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

From: [email-masked] (Markus Pargmann)
Date: Tue, 7 Oct 2014 11:07:18 +0200

Hi,

On Tue, Oct 07, 2014 at 10:35:49AM +0200, Thierry Reding wrote:
> 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 <[email-masked]>
> > ---
> >  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.

Yes your proposal looks a lot better to handle the different use cases.
The DT-binding is not a hardware description but I don't see any better
way of passing that information. So it would be good to get your
solution mainline.

Best regards,

Markus

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