do_IRQ: 0.83/0.84 No irq handler for vector

[Nikos Chantziaras <realnc@arcor.de>]

Hello list.

Kernel 2.6.26 (on x86-64) gives "do_IRQ: 0.83 No irq handler for vector" 
(and "do_IRQ: 0.83", but less often) warnings periodically.  It seems 
that the kernel's Radeon DRM driver is at fault, or at least involved 
(no warnings on a kernel without Radeon DRM).  The hardware in question 
is an R580 chip (AFAIK, R500 support was introduced with 2.6.26).

The "do_IRQ: 0.83" warning is 100% reproducible when switching from X 
(xf86-video-ati 6.9.0) to a console (vesafb):

   [drm] Setting GART location based on new memory map
   [drm] Loading R500 Microcode
   [drm] Num pipes: 4
   [drm] writeback test succeeded in 1 usecs
   do_IRQ: 0.83 No irq handler for vector
   do_IRQ: 0.83 No irq handler for vector
   do_IRQ: 0.83 No irq handler for vector

When enabling 3D (compiz fusion) this happens:

   do_IRQ: 0.83 No irq handler for vector
   do_IRQ: 0.83 No irq handler for vector
   [repeated 8 more times]
   __ratelimit: 5 messages suppressed
   do_IRQ: 0.83 No irq handler for vector
   do_IRQ: 0.83 No irq handler for vector

Some apps (glxgears) report the following when they try to do vertical 
synchronization (vblank_mode 1):

   do_wait: drmWaitVBlank returned -1, IRQs don't seem
   to be working correctly.

(the result is that vsync doesn't work as intended and rendering speed 
gets locked at exactly 0.333 FPS).

Of course I filed a bug on X.Org's bugzilla first 
(http://bugs.freedesktop.org/show_bug.cgi?id=16850), but it is not clear 
what exactly this warning means and I'm not sure if it's a problem with 
the kernel's DRM driver or with X.Org (or both.)