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

[Paul Kocialkowski]

Hi,

On Mon, 2019-04-29 at 10:41 +0200, Hans Verkuil wrote:
> 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?

Good naming is always so hard to find... I don't have anything better
to suggest off the top of my head, but will definitely keep thinking
about it.

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

Well, I no longer have time chunks allocated to the VPU topic at work,
so that means I'll have to do it on spare time and it may take me a
while to get there.

So if either one of you would like to pick it up to get it over with
faster, feel free to do that!

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com