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

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > With a feature flag both driver and device can choose whenever they want
> > support v1 or v2 or both.  With a version config field this is more
> > limited, the device can't decide to support both.  So the bonus points
> > for a smooth transition go to the feature flags not the version field ;)
> 
> I agree that feature flags would be preferable in general, but I'm
> concerned by the fact that there is (IIUC) a limited number of them.

We have 64 total, some reserved, 24 are available to devices right now,
see https://www.kraxel.org/virtio/virtio-v1.1-cs01-video-v3.html#x1-130002

> Video tends to change significantly over time, and to have optional
> features that would also be presented as feature flags. After a while
> we may run out of them, while a new protocol version would allow us to
> extend the config struct with some new flags. Or am I missing
> something?

Not everything needs a feature flag.  For example we have
VIRTIO_VIDEO_CMD_QUERY_CAPABILITY, and we can add new video formats
without needing a feature flag.  Maybe it is a good idea to explicitly
say in the specs that this can happen and that the driver should simply
ignore any unknown format returned by the device.

> I also wonder how "support v1 or v2 or both" would work with feature
> flags. In order to make it possible to opt out of v1, I guess we would
> need "v1 supported" flag to begin with?

The driver can ignore any device without v2 feature flag set.
The device can refuse to accept a driver without v2 support (don't allow
setting the FEATURES_OK bit).

A explicit v1 feature flag would allow the guest driver print a more
specific error message ("device doesn't support v1" instead of "feature
negotiation failed"), but that's it.

cheers,
  Gerd