linux-kernel - Re: [PATCH] backlight: pwm_bl: Fix GPIO out for unimplemented .get

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Date:   Thu, 23 Mar 2017 23:11:13 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
CC:     Philipp Zabel <p.zabel@...gutronix.de>,
        <linux-pwm@...r.kernel.org>, <linux-fbdev@...r.kernel.org>,
        <linux-renesas-soc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] backlight: pwm_bl: Fix GPIO out for unimplemented
 .get_direction()

On 03/22/2017 07:21 PM, Geert Uytterhoeven wrote:
> Commit 7613c922315e308a ("backlight: pwm_bl: Move the checks for initial
> power state to a separate function") not just moved some code, but made
> slight changes in semantics.
>
> If a gpiochip doesn't implement the optional .get_direction() callback,
> gpiod_get_direction always returns -EINVAL, which is never equal to
> GPIOF_DIR_IN, leading to the GPIO not being configured for output.

The platforms I have tested the gpio drivers do implement the get_direction() 
callback and I was not aware of this behaviour.

> To avoid this, invert the test and check for not GPIOF_DIR_OUT instead,
> like the original code did.
>
> This restores the display on r8a7740/armadillo.
>
> Fixes: 7613c922315e308a ("backlight: pwm_bl: Move the checks for initial power state to a separate function")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>

Reviewed-by: Peter Ujfalusi <peter.ujfalusi@...com>

> ---
>  drivers/video/backlight/pwm_bl.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index d7efcb632f7d9dde..002f1ce22bd02032 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -297,14 +297,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	}
>
>  	/*
> -	 * If the GPIO is configured as input, change the direction to output
> -	 * and set the GPIO as active.
> +	 * If the GPIO is not known to be already configured as output, that
> +	 * is, if gpiod_get_direction returns either GPIOF_DIR_IN 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) == GPIOF_DIR_IN)
> +	    gpiod_get_direction(pb->enable_gpio) != GPIOF_DIR_OUT)
>  		gpiod_direction_output(pb->enable_gpio, 1);
>
>  	pb->power_supply = devm_regulator_get(&pdev->dev, "power");
>


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