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Ville Syrjälä committed 03981c6ebec
drm/i915: Disable LP3 watermarks on all SNB machines I have a Thinkpad X220 Tablet in my hands that is losing vblank interrupts whenever LP3 watermarks are used. If I nudge the latency value written to the WM3 register just by one in either direction the problem disappears. That to me suggests that the punit will not enter the corrsponding powersave mode (MPLL shutdown IIRC) unless the latency value in the register matches exactly what we read from SSKPD. Ie. it's not really a latency value but rather just a cookie by which the punit can identify the desired power saving state. On HSW/BDW this was changed such that we actually just write the WM level number into those bits, which makes much more sense given the observed behaviour. We could try to handle this by disallowing LP3 watermarks only when vblank interrupts are enabled but we'd first have to prove that only vblank interrupts are affected, which seems unlikely. Also we can't grab the wm mutex from the vblank enable/disable hooks because those are called with various spinlocks held. Thus we'd have to redesigne the watermark locking. So to play it safe and keep the code simple we simply disable LP3 watermarks on all SNB machines. To do that we simply zero out the latency values for watermark level 3, and we adjust the watermark computation to check for that. The behaviour now matches that of the g4x/vlv/skl wm code in the presence of a zeroed latency value. v2: s/USHRT_MAX/U32_MAX/ for consistency with the types (Chris) Cc: stable@vger.kernel.org Cc: Chris Wilson <chris@chris-wilson.co.uk> Acked-by: Chris Wilson <chris@chris-wilson.co.uk> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101269 Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103713 Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20181114173440.6730-1-ville.syrjala@linux.intel.com