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
