Re: [EXT] Re: [PATCH] media: docs: dev-decoder: add restrictions about CAPTURE buffers

[Stanimir Varbanov]

On 10/29/21 5:10 AM, Ming Qian wrote:
>> -----Original Message-----
>> From: Nicolas Dufresne [mailto:nicolas@xxxxxxxxxxxx]
>> Sent: Tuesday, October 26, 2021 10:12 PM
>> To: Alexandre Courbot <acourbot@xxxxxxxxxxxx>; Mauro Carvalho Chehab
>> <mchehab@xxxxxxxxxx>; Hans Verkuil <hverkuil-cisco@xxxxxxxxx>; Tomasz Figa
>> <tfiga@xxxxxxxxxxxx>
>> Cc: linux-media@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> Subject: [EXT] Re: [PATCH] media: docs: dev-decoder: add restrictions about
>> CAPTURE buffers
>>
>> Caution: EXT Email
>>
>> Le lundi 18 octobre 2021 à 18:14 +0900, Alexandre Courbot a écrit :
>>> CAPTURE buffers might be read by the hardware after they are dequeued,
>>> which goes against the general idea that userspace has full control
>>> over dequeued buffers. Explain why and document the restrictions that
>>> this implies for userspace.
>>>
>>> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
>>> ---
>>>  .../userspace-api/media/v4l/dev-decoder.rst     | 17
>> +++++++++++++++++
>>>  1 file changed, 17 insertions(+)
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/dev-decoder.rst
>>> b/Documentation/userspace-api/media/v4l/dev-decoder.rst
>>> index 5b9b83feeceb..3cf2b496f2d0 100644
>>> --- a/Documentation/userspace-api/media/v4l/dev-decoder.rst
>>> +++ b/Documentation/userspace-api/media/v4l/dev-decoder.rst
>>> @@ -752,6 +752,23 @@ available to dequeue. Specifically:
>>>       buffers are out-of-order compared to the ``OUTPUT`` buffers):
>> ``CAPTURE``
>>>       timestamps will not retain the order of ``OUTPUT`` timestamps.
>>>
>>> +.. note::
>>> +
>>> +   The backing memory of ``CAPTURE`` buffers that are used as reference
>> frames
>>> +   by the stream may be read by the hardware even after they are
>> dequeued.
>>> +   Consequently, the client should avoid writing into this memory while the
>>> +   ``CAPTURE`` queue is streaming. Failure to observe this may result in
>>> +   corruption of decoded frames.
>>> +
>>> +   Similarly, when using a memory type other than
>> ``V4L2_MEMORY_MMAP``, the
>>> +   client should make sure that each ``CAPTURE`` buffer is always queued
>> with
>>> +   the same backing memory for as long as the ``CAPTURE`` queue is
>> streaming.
>>> +   The reason for this is that V4L2 buffer indices can be used by drivers to
>>> +   identify frames. Thus, if the backing memory of a reference frame is
>>> +   submitted under a different buffer ID, the driver may misidentify it and
>>> +   decode a new frame into it while it is still in use, resulting in corruption
>>> +   of the following frames.
>>> +
>>
>> I think this is nice addition, but insufficient. We should extend the API with a
>> flags that let application know if the buffers are reference or secondary. For the
>> context, we have a mix of CODEC that will output usable reference frames and
>> needs careful manipulation and many other drivers where the buffers *maybe*
>> secondary, meaning they may have been post-processed and modifying these
>> in- place may have no impact.
>>
>> The problem is the "may", that will depends on the chosen CAPTURE format. I
>> believe we should flag this, this flag should be set by the driver, on CAPTURE
>> queue. The information is known after S_FMT, so Format Flag, Reqbufs
>> capabilities or querybuf flags are candidates. I think the buffer flags are the
>> best named flag, though we don't expect this to differ per buffer. Though,
>> userspace needs to call querybuf for all buf in order to export or map them.
>>
>> What userspace can do with this is to export the DMABuf as read-only, and
>> signal this internally in its own context. This is great to avoid any unwanted
>> side effect described here.
> 
> I think a flag should be add to tell a buffer is reference or secondary.
> But for some codec, it's hard to determine the buffer flag when reqbufs.
> The buffer flag should be dynamically updated by driver.
> User can check the flag after dqbuf every time.

+1

I'm not familiar with stateless decoders where on the reqbuf time it
could work, debut for stateful coders it should be a dynamic flag on
every capture buffer.

> 
>>
>>>  During the decoding, the decoder may initiate one of the special
>>> sequences, as  listed below. The sequences will result in the decoder
>>> returning all the  ``CAPTURE`` buffers that originated from all the
>>> ``OUTPUT`` buffers processed
>>
> 

-- 
regards,
Stan