BUG: i830 flickering for horizontal panning

[Thomas Richter <thor@math.tu-berlin.de>]

Dear intel-gfx developers,

panning on i830 based graphics seem to be working only half-ways. 
Vertical panning works fine, but horizontal panning flickers at about 
60Hz frequency at specific pixel positions. The problem persists 
throughout kernel 3.10.9, and potentially later.

How to reproduce: Enable panning, e.g. by

xrandr --fb 2048x1536
xrandr --output DVI1 --panning 2048x1536

then scroll with the mouse slowly to the right.

The affected hardware is *at least* this one:

00:02.0 VGA compatible controller: Intel Corporation 82830M/MG 
Integrated Graphics Controller (rev 04)

as it is found in the Fujitsu-Siemens S2475 lifebook. The IBM Thinkpad 
R31 is also affected, it also contains a 830GM chipset graphics, but a 
different DVO.

There is no specific output when flickering. The screen contents seems 
to be flickering between a dark frame and a frame that is half the way 
shifted correctly (left) and half the way a copy of the leftmost pixels 
of the screen on the right hand side.

The kernel uses the i915 driver for this hardware.

I currently don't see where precisely panning in the i915 sources is 
handled, thus I cannot really say what is necessary to support this 
correctly.

Greetings,
     Thomas