 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 the enable GPIO is disabled, do not enable the backlight */
> +	if (pb->enable_gpio && gpiod_get_value(pb->enable_gpio) == 0)
> +		return FB_BLANK_POWERDOWN;
> +
> +	/* The regulator is disabled, do not enable the backlight */
> +	if (!regulator_is_enabled(pb->power_supply))
> +		return FB_BLANK_POWERDOWN;
> +
> +	return FB_BLANK_UNBLANK;
> +}
> +
>  static int pwm_backlight_probe(struct platform_device *pdev)
>  {
>  	struct platform_pwm_backlight_data *data = dev_get_platdata(&pdev->dev);
> @@ -200,7 +226,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	struct backlight_device *bl;
>  	struct device_node *node = pdev->dev.of_node;
>  	struct pwm_bl_data *pb;
> -	int initial_blank = FB_BLANK_UNBLANK;
>  	struct pwm_args pargs;
>  	int ret;
>  
> @@ -267,20 +292,13 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  		pb->enable_gpio = gpio_to_desc(data->enable_gpio);
>  	}
>  
> -	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 (node && node->phandle &&
> -		    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);
> -	}
> +	/*
> +	 * If the GPIO is configured as input, change the direction to output
> +	 * and set the GPIO as active.
> +	 */
> +	if (pb->enable_gpio &&
> +	    gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_IN)
> +		gpiod_direction_output(pb->enable_gpio, 1);

I'm confused about this. Isn't it redundant to check for the direction
if you're going to configure it as output either way? Why not just set
it as output unconditionally?

Also, is this not counterproductive? pwm_backlight_initial_power_state()
will check the value of the GPIO to determine whether or not to mark the
backlight as enabled. If we're setting this to active, then the check in
the initial state retrieval will only be false if the GPIO is an output
and inactive.

Oh wait... I guess that's exactly why you're doing this. =) Perhaps this
could be made somewhat clearer by beefing up the comment. As it is, the
comment /seems/ rather useless because it restates what the code does. I
think it'd be better to explain more verbosely what's going on, to avoid
confusing people like me.

Either way, though, the patch looks correct now that I understand it
properly, I'll leave it up to Lee if he wants to insist on a clarifying
comment.

Reviewed-by: Thierry Reding <treding@...dia.com>

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