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

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

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On Mon, Oct 05, 2015 at 03:30:24PM +0200, Boris Brezillon wrote:
> On Mon, 5 Oct 2015 14:58:03 +0200
> Thierry Reding <thierry.reding@gmail.com> wrote:
> 
> > 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@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.
> > 
> > As chance would have it, this bubbled to the top of my inbox today:
> > 
> > 	http://patchwork.ozlabs.org/patch/483993/
> 
> AFAICT, this is not valid either. This patch is assuming -EPROBE_DEFER
> can only be returned in the DT case, which is not the case: it is also
> returned if the PWMs were declared with a lookup table but the driver
> is not registered yet (module not loaded, or driver registration
> taking place after the PWM backlight driver).

Right, the non-DT, slightly less legacy case...

> If we were about to differentiate the missing PWM definition from
> the missing driver case, we should do something like this [1].
> 
> Best Regards,
> 
> Boris
> 
> [1]http://code.bulix.org/2oozbq-89125

Haha, I came up with exactly this earlier and I've been trying to think
of ways in which it could potentially break.

Thierry

--- >8 ---
From f7fee34e0c414b4268c59e97937c51e0c91a74cf Mon Sep 17 00:00:00 2001
From: Thierry Reding <thierry.reding@gmail.com>
Date: Mon, 5 Oct 2015 14:38:32 +0200
Subject: [PATCH] pwm: Return -ENODEV if no PWM lookup match is found

When looking up a PWM using the lookup table, assume that all entries
will have been added already, so failure to find a match means that no
corresponding entry has been registered.

This fixes an issue where -EPROBE_DEFER would be returned if the PWM
lookup table is empty. After this fix, -EPROBE_DEFER is reserved for
situations where no provider has yet registered for a matching entry.

Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
---
 drivers/pwm/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index 3f9df3ea3350..94e5af123660 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -719,8 +719,10 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
 		}
 	}
 
-	if (!chosen)
+	if (!chosen) {
+		pwm = ERR_PTR(-ENODEV);
 		goto out;
+	}
 
 	chip = pwmchip_find_by_name(chosen->provider);
 	if (!chip)
-- 
2.5.0


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