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

Boris Brezillon
2015-10-05T15:25:21Z

On Mon, 5 Oct 2015 16:07:41 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> 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.

>From a quick glance, I don't see any obvious problem in this approach.

>
> 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>

Not sure it has any value since I proposed the same patch, but here
is my

Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.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)

--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering