Re: [PATCH] pwm-backlight: Turn off pwm backlight in probe

[Thierry Reding <thierry.reding@gmail.com>]

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On Mon, Oct 06, 2014 at 09:22:44PM +0200, Markus Pargmann wrote:
> The backlight will be enabled by the panel again if it is used. So we
> can save the default brightness and disable the pwm backlight when
> probing.
> 
> Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
> ---
>  drivers/video/backlight/pwm_bl.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 336b83be7e2d..b4f433a6f106 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -317,9 +317,11 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  		data->dft_brightness = data->max_brightness;
>  	}
>  
> -	bl->props.brightness = data->dft_brightness;
> +	bl->props.brightness = 0;
>  	backlight_update_status(bl);
>  
> +	bl->props.brightness = data->dft_brightness;
> +
>  	platform_set_drvdata(pdev, bl);
>  	return 0;
>  

It would be nice if it was that easy. But we can't do this, because it
will regress for users of this driver that don't use a panel or DRM. If
the PWM backlight driver is used for example in conjunction with a plain
fbdev driver it isn't necessarily hooked up with anything and won't be
enabled automatically. That's really bad if fbdev is the only output you
have since you'd have to blindly type the commands to enable the
backlight. Furthermore disabling backlight isn't always what you want to
do. For example if the bootloader already turned it on and you hand over
from bootloader to kernel in a seamless way, then you absolutely want to
keep backlight on all the time.

See also[0] for a different proposal to solve the same problem. Back at
the time that received only a very few replies, but it would be nice if
Lee and Bryan could look at it again and see if we can come up with some
way to deal with this situation.

Thierry

[0]: https://lkml.org/lkml/2014/7/31/259

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