[Intel-gfx] [PATCH 4/4] drm/i915: enable forcewake voodoo also for gen6

[Chris Wilson]

On Wed, 25 Jan 2012 14:04:00 +0100, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> We still have reports of missed irqs even on Sandybridge with the
> HWSTAM workaround in place. Testing by the bug reporter gets rid of
> them with the forcewake voodoo and no HWSTAM writes.
> 
> Because I've slightly botched the rebasing I've left out the ACTHD
> readback which is also required to get IVB working. Seems to still
> work on the tester's machine, so I think we should go with the more
> minmal approach on SNB. Especially since I've only found weak evidence
> for holding forcewake while waiting for an interrupt to arrive, but
> none for the ACTHD readback.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45181
> Tested-by: Nicolas Kalkhof nkalkhof()at()web.de
> Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Whilst this patch doesn't directly enable the ACTHD w/a, I have observed
that with a very light load (drawing a single large transformed texture)
reading back the ACTHD (along with the GT forcewake dance) becomes the
predominant consumer of CPU time for the system. (The rate-limiting step
is still the GPU, it just irked me to see the kernel consume more CPU time
than X.)

This workaround appears more successful than the last, and doesn't
appear to break anything else, so

Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre