Re: [PATCH] media: s5p-mfc: set V4L2_BUF_FLAG_LAST flag on final buffer

[Hans Verkuil]

On 02/06/2020 17:09, Nicolas Dufresne wrote:
> Hi Andriy,
> 
> thanks for you patch.
> 
> Le samedi 02 mai 2020 à 15:40 -0400, Andriy Gelman a écrit :
>> From: Andriy Gelman <andriy.gelman@xxxxxxxxx>
>>
>> As per V4L2 api, the final buffer should set V4L2_BUF_FLAG_LAST flag.
>>
>> Signed-off-by: Andriy Gelman <andriy.gelman@xxxxxxxxx>
>> ---
>>  drivers/media/platform/s5p-mfc/s5p_mfc.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/media/platform/s5p-mfc/s5p_mfc.c b/drivers/media/platform/s5p-mfc/s5p_mfc.c
>> index 5c2a23b953a4..b3d9b3a523fe 100644
>> --- a/drivers/media/platform/s5p-mfc/s5p_mfc.c
>> +++ b/drivers/media/platform/s5p-mfc/s5p_mfc.c
>> @@ -614,6 +614,7 @@ static void s5p_mfc_handle_stream_complete(struct s5p_mfc_ctx *ctx)
>>  		list_del(&mb_entry->list);
>>  		ctx->dst_queue_cnt--;
>>  		vb2_set_plane_payload(&mb_entry->b->vb2_buf, 0, 0);
>> +		mb_entry->b->flags |= V4L2_BUF_FLAG_LAST;
>>  		vb2_buffer_done(&mb_entry->b->vb2_buf, VB2_BUF_STATE_DONE);
> 
> The empty buffer is only there for backward compatibility. As the spec
> says, userspace should completely ignore this buffer. I bet it will
> still have the effect to set last_buffer_dequeued = true in vb2,

Actually, no. It only tests for V4L2_BUF_FLAG_LAST, not for empty buffers.

Regards,

	Hans

> unblocking poll() operations and allowing for the queue to unblock and
> return EPIPE on further DQBUF.
> 
> Perhaps you should make sure the if both the src and dst queues are
> empty/done by the time cmd_stop is called it will still work. Other
> drivers seems to handle this, but this one does not seems to have a
> special case for that, which may hang userspace in a different way.
> 
> What you should do to verify this patch is correct, and that your
> userpace does not rely on legacy path is that it should always be able
> to detect the end of the drain with a EPIPE on DQBUF. LAST_BUF is just
> an early signalling, but may not occur if there was nothing left to
> produce (except for MFC which maybe be crafting a buffer in all cases,
> but that's going a roundtrip through the HW, which is not clear will
> work if the dst queue was empty).
> 
> As shared on IRC, you have sent these patch to FFMPEG:
> 
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20200429212942.28797-2-andriy.gelman@xxxxxxxxx/
> 
> This should have been clarified as supporting legacy drivers / older
> kernel with Samsung driver. Seems like a fair patch. And you added:
> 
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20200429212942.28797-1-andriy.gelman@xxxxxxxxx/
> 
> This one should maybe add the clarification that this is an
> optimization to skip an extra poll/dqbuf dance, but that end of
> draining will ultimately be catched by EPIPE on dqbuf for the described
> cases. Remains valid enhancement to ffmpeg imho.
> 
>>  	}
>>  
>