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Date:   Tue, 21 Aug 2018 14:29:26 +0300
From:   Stanimir Varbanov <>
To:     Tomasz Figa <>, Hans Verkuil <>
Cc:     Linux Media Mailing List <>,
        Linux Kernel Mailing List <>,
        Stanimir Varbanov <>,
        Mauro Carvalho Chehab <>,
        Pawel Osciak <>,
        Alexandre Courbot <>,,, Kyungmin Park <>,, Philipp Zabel <>,
        Tiffany Lin (林慧珊) 
        Andrew-CT Chen (陳智迪) 
        Paul Kocialkowski <>,
        Laurent Pinchart <>,,
        Ezequiel Garcia <>
Subject: Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video
 decoder interface

Hi Tomasz,

On 08/08/2018 05:55 AM, Tomasz Figa wrote:
> On Tue, Aug 7, 2018 at 4:37 PM Hans Verkuil <> wrote:

>>>>>>> +7.  If all the following conditions are met, the client may resume the
>>>>>>> +    decoding instantly, by using :c:func:`VIDIOC_DECODER_CMD` with
>>>>>>> +    ``V4L2_DEC_CMD_START`` command, as in case of resuming after the drain
>>>>>>> +    sequence:
>>>>>>> +
>>>>>>> +    * ``sizeimage`` of new format is less than or equal to the size of
>>>>>>> +      currently allocated buffers,
>>>>>>> +
>>>>>>> +    * the number of buffers currently allocated is greater than or equal to
>>>>>>> +      the minimum number of buffers acquired in step 6.
>>>>>> You might want to mention that if there are insufficient buffers, then
>>>>>> VIDIOC_CREATE_BUFS can be used to add more buffers.
>>>>> This might be a bit tricky, since at least s5p-mfc and coda can only
>>>>> work on a fixed buffer set and one would need to fully reinitialize
>>>>> the decoding to add one more buffer, which would effectively be the
>>>>> full resolution change sequence, as below, just with REQBUFS(0),
>>>>> REQBUFS(N) replaced with CREATE_BUFS.
>>>> What happens today in those drivers if you try to call CREATE_BUFS?
>>> s5p-mfc doesn't set the .vidioc_create_bufs pointer in its
>>> v4l2_ioctl_ops, so I suppose that would be -ENOTTY?
>> Correct for s5p-mfc.
> As Philipp clarified, coda supports adding buffers on the fly. I
> briefly looked at venus and mtk-vcodec and they seem to use m2m
> implementation of CREATE_BUFS. Not sure if anyone tested that, though.
> So the only hardware I know for sure cannot support this is s5p-mfc.

In Venus case CREATE_BUFS is tested with Gstreamer.


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