Re: evdev interface question: ABS_X without ABS_Y?

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

Hi Dwayne,

On Wed, Oct 26, 2011 at 05:10:23PM -0400, Dwayne Litzenberger wrote:
> I have a quick question about the /dev/input/eventX interface with
> userspace.
> 
> Let's say I have a touchscreen, and I move my finger to (200,100).
> I'll see something line this (simplified):
> 
>   EV_ABS ABS_X 200
>   EV_ABS ABS_Y 100
>   EV_SYN
> 
> Next, let's say I see something like this:
> 
>   EV_ABS ABS_Y 90
>   EV_SYN
> 
> What is the meaning of that, from the perspective of userspace?
> I see two possibilities.  Either:
> 
> 1. It's a move to (200,90), or
> 2. It's a move to (0,90)
> 
> Which is the correct interpretation? 

Door #1.

> Is this documented anywhere?

>From Documentation/input/event-codes.txt:

"The input protocol is a stateful protocol. Events are emitted only when
values of event codes have changed. However, the state is maintained
within the Linux input subsystem; drivers do not need to maintain the
state and may attempt to emit unchanged values without harm. Userspace
may obtain the current state of event code values using the EVIOCG*
ioctls defined in linux/input.h."

> 
> Apparently, the usbhid driver is generating events that imply #1, but
> xserver-xorg-input-evdev interprets these according to #2.  I'm trying
> to figure out what needs to be patched.  :)

xserver-xorg-input-evdev.

Thanks.

-- 
Dmitry