Re: [PATCH v2 5/5] drm/msm/dpu: rate limit snapshot capture for mmu faults

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

On 12/18/2024 1:33 PM, Jessica Zhang wrote:
> 
> 
> On 12/18/2024 3:20 AM, Dmitry Baryshkov wrote:
>> On Tue, Dec 17, 2024 at 04:27:57PM -0800, Jessica Zhang wrote:
>>> From: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>>
>>> There is no recovery mechanism in place yet to recover from mmu
>>> faults for DPU. We can only prevent the faults by making sure there
>>> is no misconfiguration.
>>>
>>> Rate-limit the snapshot capture for mmu faults to once per
>>> msm_atomic_commit_tail() as that should be sufficient to capture
>>> the snapshot for debugging otherwise there will be a lot of DPU
>>> snapshots getting captured for the same fault which is redundant
>>> and also might affect capturing even one snapshot accurately.
>>>
>>> Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>> ---
>>>   drivers/gpu/drm/msm/msm_atomic.c | 2 ++
>>>   drivers/gpu/drm/msm/msm_kms.c    | 5 ++++-
>>>   drivers/gpu/drm/msm/msm_kms.h    | 3 +++
>>>   3 files changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_atomic.c 
>>> b/drivers/gpu/drm/msm/msm_atomic.c
>>> index 
>>> 9c45d641b5212c11078ab38c13a519663d85e10a..9ad7eeb14d4336abd9d8a8eb1382bdddce80508a 100644
>>> --- a/drivers/gpu/drm/msm/msm_atomic.c
>>> +++ b/drivers/gpu/drm/msm/msm_atomic.c
>>> @@ -228,6 +228,8 @@ void msm_atomic_commit_tail(struct 
>>> drm_atomic_state *state)
>>>       if (kms->funcs->prepare_commit)
>>>           kms->funcs->prepare_commit(kms, state);
>>> +    kms->fault_snapshot_capture = 0;
>>> +
>>
>> - Please move it before the prepare_commit().
>> - You are accessing the same variable from different threads / cores.
>>    There should be some kind of a sync barrier.
> 
> Hi Dmitry,
> 
> Ack, will add a lock for the snapshot capture counter.
> 
> Thanks,
> 
> Jessica Zhang
> 

We cannot have a mutex lock because msm_kms_fault_handler cannot hold a 
mutex. So we need an atomic variable in this case.

>>
>>>       /*
>>>        * Push atomic updates down to hardware:
>>>        */
>>> diff --git a/drivers/gpu/drm/msm/msm_kms.c 
>>> b/drivers/gpu/drm/msm/msm_kms.c
>>> index 
>>> 78830e446355f77154fa21a5d107635bc88ba3ed..3327caf396d4fc905dc127f09515559c12666dc8 100644
>>> --- a/drivers/gpu/drm/msm/msm_kms.c
>>> +++ b/drivers/gpu/drm/msm/msm_kms.c
>>> @@ -168,7 +168,10 @@ static int msm_kms_fault_handler(void *arg, 
>>> unsigned long iova, int flags, void
>>>   {
>>>       struct msm_kms *kms = arg;
>>> -    msm_disp_snapshot_state(kms->dev);
>>> +    if (!kms->fault_snapshot_capture) {
>>> +        msm_disp_snapshot_state(kms->dev);
>>> +        kms->fault_snapshot_capture++;
>>> +    }
>>>       return -ENOSYS;
>>>   }
>>> diff --git a/drivers/gpu/drm/msm/msm_kms.h 
>>> b/drivers/gpu/drm/msm/msm_kms.h
>>> index 
>>> e60162744c669773b6e5aef824a173647626ab4e..3ac089e26e14b824567f3cd2c62f82a1b9ea9878 100644
>>> --- a/drivers/gpu/drm/msm/msm_kms.h
>>> +++ b/drivers/gpu/drm/msm/msm_kms.h
>>> @@ -128,6 +128,9 @@ struct msm_kms {
>>>       int irq;
>>>       bool irq_requested;
>>> +    /* rate limit the snapshot capture to once per attach */
>>> +    int fault_snapshot_capture;
>>> +
>>>       /* mapper-id used to request GEM buffer mapped for scanout: */
>>>       struct msm_gem_address_space *aspace;
>>>
>>> -- 
>>> 2.34.1
>>>
>>
>> -- 
>> With best wishes
>> Dmitry
>