[Intel-gfx] [PATCH] drm/i915: Workaround hang with BSD and forcewake on SandyBridge

[Daniel Vetter]

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