Re: [Freedreno] [PATCH v2 3/7] drm/msm: Fix cx collapse issue during recovery

[Akhil P Oommen]

On 7/12/2022 10:14 PM, Rob Clark wrote:
> On Mon, Jul 11, 2022 at 10:05 PM Akhil P Oommen
> <quic_akhilpo@xxxxxxxxxxx> wrote:
> > On 7/12/2022 4:52 AM, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Fri, Jul 8, 2022 at 11:00 PM Akhil P Oommen <quic_akhilpo@xxxxxxxxxxx> wrote:
> > > > There are some hardware logic under CX domain. For a successful
> > > > recovery, we should ensure cx headswitch collapses to ensure all the
> > > > stale states are cleard out. This is especially true to for a6xx family
> > > > where we can GMU co-processor.
> > > >
> > > > Currently, cx doesn't collapse due to a devlink between gpu and its
> > > > smmu. So the *struct gpu device* needs to be runtime suspended to ensure
> > > > that the iommu driver removes its vote on cx gdsc.
> > > >
> > > > Signed-off-by: Akhil P Oommen <quic_akhilpo@xxxxxxxxxxx>
> > > > ---
> > > >
> > > > (no changes since v1)
> > > >
> > > >    drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 16 ++++++++++++++--
> > > >    drivers/gpu/drm/msm/msm_gpu.c         |  2 --
> > > >    2 files changed, 14 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > index 4d50110..7ed347c 100644
> > > > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > @@ -1278,8 +1278,20 @@ static void a6xx_recover(struct msm_gpu *gpu)
> > > >            */
> > > >           gmu_write(&a6xx_gpu->gmu, REG_A6XX_GMU_GMU_PWR_COL_KEEPALIVE, 0);
> > > >
> > > > -       gpu->funcs->pm_suspend(gpu);
> > > > -       gpu->funcs->pm_resume(gpu);
> > > > +       /*
> > > > +        * Now drop all the pm_runtime usage count to allow cx gdsc to collapse.
> > > > +        * First drop the usage count from all active submits
> > > > +        */
> > > > +       for (i = gpu->active_submits; i > 0; i--)
> > > > +               pm_runtime_put(&gpu->pdev->dev);
> > > > +
> > > > +       /* And the final one from recover worker */
> > > > +       pm_runtime_put_sync(&gpu->pdev->dev);
> > > > +
> > > > +       for (i = gpu->active_submits; i > 0; i--)
> > > > +               pm_runtime_get(&gpu->pdev->dev);
> > > > +
> > > > +       pm_runtime_get_sync(&gpu->pdev->dev);
> > > In response to v1, Rob suggested pm_runtime_force_suspend/resume().
> > > Those seem like they would work to me, too. Why not use them?
> > Quoting my previous response which I seem to have sent only to Freedreno
> > list:
> >
> > "I believe it is supposed to be used only during system sleep state
> > transitions. Btw, we don't want pm_runtime_get() calls from elsewhere to
> > fail by disabling RPM here."
> The comment about not wanting other runpm calls to fail is valid.. but
> that is also solveable, ie. by holding a lock around runpm calls.
> Which I think we need to do anyways, otherwise looping over
> gpu->active_submits is racey..
>
> I think pm_runtime_force_suspend/resume() is the least-bad option.. or
> at least I'm not seeing any obvious alternative that is better
>
> BR,
> -R
We are holding gpu->lock here which will block further submissions from 
scheduler. Will active_submits still race?

It is possible that there is another thread which successfully completed 
pm_runtime_get() and while it access the hardware, we pulled the plug on 
regulator/clock here. That will result in obvious device crash. So I can 
think of 2 solutions:

1. wrap *every* pm_runtime_get/put with a mutex. Something like:
            mutex_lock();
            pm_runtime_get();
            < ... access hardware here >>
            pm_runtime_put();
            mutex_unlock();

2. Drop runtime votes from every submit in recover worker and wait/poll 
for regulator to collapse in case there are transient votes on 
regulator  from other threads/subsystems.

Option (2) seems simpler to me.  What do you think?

-Akhil.