drm/i915: detect wrong MCH watermark values

Some early bios versions seem to ship with the wrong tuning values for
the MCH, possible resulting in pipe underruns under load. Especially
on DP outputs this can lead to black screen, since DP really doesn't
like an occasional whack from an underrun.

Unfortunately the registers seem to be locked after boot, so the only
thing we can do is politely point out issues and suggest a BIOS
upgrade.

Arthur Runyan pointed us at this issue while discussion DP bugs - thus
far no confirmation from a bug report yet that it helps. But at least
some of my machines here have wrong values, so this might be useful in
understanding bug reports.

v2: After a bit more discussion with Art and Ben we've decided to only
the check the watermark values, since the OREF ones could be be a
notch more aggressive on certain machines.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Runyan, Arthur J <arthur.j.runyan@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -1235,6 +1235,10 @@
 #define   MAD_DIMM_A_SIZE_SHIFT		0
 #define   MAD_DIMM_A_SIZE_MASK		(0xff << MAD_DIMM_A_SIZE_SHIFT)
 
+/** snb MCH registers for priority tuning */
+#define MCH_SSKPD			(MCHBAR_MIRROR_BASE_SNB + 0x5d10)
+#define   MCH_SSKPD_WM0_MASK		0x3f
+#define   MCH_SSKPD_WM0_VAL		0xc
 
 /* Clocking configuration register */
 #define CLKCFG			0x10c00

drivers/gpu/drm/i915/intel_pm.c

@@ -3584,6 +3584,19 @@ static void cpt_init_clock_gating(struct drm_device *dev)
 	}
 }
 
+static void gen6_check_mch_setup(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	uint32_t tmp;
+
+	tmp = I915_READ(MCH_SSKPD);
+	if ((tmp & MCH_SSKPD_WM0_MASK) != MCH_SSKPD_WM0_VAL) {
+		DRM_INFO("Wrong MCH_SSKPD value: 0x%08x\n", tmp);
+		DRM_INFO("This can cause pipe underruns and display issues.\n");
+		DRM_INFO("Please upgrade your BIOS to fix this.\n");
+	}
+}
+
 static void gen6_init_clock_gating(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -3676,6 +3689,8 @@ static void gen6_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(GEN6_GT_MODE, _MASKED_BIT_ENABLE(GEN6_GT_MODE_HI));
 
 	cpt_init_clock_gating(dev);
+
+	gen6_check_mch_setup(dev);
 }
 
 static void gen7_setup_fixed_func_scheduler(struct drm_i915_private *dev_priv)
@@ -3861,6 +3876,8 @@ static void ivybridge_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(GEN6_MBCUNIT_SNPCR, snpcr);
 
 	cpt_init_clock_gating(dev);
+
+	gen6_check_mch_setup(dev);
 }
 
 static void valleyview_init_clock_gating(struct drm_device *dev)