[RFC wayland-protocols] inputfd - direct input access protocol

Pekka Paalanen ppaalanen at gmail.com
Mon Apr 3 08:39:13 UTC 2017

On Sat, 1 Apr 2017 10:48:40 +1000
Peter Hutterer <peter.hutterer at who-t.net> wrote:

> On Fri, Mar 31, 2017 at 12:23:49PM +0300, Pekka Paalanen wrote:
> > > +      <description summary="device capability notification">
> > > +	This event is sent to notify the client of a custom property that
> > > +	applies to this device. The property is a standard key/value store
> > > +	in UTF-8 format, interpretation of both strings is left to the
> > > +	client. The wayland protocol makes no guarantees about the content
> > > +	of each string beyond its text encoding.
> > > +
> > > +	Compositors and clients need to agree on a dictionary of properties.
> > > +	For example, a compositor may designate the device to be of
> > > +	'joystick-type' 'gamepad'. This dictionary is out of the scope of
> > > +	this protocol.
> > > +      </description>
> > > +      <arg name="property" type="string" summary="A UTF-8 encoded property name"/>
> > > +      <arg name="value" type="string" summary="A UTF-8 encoded property value"/>
> > > +    </event>  
> > 
> > While I understand the desire to leave the dictionary for others to
> > specify, this event is essentially useless without it. The dictionary
> > really is part of the protocol, even if you didn't want it. I'm not
> > sure it's helpful to leave the authority on the dictionary unspecified.
> > As you wrote, they need to be agreed somewhere.
> > 
> > This raises a couple of questions for the protocol itself:
> > 
> > - What should the client do with a property or value it does not
> >   recognize?
> > 
> > - If the client does not recognize a property or a value, can it still
> >   use the device? Could this vary per property?
> > 
> > - Should the compositor be sending only properties and values the
> >   client can understand?
> > 
> > If this protocol does not define a single authority on the dictionary,
> > should it then carry a dictionary type, id or namespace?
> > 
> > Should properties be namespaced?  
> ok, easy answers first :)
> clients must ignore property names or values that they do not understand,
> the compositor may send any value (it won't know what the client supports),
> the protocol does not enforce namespacing, but the dictionary authority may
> do so.
> one more thing I thought of: properties key/values are *not* singular, a
> client may receive the same property with multiple different values (e.g.
> that's allowed for any property depends on the dictionary.

Hi Peter,

cool, I expect these to be written down in v2. :-)

> Now to the difficult answers:
> I think it'll be impossible to *not* have this in an external dictionary.
> The needs of the compositor and the clients are both the same and completely
> different for this protocol to work correctly. Let's take the example of
> tagging a device as gamepad. The default udev rules are unreliable,
> ID_INPUT_JOYSTICK has too many false positives. So we need some database
> that tags the device as JOYSTICK_TYPE=gamepad, JOYSTICK_TYPE=wheel, etc.
> That can be done in udev, but needs some external authority anyway -
> libwacom style. Without that, you'll have a mismatch in what compositors
> detect as gaming devices vs what clients can handle (assuming the mythical
> libgamingdevice exists).
> The compositor doesn't care about the actual value though, anything with
> JOYSTICK_TYPE is a gaming device and will be accessible through inputfd.
> Adding new joystick types in the future does not change the protocol or the
> compositor implementation. A client should not need to wait for protocol bumps just
> to get access to a new type that the compositor doesn't care about anyway.
> Just as a general reminder, the whole point of this protocol is for the
> compositor to have some say in what's available but otherwise get out of the
> way.
> If we don't have the generic properties, we need the client to have some
> other way to access information about the device. In the Wacom case we have
> 'path' which gives the client a syspath and thus the udev device and access
> to udev properties. Bastien wasn't too happy about this for GPIO devices
> where it may not be possible, so the property approach was the first thing
> that came to mind.
> Having said this - while I don't want this in the protocol, there's no
> reason we can't define that dictionary authority ourselves. I just want it
> external to the protocol. ICCCM/EMWH-style, effectively. But I would feel
> uncomfortable *being* the authority, given that this is too far outside my
> core knowledge. Happy to be the maintainer for this though, an initial
> implementation would be largely boilerplate.
> I still think that there's a market for a generic libgamingdevice library
> that works libinput-style for gaming devices and provides the database that
> matches this dictionary. 

Thanks for the explanation, it makes sense, particularly if the
properties might be useful with other window systems, protocols, or
even to-bare-hardware applications (libSDL on bare DRM/evdev?). Like
Jonas said, the minimum we need in the protocol spec is a pointer to
the authoritative property specification.

> > This reminded me, we should probably check whether libwayland-client
> > can handle 'focus_in' racing with 'destroy' without leaking the fd. I
> > have no recollection if it does. That is, when the client sends
> > 'destroy' at the same time the server sends 'focus_in', the client no
> > longer has a live proxy (has a zombie instead), does libwayland-client
> > while dropping the 'focus_in' on the floor also close the received fd.  
> implementation-detail? :)
> but yeah, needs fixing.

Indeed a side comment. :-)

> > > +
> > > +    <event name="focus_out">
> > > +      <description summary="input fd device focus out event">
> > > +	Notification that this client no longer has focus and/or access to
> > > +	this device. Further reads from this device's file descriptor
> > > +	will fail. The client must close(2) the file descriptor received in  
> > 
> > "will fail" seems a bit strong.  
> well, in the case of EVIOCMUTE that's exactly what will happen, ENODEV
> galore for anything after the ioctl. Having this in the protocol makes it
> unambiguous what's going to happen and clear that any client that cannot
> handle this case is buggy. "will fail soon" might be more precise, but I'm
> not sure it's as clear.

We still allow to add other fd types in the future. We don't know if
those will actually fail or just go silent.

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