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

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Message-ID: <9b57b254-8d67-9937-5cec-783e7a4d8f56@amd.com>
Date:   Wed, 7 Sep 2022 08:37:46 +0200
From:   Christian König <christian.koenig@....com>
To:     Andrey Grodzovsky <andrey.grodzovsky@....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

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.

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