Need some help getting a touchscreen working...

Hello,

I'm trying to get a USB IR Touchscreen working, and I need some help  
filling some gaps in my understanding of how everything works  
together.  This might actually be an XFree86 question, but I figured  
I'd start at the bottom of the stack, and hopefully you call can help  
me work my way up to the right experts.

I'm using Slackware, with the 2.6.20 Kernel.  (Strictly speaking, I'm  
only using Slackware to build, and I'm actually deploying SLAX with  
that kernel booting off a USB flash drive.)  I'm running X with twm.

I have driver source code from the manufacturer, which was apparently  
written for an earlier version of the kernel, judging from this  
method signature:

   static void unitouchusb_irq(struct urb *urb, struct pt_regs *regs)

By referring to the usbtouchscreen.c source, I was able to pretty  
easily modify the vendor's sources such that they compile, and the  
drivers basically work now (touching the screen does move the mouse  
pointer around).

The drivers take a bunch of module params for calibration, and  
translate the raw signals into calls like:

	input_report_abs(unitouch->input, ABS_X, translatedX);

What I'm trying to understand is how the value passed as ABS_X  
eventually turns into a mouse pointer position in X Windows.   
Somewhere along the way it is getting normalized against values that  
were passed to input_set_abs_params, and then that fractional value  
eventually gets scaled to screen coordinates.  But something is going  
awry in this process: over time, the same value passed to  
input_report_abs is resulting in different mouse pointer positions.

What I *think* is happening, is that X has some sort of virtual  
screen center, which gets moved (perhaps by reporting ABS_X values  
which are close to the min/max provided to set_abs_params?).

Can someone here explain what's happening?

(Note: My xorg.conf file doesn't mention USB at all:

Section "InputDevice"

     # generated from default
     Identifier     "Mouse0"
     Driver         "mouse"
     Option         "Protocol" "auto"
     Option         "Device" "/dev/psaux"
     Option         "Emulate3Buttons" "no"
     Option         "ZAxisMapping" "4 5"
EndSection

Should it?  A USB mouse plugged in to the same system is working fine.)

Thanks in advance!!!

-Joshua


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