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

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Date:   Wed, 20 Jul 2022 11:06:45 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Akhil P Oommen <quic_akhilpo@...cinc.com>
Cc:     Doug Anderson <dianders@...omium.org>, Sean Paul <sean@...rly.run>,
        Jonathan Marek <jonathan@...ek.ca>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jordan Crouse <jordan@...micpenguin.net>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Chia-I Wu <olvaffe@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Freedreno] [PATCH v2 3/7] drm/msm: Fix cx collapse issue during recovery

On Tue, Jul 12, 2022 at 12:15 PM Akhil P Oommen
<quic_akhilpo@...cinc.com> wrote:
>
> On 7/12/2022 10:14 PM, Rob Clark wrote:
> > On Mon, Jul 11, 2022 at 10:05 PM Akhil P Oommen
> > <quic_akhilpo@...cinc.com> 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@...cinc.com> 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@...cinc.com>
> >>>> ---
> >>>>
> >>>> (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?
>

But I think without #1 you could still be racing w/ some other path
that touches the hw, like devfreq, right.  They could be holding a
runpm ref, so even if you loop over active_submits decrementing the
runpm ref, it still doesn't drop to zero

BR,
-R