linux-kernel - Re: [PATCH v2 RESEND] video: backlight: pwm_bl: Initialize fb_bl_on[x] and use_count during pwm_backlight

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Date:   Mon, 23 Jan 2017 17:09:17 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Lukasz Majewski <l.majewski@...ess.pl>
Cc:     Lee Jones <lee.jones@...aro.org>, linux-pwm@...r.kernel.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Liu Ying <Ying.Liu@...escale.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Fabio Estevam <festevam@...il.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v2 RESEND] video: backlight: pwm_bl: Initialize
 fb_bl_on[x] and use_count during pwm_backlight_probe()

On Mon, Dec 26, 2016 at 11:21:36PM +0100, Lukasz Majewski wrote:
> The commit a55944ca82d2 
> ("backlight: update bd state & fb_blank properties when necessary")
> has posed some extra restrictions on blanking and unblanking frame buffer
> device.
> 
> Unfortunately, pwm_bl driver's probe did not initialize members of
> struct backlight_device necessary for further blank/unblank operation.
> 
> This code in case of initial unblank of backlight device (default 
> behaviour) sets use_count to 1 and marks this particular backlight device
> as used by all available fb devices (since it is not known during probe 
> how much and which fb devices will be assigned).
> 
> Without this code, the backlight works properly until one tries to blank it
> manually from sysfs with "echo 1 > /sys/class/graphics/fb0/blank".
> Since fb_bl_on[0] and use_count were both set to 0, the logic at
> fb_notifier_callback (@backlight.c) thought that we didn't turn on
> (unblanked) the backlight device and refuses to disable (blank) it.
> As a result we see garbage from fb displayed.
> 
> Signed-off-by: Lukasz Majewski <l.majewski@...ess.pl>
> ---
> The patch has been tested on i.MX6q with vanilla 4.9 kernel.
> It applies on 4.10-rc1.
> ---
>  drivers/video/backlight/pwm_bl.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)

It's difficult to wrap one's head around the above-mentioned commit
a55944ca82d2 ("backlight: update bd state & fb_blank properties when
necessary"). The solution borders on crazy. That said, this patch
builds on top of crazy, and given that it looks okay to me.

Acked-by: Thierry Reding <thierry.reding@...il.com>

> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 1261400..6859ba0 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -202,7 +202,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	struct pwm_bl_data *pb;
>  	int initial_blank = FB_BLANK_UNBLANK;
>  	struct pwm_args pargs;
> -	int ret;
> +	int ret, i;
>  
>  	if (!data) {
>  		ret = pwm_backlight_parse_dt(&pdev->dev, &defdata);
> @@ -348,6 +348,14 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  
>  	bl->props.brightness = data->dft_brightness;
>  	bl->props.power = initial_blank;
> +
> +	if (initial_blank == FB_BLANK_UNBLANK) {
> +		for (i = 0; i < FB_MAX; i++)
> +			bl->fb_bl_on[i] = true;
> +
> +		bl->use_count = 1;
> +	}
> +
>  	backlight_update_status(bl);
>  
>  	platform_set_drvdata(pdev, bl);
> -- 
> 2.1.4
> 

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