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

[Dmitry Sepp <dmitry.sepp@opensynergy.com>]

Hi Keiichi,

On Montag, 23. März 2020 16:48:14 CET Keiichi Watanabe wrote:
> Hi,
> 
> On Mon, Mar 23, 2020 at 10:28 PM Dmitry Sepp
> 
> <dmitry.sepp@opensynergy.com> wrote:
> > Hi Keiichi,
> > 
> > On Montag, 23. März 2020 13:07:54 CET Keiichi Watanabe wrote:
> > > Hi everyone,
> > > 
> > > I have implemented a virtio-video device following my v3 spec in
> > > crosvm, which worked well together with Dmitry's driver [1]. I've
> > > started preparing v4 proposal to address problems found while
> > > implementing the driver and the devices.
> > 
> > Great news!
> > 
> > > Regarding v3 protocol, I'm thinking about how we can differentiate
> > > 'parameters' and 'controls' in the virtio-video spec?
> > > In the previous discussion, we decided to have a profile, level and
> > > bitrate as controls because we want to query supported values for each
> > > field.
> > > But, I don't think it's a good criteria because it'd be possible to
> > > query other values in params.
> > 
> > Could you elaborate on this? Do you now how the design could look like or
> > it is just an idea? AFAIR during the discussion of OpenSynergy's original
> > v1 spec your point was to separate something that we call 'controls' now
> > to reduce the command data size and make command handling less error
> > prone.
> 
> The problem in v3 is that if we want to add a new value to be set it'd
> be unclear which params or controls is better to be extended.
> One possible rule may be "if a value can be queried by the driver, it
> should be a control". However, this rule doesn't explain why we have
> "format" in params for example. So, I think we need a discussion and
> may want to rearrange the structures.
> 
> Yeah, in the previous discussion, I suggested to have profile, level
> and bitrate as control values instead of members of params. Now, I'm
> wondering whether we can have every values as control values.
> I don't think it's a perfect idea, but I haven't come up with any
> better concrete design yet. So, I'd really appreciate if you could
> share your thoughts.
> 

Ok, we can for example add more precise definition to input and output. Let's 
say we have 'bitstream' format structure and a 'image' format structure. E.g. 
for decoder obviously bitstream is input and image is output.

Then instead of params and controls we can define some abstract 'properties'. 
And make some of the properties assigned/mapped/available to bitstream and 
some to image, depending on the current function. I think that could make 
sense as for example for decoder 'bitstream' probably requires very few basic 
'properties' like fourcc format, in contrast to 'image'. But for encoder 
'bitstream' will also have the bitrate 'property' set.

> > On one hand if don't really see any difference in params vs controls it
> > would for sure make sense to remove one of the two. On the other hand I'd
> > of course like to avoid moving back in forth, especially when it comes to
> > such a major driver rework.
> 
> Yes, I understand that it may require a big change in the implementation.
> I'm sorry for not being able to think of this point seriously in the
> previous thread.
> 
> Of course, I'd also really like to avoid rework, but I believe we
> shouldn't give up defining a clean and reasonable specification.
> Let's find a clear definition in this cycle to avoid future rework.
> 
> > > So, I'm thinking about what should be the difference between params
> > > and controls. If no difference, we should deprecate
> > > virtio_video_params and have every field there as a control value
> > > instead.
> > 
> > I deem we should then deprecate controls instead. Params seem to be more
> > abstract. Width and height don't sound like a control for me.
> 
> Though this is actually one of options, how can we query profiles and
> levels if they are in params?
> This is why we decided them as controls.
> 
> Best regards,
> Keiichi
> 
> > > If we add a new command to get and set multiple controls at once, this
> > > change won't cause overhead.
> > 
> > How would we do this? Provide a flexible array member where each entry has
> > a type field first?
> 
> Yeah, something like the idea. But, I haven't designed an actual structure
> yet.
> > What can also make sense is to potentially join set and get calls
> > (probably
> > provide 'get' stuff automatically within a response to 'set'). Anyway set
> > and get are currently used in conjunction all the time.
> 
> It'd make sense to return new input and output params when one of
> params is updated.
> But, if we choose this design, we need to assume one device has just
> two params; input and output.
> 
> This is okay for video decoder and encoder, but it may become a
> problem if we want to support other types of video device that has
> only one direction. (e.g. video capture device)
> Though we have no plan for supporting this, OpenSynergy's v1 proposal
> contained this type IIRC.

Honestly speaking, the idea is not completely abandoned. The spec and the 
driver has more than enough functionality to handle a simple Android EVS 
camera use-case. But I think let's discuss this separately later.

Best regards,
Dmitry.

> 
> Best regards,
> Keiichi
> 
> > Best regards,
> > Dmitry.
> > 
> > > What do you think? Is there anything I missed?
> > > If it sounds fine, I'll remove virtio_video_params from the v4 spec
> > > proposal.
> > > 
> > > Best regards,
> > > Keiichi
> > > 
> > > [1]: https://patchwork.linuxtv.org/patch/61717/