tablet virtual mouse (aka evdev's EV_ABS) is broken

[stsp]

0001-evdev-scale-tablet-coords-for-mouse.patch.txt

Hello.

This patch fixes the mouse under
qemu when tablet emulation is used
for passing abs coords.
Abs coords need to be scaled
from 32767x32767 to the window
resolution, see this:
telmich/gpm@99e29cb
as a reference.
I am not saying my patch is the
best possible solution. In particular
the one may try to obtain these
32767 values from evdev, rather
than to hard-code.
So please either apply it or improve
the way you think suits.

[sezero]

@icculus or @slouken to review.

[slouken]

Let's take a look at this for 2.0.20.

[cgutman]

We should use EVIOCGABS to get the min and max axis values like we do for touchscreens

[stsp]

In SDL_udev.c I tried to
identify that device with
"ID_INPUT_TABLET"
ID just to find out whether
we need to use EVIOCGABS,
but it doesn't accept that
ID. I suppose some udev
fix is needed to identify
that device correctly before
we can handle it as a tablet...

[cgutman]

Hmm, I believe EVIOCGABS support is mandatory for ABS axes. Without that range information, it’s impossible to properly interpret the axis values.

Libevdev definitely seems to expect no failures for that ioctl on any ABS axis that is present on the device:
https://github.com/freedesktop/libevdev/blob/066d9caf551b4d4662a776e3e3702918b0b8f3a7/libevdev/libevdev.c#L507-L518

[stsp]

Yes, but do you suggest
doing EVIOCGABS when
we already got EV_ABS?
I supposed we need to
first identify and query the
device similar to SDL currently
deals with touchpad, and
then have the resolution
handly for us in some struct.

[slouken]

@cgutman, is this something you'd like to get in for 2.0.22?

[cgutman]

I had a PoC branch, but I wasn't able to get VirtualBox to cooperate to be able to test it.

We can move this to 2.0.24.

[stsp]

Please let me know if any
testing is needed on qemu.