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. > > 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. > > Regards, > > Hans
