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