Re: [PATCH] Revert "backlight: pwm: Handle EPROBE_DEFER while requesting the PWM"

[Boris Brezillon <boris.brezillon@free-electrons.com>]

Hi Thierry,

On Mon, 5 Oct 2015 11:35:43 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Thu, Oct 01, 2015 at 10:00:22AM +0200, Nicolas Ferre wrote:
> > Le 30/09/2015 21:29, Robert Jarzmik a écrit :
> > > Robert Jarzmik <robert.jarzmik@free.fr> writes:
> > > 
> > >> This reverts commit 68feaca0b13e453aa14ee064c1736202b48b342f.
> > >> This commit breaks legacy platforms, for which :
> > >>  (a) no pwm table is added (legacy platforms)
> > >>  (b) in this case, in pwm_get(), pmw_lookup_list is empty, and therefore
> > >>      chosen = NULL, and therefore pwm_get() returns NULL, and pwm_get()
> > >>      returns -EPROBE_DEFER
> > >>  (c) as a consequence, this code is unreachable in pwm_bl.c :
> > >>      if (IS_ERR(pb->pwm)) {
> > >> 	ret = PTR_ERR(pb->pwm);
> > >>  	dev_info(&pdev->dev, "%s:%d(): %d\n", __func__, __LINE__, ret);
> > >>  	if (ret = -EPROBE_DEFER)
> > >>  		goto err_alloc;
> > >>
> > >>  	dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
> > >>  	pb->legacy = true;
> > >>  	pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
> > >>
> > >> As this code is unreachable, all legacy platforms relying on pwm_id are
> > >> broken, amongst which pxa have been tested as broken.
> > >>
> > >> Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
> > > Thierry, would you have a look please ?
> > > As I said before, all legacy platform relying on pwm_id are broken. I'd like to
> > > be sure this lands in the next -rc series.
> > 
> > Well, as I answered on the linux-pwm mailing-list (I was not in copy) here:
> > http://article.gmane.org/gmane.linux.pwm/2744
> > I wonder if it's not easier to fix the platforms and add the pwm tables...
> > 
> > Otherwise, Boris proposed this fix:
> > 8<-----------------------------------------------------------
> > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > index eff379b..00483d4 100644
> > --- a/drivers/video/backlight/pwm_bl.c
> > +++ b/drivers/video/backlight/pwm_bl.c
> > @@ -273,15 +273,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
> >  	pb->pwm = devm_pwm_get(&pdev->dev, NULL);
> >  	if (IS_ERR(pb->pwm)) {
> >  		ret = PTR_ERR(pb->pwm);
> > -		if (ret = -EPROBE_DEFER)
> > -			goto err_alloc;
> >  
> >  		dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
> >  		pb->legacy = true;
> >  		pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
> >  		if (IS_ERR(pb->pwm)) {
> >  			dev_err(&pdev->dev, "unable to request legacy PWM\n");
> > -			ret = PTR_ERR(pb->pwm);
> > +			if (ret != -EPROBE_DEFER)
> > +				ret = PTR_ERR(pb->pwm);
> > +
> >  			goto err_alloc;
> >  		}
> >  	}
> > 
> > which is not tested and may add an extra non-valid error log.
> 
> This is a little risky in my opinion. Not only does it print two error
> messages for non-legacy platforms (that would be another regression if
> you want to be nit-picking), but it is subtly buggy. If you have a
> system with multiple PWM providers, you could end up failing the first
> pwm_get() with -EPROBE_DEFER but then continue to the legacy case, and
> this could succeed because data->pwm_id = 0, and that other provider
> could be exporting the PWM with this ID. If I remember correctly this
> was one of the reasons why the offending commit was merged in the first
> place.

Just for the record, when I proposed this fix to Nicolas, I clearly
stated that this was not the way to go, and that fixing the offending
platforms to use PWM lookup table was the only sane solution, though I
didn't thought about the invalid PWM id case leading to buggy behavior.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com