Bug 245193

+++ This bug was initially created as a clone of Bug #243920 +++

Description of problem:
I setup an Xorg config under a Xen guest to use the evdev driver for input
instead of /dev/input/mice, since the former honours absolute pointer events &
thus avoids the insane 'double mouse' issues.

There appears to be a bug in the evdev driver though. If moving the cursor in
the X-axis only, or in the Y-axis only it works fine. If I move the cursor in
the X & Y axis at the same time (eg bottom-left -> top-right of screen), the
position of the cursor on screen seems to 'loose' the X co-ordinate updates
until I finish moving the mouse.

eg, move from bottom-left -> top-right of screen, while the mouse it moving the
cursor will only update the Y co-ordinate. When I stop moving the mouse, it
finally updates the X co-ordinate

Version-Release number of selected component (if applicable):
xorg-x11-drv-evdev-1.1.2-3.fc7

How reproducible:
Always

Steps to Reproduce:
1. Boot a Fedora 7 paravirt Xen guest
2. Configure it to use attached Xorg config file
3. Start X
4. Move mouse at a medium speed from bottom-left to top-right corner of screen
  
Actual results:
Mouse moves up left-hand side of screen, and then at last movement flips over to
top-right corner

Expected results:
Mouse moves smoothly diagonally across the screen from bottom-left to top-right.

Additional info:

-- Additional comment from berrange on 2007-06-12 14:59 EST --
Created an attachment (id=156818)
Sample Xorg config to use  evdev with Xen guest framebuffer


-- Additional comment from berrange on 2007-06-12 15:04 EST --
Looking at the way the evdev events are fed from kernel upto X, the data stream
has 3 core types of event

 - X axis update
 - Y axis update
 - Sync event

The kernel normally sends a sequence of

 - X axis update + Y axis update + sync event
 - X axis update + sync event
 - Y axis update + sync event

There is some extra code in the evdev driver, however, which does an explicit
sync after every individual X or Y axis event. This seems to be causing the X
axis update to be lost - resulting in the cursor travelling up the left-hand
side of the screen when moved diagonally. This manual sync looks unneccessary
since the kernel sends an explicit sync event whenever one ie needed. 

So I wrote the attached patch which removed the two redundant calls to sync
pointer position. This resulted in smooth mouse movement expected.

I'm a little puzzelled why these explicit sync calls are in the driver at all -
not clear they could ever have worked correctly ?!?!


-- Additional comment from berrange on 2007-06-12 15:05 EST --
Created an attachment (id=156820)
Remove redundant pointer sync calls