[PATCH libinput 0/24] Tablet support

Thu May 22 23:44:01 PDT 2014

On Fri, May 23, 2014 at 02:30:45AM -0400, Jasper St. Pierre wrote:
> On Fri, May 23, 2014 at 2:19 AM, Peter Hutterer <peter.hutterer at who-t.net>wrote:
> 
> > On Fri, May 23, 2014 at 01:40:49AM +0200, Carlos Garnacho wrote:
> > > Hi,
> > >
> > > First of all, I'm sorry I dropped the ball this long. It's great to see
> > > you've been doing some progress.
> > >
> > > On jue, 2014-05-22 at 01:17 -0400, Chandler Paul wrote:
> > > > Hi! Sorry this took so long to write, I've been spending a lot of my
> > > > time recently trying to understand the libinput code and all of that
> > > > good stuff, and I wanted to make sure I had a decent understanding of
> > it
> > > > before I actually wrote up a response.
> > > >
> > > > On Mon, 2014-04-21 at 19:11 +0200, Carlos Garnacho wrote:
> > > > > Hey there!,
> > > > >
> > > > > Here's a few patches to have libinput handle events from tablets,
> > > > > these devices are basically pointer devices, with a varying range
> > > > > of extra buttons (either stylus or "pad" buttons), and extra ABS_*
> > > > > axes. These devices also often offer information about the stylus
> > > > > in use, and its BTN_TOOL_* codes.
> > > > >
> > > > > So I've gone for reusing and extending libinput_event_pointer, adding
> > > > > extra libinput_pointer_axis values, and adding an "axis_frame" event
> > > > > to mark the end of a set of simultaneous axis changes, and a
> > "tool_update"
> > > > > event to mark tool changes (and delimit proximity). These features
> > are
> > > > > only triggered if a new LIBINPUT_DEVICE_CAP_STYLUS capability is set.
> > > > I'm with Peter on splitting these up. It seems kind of inconsistent
> > with
> > > > the rest of libinput (with what I've gathered, anyway). A
> > > > tablet-specific event sounds interesting too, but I feel like
> > > > compressing all of the axis changes into. For now I'm going to work on
> > > > having all the axis changes reported as separate POINTER_AXIS events.
> > >
> > > I'm not against making this a separate set of event types. I would
> > > advise though against attempting to compress axis changes into a single
> > > event, it puts certain processing burden on clients, and most of those
> > > not always want as much data.
> >
> > can you elaborate on what the processing burdens are?
> >
> > there are two options for handling compressed events in the library: one is
> > the approach XI2 uses, or always guaranteeing full data.
> > in the XI2 approach, we only forward data that is set in the event which
> > puts some burden on the client to calculate and keep track of the data. I
> > don't think that's the best aproach for libinput. Rather, we should
> > guarantee that all axes are always set to the current value, so most
> > clients
> > could then just call get_pressure() to get the pressure, regardles of
> > whether that changed with that event. if needed, we can export some extra
> > mask to tell if a particular value was updated, but just having the full
> > event should take some burden away.
> >
> 
> The target use case for libinput is Wayland compositors, not clients
> directly. Wayland's tablet protocol sends events when values change -- edge
> triggered, not level triggered.

ok, that's a definitive requirement for the bitmask then, still doesn't
explain why not to pack the whole event into a single struct.
we don't have agreed-on wayland tablet protocol yet so what goes down the
protocol isn't clear yet, though the axis frame event may be simpler on the
protocol-level.

but on the compositor side - you at least always want x/y for pointer
placement, other axes aren't so clear-cut e.g. do you want pointer movement
based on proximity or based on touch? is this decided by libinput or by the
compositor? on top of that - we can't process x/y in libinput until we get
the EV_SYN, so whether we shove everything into one event or across separate
event makes little difference in libinput. It would reduce the need of the
compositor to re-assemble an event that's already been assembled and then
split up again.

Cheers,
   Peter

PS: we really need to stop using "client" and should start using "caller"
when talking about libinput, because this is the source of much
misunderstanding.

> > > Independently of this being a separate set of events, IMO the axis frame
> > > event is still useful for pointer events, this might help compressing
> > > the processing of dx/dy for scrolling in clients for example, instead of
> > > processing 2 events that queue the redraw of different areas, some
> > > unneeded in the end.
> >
> > I agree, this is something that has value.
> >
> > Cheers,
> >    Peter
> >
> >
> >
> > > > > Caveats:
> > > > >
> > > > > * Many of these devices have also tactile strips or wheels, but
> > these are
> > > > >   unhandled so far... On the devices I've got available for testing,
> > current
> > > > >   kernel support seems varyingly inconsistent:
> > > > >
> > > > >   - One device has 2 strips, which report on ABS_RX/RY (radial??).
> > Min/max
> > > > >     are 0..4096, but the reported values are 1,2,4,8,16... So
> > effectively
> > > > >     a log2 scale, or more graphically a bit shifting over a bunch of
> > 0s,
> > > > >     which is somewhat more resembling to the physical action on the
> > strip.
> > > > Since I'm on a deadline for this and making changes to this in the
> > > > kernel would take too long, I don't think I'm going to advocate for
> > > > changing this behavior right now. Although I do agree that eventually
> > it
> > > > should be changed. Graphically, a bit moving across a field like that
> > > > makes sense, but I think that would be a difficult value to make
> > > > practical use of in an application without changing it to a simple 1-13
> > > > value. If I get far enough that I can start implementing support for
> > > > tactile strips and all those other fancy features some of these tablets
> > > > have I might have it convert the values for tactile strips like that to
> > > > something more usable by default and leave the other axes as-is. I'm
> > > > curious to hear Ping and Jason's opinion on this though, and what kind
> > > > of advantages
> > > >
> > > > >   - Another device has one wheel, reported through ABS_WHEEL. Even
> > though
> > > > >     min/max are reported as [0..1023], on interaction it goes
> > [0..71] (funky
> > > > >     range too)
> > > > >
> > > > >   We could just forward this as-is, but seems hindering enough to do
> > anything
> > > > >   useful with those unless that behavior is corrected.
> > > > >
> > > > >   When supported, IMO it'd make sense to have those axes behave
> > similar to
> > > > >   scroll axes, so the axis value increments or decrements depending
> > on the
> > > > >   direction. I'm not sure if there would be cases where the absolute
> > value
> > > > >   matters here?
> > > > >
> > > > > * One thing worth noting is that axes are currently normalized, to
> > [-1..1]
> > > > >   for stylus tilt, and [0..1] for everything else. I remember Peter's
> > > > >   tablet wayland protocol proposal basically forwarded
> > input_absinfo, this
> > > > >   might not be fully compatible with that, although TBH I'm unsure
> > > > >   clamping/normalization should take place so high in the stack...
> > > > I'm with Peter on this actually. If the axes were used for something
> > > > else I might approve of normalization in libinput but I think having
> > > > absolute values fits more of the use cases for the extra axes found on
> > > > many tablets, especially since Ping said that some of Wacom's in-house
> > > > applications actually need these. I do think however, that maybe we
> > > > should consider clamping axis values with libinput even if we don't
> > > > normalize the axes by default.
> > > >
> > > > I've forked libinput and I have a branch where I'm fixing up the
> > patches
> > > > Carlos sent in based on the feedback from Peter. You can find it here:
> > > >
> > > >     https://github.com/Lyude1337/libinput/tree/carlos_cleanup
> > >
> > > Thanks for the cleanup and appliying the recommended fixes, the changes
> > > look good so far.
> > >
> > > >
> > > > The history is messy on this, but once this is ready to get sent in as
> > > > actual patches I'll be rebasing the history.
> > > >
> > > > Right now I've removed normalization from my branch, but if someone
> > > > brings up a good reason to actually have libinput handle that then I
> > can
> > > > revert the change.
> > >
> > > I agree, this should just probably be some helper code.
> > >
> > > Cheers,
> > >   Carlos
> > >
> > _______________________________________________
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> > wayland-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/wayland-devel
> >
> 
> 
> 
> -- 
>   Jasper