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Subject: Re: [PATCH v3 1/2] virtio-video: Add virtio video device specification


On Donnerstag, 9. April 2020 12:46:27 CEST Keiichi Watanabe wrote:
> Currently, we have three options of the design of per-stream properties:
> 1. Have two structs for image format and bitstream format.
> Pros:
> Well structured. Easy to support uni-directional stream.
> Cons:
> Not all properties may not be classified well. For example, bitrate in
> encoder is about "how we encode it" rather than "what we encode it
> into". So, it may be a bit strange to have it in the bitstream
> information.
> 2. Have one struct that contains all properties.
> Pros:
> Well structured. Since updating one format affects the other format,
> it may make more sense to have everything in one struct.
> Also, we can have any per-stream properties there that may be tied to
> neither image format nor bitstream format.
> Cons:
> If we want to support uni-directional streams in the virtio-video
> protocol, we may be going to have many unused fields in that struct.
> 3. Have every property as a separate subcommand like v3's controls
> Pros:
> Easy to add more properties in the future.
> Cons:
> Less structured. So, we need to be careful not to overlook mandatory
> properties when we implement the driver and device.
> IMHO, I'm relatively supportive of 2, but we might need to rethink
> whether we really want to support uni-directional streams in
> virtio-video.

Ok, let's assume we keep it is one struct. Anyway, we indeed can just ignore 
some of the fields if we want so. We need to define some conventions for the 
struct. Like whether we should fill all the fields all the time when sending 
set_params() and so on.

I want to ask you about the frame-level bitrate control here [1]. Is it 
planned to support it? If yes, we also need a control to enable that and a way 
to pass minimum and maximum value for the quantization parameter.

> I guess it's worthwhile to create a separate protocol like
> virtio-camera even if it somehow overlaps with virtio-video.
> Only for a simple video capture scenario, extending the virtio-video
> protocol can be one of simple solutions. However, if we want to extend
> it for more features like MIPI cameras, it's not hard to imagine the
> protocol becoming very complicated.
> I wonder if we can keep virtio protocols simple and clean rather than
> making an all-in-one protocol for media devices like V4L2.
> > I could be useful if it was possible to store the structure in the config
> > space, but that won't fly as we have multiple streams with different
> > settings. Also one device can support multiple formats, so we won't be
> > able to handle the unions.
> Yeah, this structure should be per-stream properties but virtio's
> config is for per-device properties.
> So, it doesn't work as you said.
> > > > The idea being that depending on the value of "format", only the
> > > > relevant member of the union becomes meaningful. This ensures that the
> > > > device/driver does not need to check for every control whether it is
> > > > valid in the current context ; it just needs to check what "format" is
> > > > and take the values from the relevant members.
> > > 
> > > I like this idea to use union to make it more structured.
> > 
> > I don't really have any strong objections agains unions per se, but I deem
> > we need to keep the structure flexible. At the very beginning of the
> > development there was a discussion about stream priority. If I add a
> > 'prio' field to this structure it will break the binary compatibility
> > with older versions.
> Hmm, I don't think unions can incur any extra binary compatibility
> issues compared with designs using only structs.
> Since alignment rules are well-defined for unions as well as structs,
> it'd be okay.
> We might want to have an explicit field to show the size of a union like:
> union {
>   struct {
>     ...
>   } h264;
>   struct {
>     ...
>   } vp8;
>   ...
> }
> > > > I don't think we even need to have a different queue for both structs
> > > > (which is what V4L2 does, but again for its own reasons). Just a
> > > > single one per coding or decoding context could be enough AFAICT.
> > > > 
> > > > Depending on whether we are decoding or encoding, access to some
> > > > members would be restricted to read/only for the device or driver. For
> > > > instance, when encoding the driver can set "bitrate" to the desired
> > > > encoding rate. When decoding, the decoder can report the video's
> > > > bitrate there.
> > > > 
> > > > Now I'm not sure what would be the best way to share this structure.
> > > > Either a memory area shared between the driver and device, with
> > > > commands/messages sent to notify that something has changed, or
> > > > sending the whole structure with each command/message.
> > > 
> > > I don't think the first idea is feasible in the virtio mechanism. So,
> > > we can utilize the same way as the previous proposal; SET_PARAMS and
> > 
> > For similar thing the virtio config space exists, but as I mentioned
> > above, it won't fit the current virtio-video design (or we probably can
> > pre-define the max number of streams on the device side and have params
> > for each stream in the config space, but this looks clumsy).
> > 
> > > We also need to think of how to advertise supported profiles and
> > > levels. Probably, we might want to extend struct
> > > virtio_video_format_desc to include supported profiles/levels in a
> > > response of QUERY_CAPABLITY.
> > 
> > It would mean back to spec v1 AFAIR. We probably need to recall why we got
> > rid of that.
> Probably, this reply:
> https://markmail.org/thread/zr3ycvxixnwi5agt
> At that time, I assumed that we'll have profiles/levels/bitrates as
> controls, aparting from other per-stream properties. In that
> situation, it made sense to have a separate mechanism to get/set/query
> these properties.
> However, we are likely not to end up distinguishing these properties
> from other properties. If so, we don't need any other querying
> mechanism other than QUERY_CAPABILITY.
> To support profiles, we can extend virtio_video_format_desc to
> (a) add fields like "profiles" and "levels" that shows supported
> values as bit mask, or
> (b) add fields like "num_profiles" and "num_levels" that describes the
> lengths of arrays that follows.
> My personal preference is (a).

Yes, this should be ok. We had arrays in the spec v1, but as we have now 
bitmasks for formats, we can do so for profiles and levels.

We currently have two problems with capabilities when it comes to the real 

1. If we want to avoid calling stream_create in open(), we need to have 
bitrates already on per-format basis in capabilities.
2. We also need to know min input and output buffer count in advance, i.e. it 
should not be in params, especially for encoder that won't report it after 
metadata parsing like decoder, please see [2] (thanks Nicolas Dufresne for 
helping with that issue).

[1] https://github.com/chromium/chromium/blob/master/media/gpu/v4l2/
[2] https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/issues/672

Best regards,

> Best regards,
> Keiichi
> > > > Now the parameters I have listed above are not subject to changing a
> > > > lot, but there are also parameters that we may want to specify/be
> > > > notified on with each frame. For instance, whether we want a frame to
> > > > be forcibly encoded as a keyframe. V4L2 uses a control for this, but
> > > > we could probably do better if we can pass this information with each
> > > > frame to be encoded. Maybe we can implement that by using different
> > > > QUEUE commands for encoder and decoder, or again by using a union.
> > > 
> > > Ah, I haven't come up with such a kind of parameter. Perhaps, we can
> > > extend struct virtio_video_resource_queue to have this flag.
> > 
> > This looks sane for me.
> > 
> > Best regards,
> > Dmitry.

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