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

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

On 7/20/2022 11:36 PM, Rob Clark wrote:
> On Tue, Jul 12, 2022 at 12:15 PM Akhil P Oommen
> <quic_akhilpo@quicinc.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@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?
>>
> 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
Yes, you are right. There could be some transient votes from other 
threads/drivers/subsystem. This is the reason we need to poll for cx 
gdsc collapse in the next patch.Even with #1, it is difficult to 
coordinate with smmu driver and close to impossible with tz/hyp.

-Akhil.