Re: [PATCH v4] media: docs-rst: Document m2m stateless video decoder interface

[Hans Verkuil]

On 4/27/19 2:06 PM, Nicolas Dufresne wrote:
> Le vendredi 26 avril 2019 Ã 16:18 +0200, Hans Verkuil a Ãcrit :
>> On 4/16/19 9:22 AM, Alexandre Courbot wrote:
>>
>> <snip>
>>
>>> Thanks for this great discussion. Let me try to summarize the status
>>> of this thread + the IRC discussion and add my own thoughts:
>>>
>>> Proper support for multiple decoding units (e.g. H.264 slices) per
>>> frame should not be an afterthought ; compliance to encoded formats
>>> depend on it, and the benefit of lower latency is a significant
>>> consideration for vendors.
>>>
>>> m2m, which we use for all stateless codecs, has a strong assumption
>>> that one OUTPUT buffer consumed results in one CAPTURE buffer being
>>> produced. This assumption can however be overruled: at least the venus
>>> driver does it to implement the stateful specification.
>>>
>>> So we need a way to specify frame boundaries when submitting encoded
>>> content to the driver. One request should contain a single OUTPUT
>>> buffer, containing a single decoding unit, but we need a way to
>>> specify whether the driver should directly produce a CAPTURE buffer
>>> from this request, or keep using the same CAPTURE buffer with
>>> subsequent requests.
>>>
>>> I can think of 2 ways this can be expressed:
>>> 1) We keep the current m2m behavior as the default (a CAPTURE buffer
>>> is produced), and add a flag to ask the driver to change that behavior
>>> and hold on the CAPTURE buffer and reuse it with the next request(s) ;
>>> 2) We specify that no CAPTURE buffer is produced by default, unless a
>>> flag asking so is specified.
>>>
>>> The flag could be specified in one of two ways:
>>> a) As a new v4l2_buffer.flag for the OUTPUT buffer ;
>>> b) As a dedicated control, either format-specific or more common to all codecs.
>>>
>>> I tend to favor 2) and b) for this, for the reason that with H.264 at
>>> least, user-space does not know whether a slice is the last slice of a
>>> frame until it starts parsing the next one, and we don't know when we
>>> will receive it. If we use a control to ask that a CAPTURE buffer be
>>> produced, we can always submit another request with only that control
>>> set once it is clear that the frame is complete (and not delay
>>> decoding meanwhile). In practice I am not that familiar with
>>> latency-sensitive streaming ; maybe a smart streamer would just append
>>> an AUD NAL unit at the end of every frame and we can thus submit the
>>> flag it with the last slice without further delay?
>>>
>>> An extra constraint to enforce would be that each decoding unit
>>> belonging to the same frame must be submitted with the same timestamp,
>>> otherwise the request submission would fail. We really need a
>>> framework to enforce all this at a higher level than individual
>>> drivers, once we reach an agreement I will start working on this.
>>>
>>> Formats that do not support multiple decoding units per frame would
>>> reject any request that does not carry the end-of-frame information.
>>>
>>> Anything missing / any further comment?
>>>
>>
>> After reading through this thread and a further irc discussion I now
>> understand the problem. I think there are several ways this can be
>> solved, but I think this is the easiest:
>>
>> Introduce a new V4L2_BUF_FLAG_HOLD_CAPTURE_BUFFER flag.
>>
>> If set in the OUTPUT buffer, then don't mark the CAPTURE buffer as
>> done after processing the OUTPUT buffer.
>>
>> If an OUTPUT buffer was queued with a different timestamp than was
>> used for the currently held CAPTURE buffer, then mark that CAPTURE
>> buffer as done before starting processing this OUTPUT buffer.
> 
> Just a curiosity, can you extend on how this would be handled. If there
> is a number of capture buffer, these should have "no-timestamp". So I
> suspect we need the condition to differentiate no-timestamp from
> previous timestamp. What I'm unclear is to what does it mean "no-
> timestamp". We already stated the timestamp 0 cannot be reserved as
> being an unset timestamp.

For OUTPUT buffers there is no such thing as 'no timestamp'. They always
have a timestamp (which may be 0). The currently active CAPTURE buffer
also always has a timestamp as that was copied from the first OUTPUT buffer
for that CAPTURE buffer.

> 
>>
>> In other words, for slicing you can just always set this flag and
>> group the slices by the OUTPUT timestamp. If you know that you
>> reached the last slice of a frame, then you can optionally clear the
>> flag to ensure the CAPTURE buffer is marked done without having to wait
>> for the first slice of the next frame to arrive.
>>
>> Potential disadvantage of this approach is that this relies on the
>> OUTPUT timestamp to be the same for all slices of the same frame.
>>
>> Which sounds reasonable to me.
>>
>> In addition add a V4L2_BUF_CAP_SUPPORTS_HOLD_CAPTURE_BUFFER
>> capability to signal support for this flag.
>>
>> I think this can be fairly easily implemented in v4l2-mem2mem.c.
>>
>> In addition, this approach is not specific to codecs, it can be
>> used elsewhere as well (composing multiple output buffers into one
>> capture buffer is one use-case that comes to mind).
>>
>> Comments? Other ideas?
> 
> Sounds reasonable to me. I'll read through Paul's comment now and
> comment if needed.

Paul's OK with it as well. The only thing I am not 100% happy with is
the name of the flag. It's a very low-level name: i.e. it does what it
says, but it doesn't say for what purpose.

Does anyone have any better suggestions?

Also, who will implement this in v4l2-mem2mem? Paul, where you planning to do that?

Regards,

	Hans