linux-kernel - Re: [PATCH] Revert "backlight: pwm: Handle EPROBE

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Date:	Mon, 5 Oct 2015 14:58:03 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	linux-pwm@...r.kernel.org,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "backlight: pwm: Handle EPROBE_DEFER while
 requesting the PWM"

On Mon, Oct 05, 2015 at 01:19:12PM +0200, Boris Brezillon wrote:
> Hi Thierry,
> 
> On Mon, 5 Oct 2015 11:35:43 +0200
> Thierry Reding <thierry.reding@...il.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@...e.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@...e.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.

As chance would have it, this bubbled to the top of my inbox today:

	http://patchwork.ozlabs.org/patch/483993/

I think that conflicts with Nicolas' -EPROBE_DEFER patch, but I think
reverting Nicolas' patch and then applying the above on top might fix
this nicely for everybody.

Thierry

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