Re: [virtio-dev] Re: [PATCH] Update virtio input device specification

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 06, 2017 at 03:21:26PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > The downside is that this is hard to specify formally, the same way we
> > do other devices. Say we do that and point to a fixed version of the
> > relevant Linux headers and the headers evolve and add or change
> > something. Then we would either go change the spec to be in sync with
> > the headers or build a compatibility layer in the implementation.
> 
> Highly unlikely that the structs and ioctls in linux header change.
> It's userspace <=> kernel abi.  And even if they change some days it
> would have to happen in a backward compatible way.  So I think
> documenting the ioctl structs we have today is reasonable.  If new
> ioctls for new device types show up we probably need a virtio feature
> flag anyway to properly support them.
> 
> Events (new key codes for example) are added now and then.  So for them
> we might continue referencing the include file with the codes instead of
> copying them into the spec.
> 
> > because evdev
> > needs to maintain some reasonable backwards compatibility by itself so
> > it's very unlikely that things would break.
> 
> Yep.  Unknown events can simply be ignored.  And you can query the
> device to figure which events (aka keys / axis / ...) are supported
> (ioctl in evdev, config space in virtio).  So that level of
> compatibility is already taken care of, in both evdev and virtio.
> 
> > > Let's assume linux gains ability to generate new events on top of old
> > > ones.
> > > Do you want to send both in all cases?
> > > If you only send multitouch how do you handle old guests?
> > > If you send both how does new guest know it should
> > > ignore old  events?
> > 
> > I think that these are valid concerns but they need to be addressed at
> > the evdev layer, not in virtio.
> 
> Indeed.  And I think multitouch is addressed by simply sending both.
> Apps without multitouch support simply ignore the multitouch events.
> Apps with multitouch support can figure this is a multitouch-capable
> device and ignore the old events on those.
> 
> cheers,
>   Gerd


Gerd is there a chance you could try and address whatever
you think needs to be addressed and repost?

Would be nice to have this in 1.1 and I think it's better
to have an incomplete description than none.


> 
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