[PATCH libinput 0/24] Tablet support

[Carlos Garnacho]

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.

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.

>  
> > 
> > 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