linux-kernel - Re: [PATCH v2 1/4] drm/sched: Enable signaling for finished fence

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Date:   Wed, 7 Sep 2022 12:18:37 -0400
From:   Andrey Grodzovsky <andrey.grodzovsky@....com>
To:     Christian König <christian.koenig@....com>,
        Arvind Yadav <Arvind.Yadav@....com>, shashank.sharma@....com,
        amaranath.somalapuram@....com, Arunpravin.PaneerSelvam@....com,
        sumit.semwal@...aro.org, gustavo@...ovan.org, airlied@...ux.ie,
        daniel@...ll.ch, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] drm/sched: Enable signaling for finished fence


On 2022-09-07 02:37, Christian König wrote:
> Am 06.09.22 um 21:55 schrieb Andrey Grodzovsky:
>>
>> On 2022-09-06 02:34, Christian König wrote:
>>> Am 05.09.22 um 18:34 schrieb Arvind Yadav:
>>>> Here's enabling software signaling for finished fence.
>>>>
>>>> Signed-off-by: Arvind Yadav <Arvind.Yadav@....com>
>>>> ---
>>>>
>>>> Changes in v1 :
>>>> 1- Addressing Christian's comment to remove CONFIG_DEBUG_FS check from
>>>> this patch.
>>>> 2- The version of this patch is also changed and previously
>>>> it was [PATCH 2/4]
>>>>
>>>> ---
>>>>   drivers/gpu/drm/scheduler/sched_main.c | 2 ++
>>>>   1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index e0ab14e0fb6b..fe72de0e2911 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -962,6 +962,8 @@ static int drm_sched_main(void *param)
>>>>               /* Drop for original kref_init of the fence */
>>>>               dma_fence_put(fence);
>>>>   + dma_fence_enable_sw_signaling(&s_fence->finished);
>>>
>>> Ok, this makes it a lot clearer. Previously I though that we have 
>>> some bug in dma_fence_add_callback().
>>>
>>> This is essentially the wrong place to call this, the finished fence 
>>> should be enabled by the caller and not here.
>>>
>>> There is also another problem in dma_fence_enable_sw_signaling(), it 
>>> returns early when the fence is already signaled:
>>>
>>>         if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>>>                 return;
>>>
>>> Please remove that one first.
>>
>>
>> Why we even need this explicit call if dma_fence_add_callback calls 
>> __dma_fence_enable_signaling anyway ?
>
> Two different fence objects.
>
> The dma_fence_add_callback() is done on the hw fence we get in return 
> of submitting the job.
>
> The dma_fence_enable_sw_signaling() here is done on the finished fence 
> we use to signal the completion externally.
>
> Key point is the finished fence should be used by the frontend drivers 
> which uses the scheduler and not by the scheduler itself.
>
> Christian.


Oh, so we need to explicitly call this because dma_fence_add_callback is 
not always used for finished fence right ?

Yea, then it makes sense that the client needs to manage this since each 
one has his own logic what to do with it.

Andrey


>
>>
>> Andrey
>>
>>
>>>
>>> Thanks,
>>> Christian.
>>>
>>>
>>>> +
>>>>               r = dma_fence_add_callback(fence, &sched_job->cb,
>>>>                              drm_sched_job_done_cb);
>>>>               if (r == -ENOENT)
>>>
>