Re: New "ABS_WHEEL2" axis?

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, Nov 30, 2011 at 02:42:09PM -0800, Ping Cheng wrote:
> > On Thu, Oct 06, 2011 at 01:31:09PM -0700, Jason Gerecke wrote:
> >> On Wed, Oct 5, 2011 at 8:34 PM, Dmitry Torokhov
> >> <dmitry.torokhov@gmail.com> wrote:
> >> > On Wednesday, October 05, 2011 12:44:40 PM Jason Gerecke wrote:
> >> >> I'm working on adding support for the recently-announced Cintiq 24HD,
> >> >> and am hashing out most of the details on the linuxwacom mailing list.
> >> >> In the discussions there, it was suggested that we add a new e.g.
> >> >> "ABS_WHEEL2" axis to the input.h header to represent the second touch
> >> >> ring available on the 24HD. I think it's a good idea, but wanted to
> >> >> get some opinions from over here before making a patch. The other
> >> >> option we have available is to use an axis not already in use by the
> >> >> wacom driver (such as ABS_RUDDER), but none have a matching semantic
> >> >> meaning.
> >
> > Looking at this once more ABS_WHEEL does not semantically match to what
> > Wacom driver users it for either. ABS_WHEEL is not "an abstract
> > circular-shaped sensor on a device" for rather a "steering wheel"-like
> > control on a game controller. So Wacom needs to move away from using this
> > event.
> 
> We can move away from using the WHEEL. But, we do still need to use
> something to report the values. We need a constructive suggestion to
> "move away" ;).
> 
> >> The rings are controls built into the "pad" -- the hardware you bring
> >> the stylus in proximity to. Along with the touch rings, there are also
> >> buttons and touch strips. Now, some devices have controls on only the
> >> left-hand side of the pad, while others have the controls on both
> >> sides of the pad. While one could argue that this makes each "side" a
> >> complete context that can be broken apart into separate devices, that
> >> doesn't really solve anything. What's stopping us from grouping
> >> several rings together?
> >
> > Well, there are several topics here...
> >
> > What you apparently have is a group of unclassified, as far as Linux
> > input subsystem concerned, sensors. When I say unclassified I mean that
> > there is no appropriate event code we can assign to the control to allow
> > a generic consumer determine the purpose of that sensor. You need help
> > of a specialized driver to decide what to do with the data or, in case
> > of generic driver, user has to explicitly map it to some action, but
> > there is no way for automatic discovery/configuration which is the goal
> > of input core. The only and closest event that we have that is suitable
> > here is ABS_MISC.
> >
> > Here however is a problem: there is only one ABS_MISC and we often have
> > several such sensors on a device. I do not want to have ABS_MISCx as
> > again, it does not solve anything, manufactures always come with bigger
> > and bigger devices (there some music controllers with dozens of
> > sliders). Encoding data into ABS_MISC (let's say highest byte denotes
> > sensor number) is awkward as well. So that is why I propose splitting
> > them into separate input devices and have driver discover and handle
> > them as it sees fit.
> 
> The rings are on the tablet. Events from them are combined with the
> events from the other tools moving on the tablet. Splitting the
> wheels/expresskeys from the tablet would only complicate the
> situation. We would have to link the wheels back to the tablet in the
> user land, an unnecessary step that we should/could avoid in the
> kernel.

The fact that they are in the same physical package does not mean that
they are necessarily mapped to a single input device. For example my USB
keyboard consists of 3 logical devices.

I understand that having data from them in the same event stream would
be nice and if you have an idea how to achieve that in a generic way
without resorting to ABS_MISC55 - that would be great. Maybe we
need something similar to multitouch protocol solution, but for
unclassified data.

OTOH separate input devices complicate userland in cases when driver
wants to handle them together but is really flexible.

-- 
Dmitry