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SubjectRe: Re: [PATCH] drm/panfrost: fix runtime pm imbalance on error

Thank you for your further explanation! It's all clear for me and I
will write a new patch to fix this imbalance.

Regards,
Dinghao
> On 21/05/2020 08:00, dinghao.liu@zju.edu.cn wrote:
> > Hi Steve,
> >
> > There are two bailing out points in panfrost_job_hw_submit(): one is
> > the error path beginning from pm_runtime_get_sync(), the other one is
> > the error path beginning from WARN_ON() in the if statement. The pm
> > imbalance fixed in this patch is between these two paths. I think the
> > caller of panfrost_job_hw_submit() cannot distinguish this imbalance
> > outside this function.
>
> My point is the caller expects panfrost_job_hw_submit() to increase the
> PM reference count. Since panfrost_job_hw_submit() cannot return an
> error (it's void return) we cannot signal to the caller that the
> reference hasn't been taken.
>
> > panfrost_job_timedout() calls pm_runtime_put_noidle() for every job it
> > finds, but all jobs are added to the pfdev->jobs just before calling
> > panfrost_job_hw_submit(). Therefore I think the imbalance still exists.
>
> My point's exactly that - the "jobs are added to pfdev->jobs just before
> calling panfrost_job_hw_submit()". Since we don't have a way for
> panfrost_job_hw_submit() to fail it must unconditionally take any
> references that will then be freed later on.
>
> > But I'm not very sure if we should add pm_runtime_put on the error path
> > after pm_runtime_get_sync(), or remove pm_runtime_put one the error path
> > after WARN_ON().
>
> The pm_runtime_put after the WARN_ON() is a bug. Sorry this is probably
> what confused you - clearly the WARN_ON() situation is never meant to
> happen in the first place, so hopefully this isn't actually possible.
>
> Feel free to send a patch removing it! ;)
>
> > As for the problem about panfrost_devfreq_record_busy(), this may be a
> > new bug and requires independent patch to fix it.
>
> Indeed, I'll post a proper patch for that later - I just spotted it
> while looking at the code.
>
> Thanks,
>
> Steve
>
> > Regards,
> > Dinghao
> >
> >
> >> On 20/05/2020 12:05, Dinghao Liu wrote:
> >>> pm_runtime_get_sync() increments the runtime PM usage counter even
> >>> the call returns an error code. Thus a pairing decrement is needed
> >>> on the error handling path to keep the counter balanced.
> >>>
> >>> Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
> >>
> >> Actually I think we have the opposite problem. To be honest we don't
> >> handle this situation very well. By the time panfrost_job_hw_submit() is
> >> called the job has already been added to the pfdev->jobs array, so it's
> >> considered submitted even if it never actually lands on the hardware. So
> >> in the case of this function bailing out early we will then (eventually)
> >> hit a timeout and trigger a GPU reset.
> >>
> >> panfrost_job_timedout() iterates through the pfdev->jobs array and calls
> >> pm_runtime_put_noidle() for each job it finds. So there's no inbalance
> >> here that I can see.
> >>
> >> Have you actually observed the situation where pm_runtime_get_sync()
> >> returns a failure?
> >>
> >> HOWEVER, it appears that by bailing out early the call to
> >> panfrost_devfreq_record_busy() is never made, which as far as I can see
> >> means that there may be an extra call to panfrost_devfreq_record_idle()
> >> when the jobs have timed out. Which could underflow the counter.
> >>
> >> But equally looking at panfrost_job_timedout(), we only call
> >> panfrost_devfreq_record_idle() *once* even though multiple jobs might be
> >> processed.
> >>
> >> There's a completely untested patch below which in theory should fix that...
> >>
> >> Steve
> >>
> >> ----8<---
> >> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c
> >> b/drivers/gpu/drm/panfrost/panfrost_job.c
> >> index 7914b1570841..f9519afca29d 100644
> >> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> >> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> >> @@ -145,6 +145,8 @@ static void panfrost_job_hw_submit(struct
> >> panfrost_job *job, int js)
> >> u64 jc_head = job->jc;
> >> int ret;
> >>
> >> + panfrost_devfreq_record_busy(pfdev);
> >> +
> >> ret = pm_runtime_get_sync(pfdev->dev);
> >> if (ret < 0)
> >> return;
> >> @@ -155,7 +157,6 @@ static void panfrost_job_hw_submit(struct
> >> panfrost_job *job, int js)
> >> }
> >>
> >> cfg = panfrost_mmu_as_get(pfdev, &job->file_priv->mmu);
> >> - panfrost_devfreq_record_busy(pfdev);
> >>
> >> job_write(pfdev, JS_HEAD_NEXT_LO(js), jc_head & 0xFFFFFFFF);
> >> job_write(pfdev, JS_HEAD_NEXT_HI(js), jc_head >> 32);
> >> @@ -410,12 +411,12 @@ static void panfrost_job_timedout(struct
> >> drm_sched_job *sched_job)
> >> for (i = 0; i < NUM_JOB_SLOTS; i++) {
> >> if (pfdev->jobs[i]) {
> >> pm_runtime_put_noidle(pfdev->dev);
> >> + panfrost_devfreq_record_idle(pfdev);
> >> pfdev->jobs[i] = NULL;
> >> }
> >> }
> >> spin_unlock_irqrestore(&pfdev->js->job_lock, flags);
> >>
> >> - panfrost_devfreq_record_idle(pfdev);
> >> panfrost_device_reset(pfdev);
> >>
> >> for (i = 0; i < NUM_JOB_SLOTS; i++)
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