Re: [PATCH v2 3/8] drm/panfrost: Hold runtime PM reference until jobs complete

[Rob Herring <robh@kernel.org>]

On Fri, Aug 23, 2019 at 9:50 AM Steven Price <steven.price@arm.com> wrote:
>
> On 23/08/2019 03:12, Rob Herring wrote:
> > Doing a pm_runtime_put as soon as a job is submitted is wrong as it should
> > not happen until the job completes. It works because we are relying on the
> > autosuspend timeout to keep the h/w enabled.
> >
> > Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> > Cc: Steven Price <steven.price@arm.com>
> > Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > Signed-off-by: Rob Herring <robh@kernel.org>
>
> Nice find, but I'm a bit worried about one thing.
>
> > ---
> > v2: new patch
> >
> >   drivers/gpu/drm/panfrost/panfrost_job.c | 16 +++++++++-------
> >   1 file changed, 9 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> > index 05c85f45a0de..80c9cab9a01b 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> > @@ -151,7 +151,7 @@ static void panfrost_job_hw_submit(struct panfrost_job *job, int js)
> >               return;
> >
> >       if (WARN_ON(job_read(pfdev, JS_COMMAND_NEXT(js))))
> > -             goto end;
> > +             return;
> >
> >       cfg = panfrost_mmu_as_get(pfdev, &job->file_priv->mmu);
> >
> > @@ -187,10 +187,6 @@ static void panfrost_job_hw_submit(struct panfrost_job *job, int js)
> >       job_write(pfdev, JS_COMMAND_NEXT(js), JS_COMMAND_START);
> >
> >       spin_unlock_irqrestore(&pfdev->hwaccess_lock, flags);
> > -
> > -end:
> > -     pm_runtime_mark_last_busy(pfdev->dev);
> > -     pm_runtime_put_autosuspend(pfdev->dev);
> >   }
> >
> >   static void panfrost_acquire_object_fences(struct drm_gem_object **bos,
> > @@ -388,9 +384,13 @@ static void panfrost_job_timedout(struct drm_sched_job *sched_job)
> >
> >       mutex_lock(&pfdev->reset_lock);
> >
> > -     for (i = 0; i < NUM_JOB_SLOTS; i++)
> > +     for (i = 0; i < NUM_JOB_SLOTS; i++) {
> >               drm_sched_stop(&pfdev->js->queue[i].sched, sched_job);
> > -
> > +             if (pfdev->jobs[i]) {
> > +                     pm_runtime_put_noidle(pfdev->dev);
> > +                     pfdev->jobs[i] = NULL;
>
> I can't see what prevents this racing with panfrost_job_irq_handler() -
> the job could be completing at the same time as we assign NULL. Then
> panfrost_job_irq_handler() will happily dereference the NULL pointer...

The fact that 1 job's timeout handler is cleaning up all the other
jobs is messy. I'm not sure if there's a better way...

We could perhaps disable the job interrupt at the beginning though I
think we can still have a race as we can't use disable_irq() with a
shared irq. Or do this after the reset.

> Admittedly this patch is an improvement over the situation before :)

Yes, and I'd like to stop digging a deeper hole...

Rob
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