[Intel-gfx] [PATCH] drm/i915: fix forcewake related hangs on snb

[Daniel Vetter]

On Thu, Jul 26, 2012 at 03:50:02PM +0100, Chris Wilson wrote:
> On Thu, 26 Jul 2012 16:24:50 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > ... by adding seemingly redudant posting reads.
> > 
> > This little dragon lair exploded the first time around when we've
> > refactored the code a bit to use the common wait_for_atomic_us in
> > "drm/i915: Group the GT routines together in both code and vtable",
> > which caused QA to file fdo bug #51738.
> > 
> > Chris Wilson entertained a few approaches to fixing #51738: Replacing
> > the udelay(1) with the previously-used udelay(10) (or any other
> > "sufficiently larger" delay), adding a posting read, or ditching the
> > delay completely and using cpu_relax. We went with the cpu_relax and
> > "915: Workaround hang with BSD and forcewake on SandyBridge". Which
> > blew up in fdo bug #52424, but adding the posting read while still
> > using cpu_relax seems to also fix that, it looks like the
> > posting read is the important ingriedient to fix these rc6 related
> > hangs on snb.
> > 
> > Popular theories as to why this is like it is include:
> > - A herd of pink elephants got royally angered somehow.
> > 
> > - The gpu has internally different functional units and judging by the
> >   register offsets, the forcewake request register and the forcewake
> >   ack registers are _not_ in the same functional unit (or at least
> >   aren't reached through the same routes). Hence the posting read
> >   syncs up with the wrong block and gets the entire gpu confused.
> > 
> > - ...
> > 
> > As a minimal ducttape fix for 3.6, let's just put these posting reads
> > into place again. We can try fancier approaches (like adding back the
> > cpu_relax instead of the udelay) in -next.
> > 
> > This (re-)fixes a regression introduced in
> > 
> > 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
> > 
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52424
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51738u
> > Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> No change on IVB, fixes the dummy_reloc_loop hang on SNB.
> 
> Tested-by: Chris Wilson <chris at chris-wilson.co.uk>
I've picked this up for -fixes, thanks for testing. I'll send a pull to
Dave tomorrow, assuming QA doesn't complain about things any more, too.
-Daniel
-- 
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48