On Sun, Jul 15, 2012 at 09:42:38AM +0100, Chris Wilson wrote: > For reasons that are not apparent to anybody, 990bbdadaba (drm/i915: > Group the GT routines together in both code and vtable) breaks the use > of the BitStream Decoder ring on SandyBridge. The active ingredient of > that patch is the conversion from a udelay(10) to a udelay(1) in the > busy-wait loop of waiting for the forcewake acknowledge. If we restore > that udelay(10) or insert another udelay(1) afterwards (or any wait > longer than 250ns) everything works again. An alternative is also to > remove any delay from the busy-wait loop. > > Given that in the atomic sections we want to complete the wait as quick > as possible to avoid blocking the CPU for too long, it makes sense to > remove the delay altogether and simply spin on the exit condition until > it completes. So we replace the udelay(1) with cpu_relax(). > > Papers over regression from > > commit 990bbdadabaa51828e475eda86ee5720a4910cc3 > Author: Chris Wilson <chris at chris-wilson.co.uk> > Date: Mon Jul 2 11:51:02 2012 -0300 > > drm/i915: Group the GT routines together in both code and vtable > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51738 > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk> Dragon, iceberg or elephant, that's the question ... Patch merged to dinq, thanks a lot for wrestling the strange things in this dungeon. -Daniel -- Daniel Vetter Mail: daniel at ffwll.ch Mobile: +41 (0)79 365 57 48
