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

[Akhil P Oommen <quic_akhilpo@quicinc.com>]

On 7/12/2022 10:14 PM, Rob Clark wrote:
> On Mon, Jul 11, 2022 at 10:05 PM Akhil P Oommen
> <quic_akhilpo@quicinc.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@quicinc.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@quicinc.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?

-Akhil.