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 à 16:18 +0200, Hans Verkuil a écrit : > > > 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
