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      drm/i915: fix FORCEWAKE posting reads · b5144075
      Jani Nikula 提交于
      We stopped reading FORCEWAKE for posting reads in
      
      commit 8dee3eea
      Author: Ben Widawsky <ben@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. On the
      bug reporter's machine this broke entering rc6 states after a
      suspend/resume cycle. 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=52411Reported-and-tested-by: NAlexander Bersenev <bay@hackerdom.ru>
      CC: Ben Widawsky <ben@bwidawsk.net>
      Signed-off-by: NJani Nikula <jani.nikula@intel.com>
      Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
      Cc: stable@vger.kernel.org
      [danvet: add cc: stable and make the commit message a bit clearer that
      this is a regression fix and what exactly broke.]
      Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
      b5144075