drm/i915: Make RPS EI/thresholds multiple of 25 on SNB-BDW

Somehow my SNB GT1 (Dell XPS 8300) gets very unhappy around
GPU hangs if the RPS EI/thresholds aren't suitably aligned.
It seems like scheduling/timer interupts stop working somehow
and things get stuck eg. in usleep_range().

I bisected the problem down to
commit 8a5864377b12 ("drm/i915/skl: Restructured the gen6_set_rps_thresholds function")
I observed that before all the values were at least multiples of 25,
but afterwards they are not. And rounding things up to the next multiple
of 25 does seem to help, so lets' do that. I also tried roundup(..., 5)
but that wasn't sufficient. Also I have no idea if we might need this sort of
thing on gen9+ as well.

These are the original EI/thresholds:
 LOW_POWER
  GEN6_RP_UP_EI          12500
  GEN6_RP_UP_THRESHOLD   11800
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 21250
 BETWEEN
  GEN6_RP_UP_EI          10250
  GEN6_RP_UP_THRESHOLD    9225
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 18750
 HIGH_POWER
  GEN6_RP_UP_EI           8000
  GEN6_RP_UP_THRESHOLD    6800
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 15000

These are after 8a5864377b12:
 LOW_POWER
  GEN6_RP_UP_EI          12500
  GEN6_RP_UP_THRESHOLD   11875
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 21250
 BETWEEN
  GEN6_RP_UP_EI          10156
  GEN6_RP_UP_THRESHOLD    9140
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 18750
 HIGH_POWER
  GEN6_RP_UP_EI           7812
  GEN6_RP_UP_THRESHOLD    6640
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 15000

And these are what we have after this patch:
 LOW_POWER
  GEN6_RP_UP_EI          12500
  GEN6_RP_UP_THRESHOLD   11875
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 21250
 BETWEEN
  GEN6_RP_UP_EI          10175
  GEN6_RP_UP_THRESHOLD    9150
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 18750
 HIGH_POWER
  GEN6_RP_UP_EI           7825
  GEN6_RP_UP_THRESHOLD    6650
  GEN6_RP_DOWN_EI        25000
  GEN6_RP_DOWN_THRESHOLD 15000

Cc: stable@vger.kernel.org
Cc: Akash Goel <akash.goel@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Testcase: igt/kms_pipe_crc_basic/hang-read-crc-pipe-B
Fixes: 8a5864377b12 ("drm/i915/skl: Restructured the gen6_set_rps_thresholds function")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461159836-9108-1-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -2948,7 +2948,14 @@ enum skl_disp_power_wells {
 #define GEN6_RP_STATE_CAP	_MMIO(MCHBAR_MIRROR_BASE_SNB + 0x5998)
 #define BXT_RP_STATE_CAP        _MMIO(0x138170)
 
-#define INTERVAL_1_28_US(us)	(((us) * 100) >> 7)
+/*
+ * Make these a multiple of magic 25 to avoid SNB (eg. Dell XPS
+ * 8300) freezing up around GPU hangs. Looks as if even
+ * scheduling/timer interrupts start misbehaving if the RPS
+ * EI/thresholds are "bad", leading to a very sluggish or even
+ * frozen machine.
+ */
+#define INTERVAL_1_28_US(us)	roundup(((us) * 100) >> 7, 25)
 #define INTERVAL_1_33_US(us)	(((us) * 3)   >> 2)
 #define INTERVAL_0_833_US(us)	(((us) * 6) / 5)
 #define GT_INTERVAL_FROM_US(dev_priv, us) (IS_GEN9(dev_priv) ? \