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Daniel Vetter authored
... 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 990bbdad Author: Chris Wilson <chris@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@chris-wilson.co.uk> Tested-by:
Chris Wilson <chris@chris-wilson.co.uk> Tested-by:
Du Yan <yanx.du@intel.com> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52424 Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51738uSigned-off-by:
Daniel Vetter <daniel.vetter@ffwll.ch>
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