On Thu, 17 Jan 2013 10:24:09 +0200, Jani Nikula <jani.nikula at intel.com> wrote: > We stopped reading FORCEWAKE for posting reads in > > commit 8dee3eea3ccd3b6c00a8d3a08dd715d6adf737dd > Author: Ben Widawsky <ben at bwidawsk.net> > Date: Sat Sep 1 22:59:50 2012 -0700 > > drm/i915: Never read FORCEWAKE > > and started using something from the same cacheline instead. It turns out > reading ECOBUS as posting read worked fine, while GTFIFODBG did not, > preventing RC6 states after suspend/resume per the bug report referenced > below. It's not entirely clear why, but clearly GTFIFODBG was nowhere near > the same cacheline or address range as FORCEWAKE. > > Trying out various registers for posting reads showed that all tested > registers for which NEEDS_FORCE_WAKE() (in i915_drv.c) returns true > work. Conversely, most (but not quite all) registers for which > NEEDS_FORCE_WAKE() returns false do not work. Details in the referenced > bug. > > Based on the above, add posting reads on ECOBUS where GTFIFODBG was > previously relied on. > > In true cargo cult spirit, add posting reads for FORCEWAKE_VLV writes as > well, but instead of ECOBUS, use FORCEWAKE_ACK_VLV which is in the same > address range as FORCEWAKE_VLV. > > v2: Add more details to the commit message. No functional changes. > > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=52411 > Reported-and-tested-by: Alexander Bersenev <bay at hackerdom.ru> > CC: Ben Widawsky <ben at bwidawsk.net> > Signed-off-by: Jani Nikula <jani.nikula at intel.com> You don't mention at all why you consider this to be important at all. A hint that the extra read makes rc6 after resume more stable on one machine would be useful. So a missing flush of the write in _put() makes sense for rc6 staying active. (If it happens once, it can happen everytime we do the forcewake dance and so stay asserted indefinitely). And I can quite believe that the hw has separate queues for the GT powerwell that require a read from within that powerwell to flush, so the magic pixie dust looks consistent. Ok, I'm happy that I can rationalise that this does indeed fix a genuine quirk of our hw. The cargo cult survives. Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk> -Chris -- Chris Wilson, Intel Open Source Technology Centre
