drm/i915: use gtfifodbg

Add register definitions for GTFIFODBG, and clear it during init time to
make sure state is correct.

This register tells us if either a read, or a write occurred while the
fifo was full. It seems like bit 2 is an OR of bit 0 and bit 1, so we
check that as well, but the documents are not quite clear.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by (v1): Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -3703,6 +3703,12 @@
 #define  ECOBUS					0xa180
 #define    FORCEWAKE_MT_ENABLE			(1<<5)
 
+#define  GTFIFODBG				0x120000
+#define    GT_FIFO_CPU_ERROR_MASK		7
+#define    GT_FIFO_OVFERR			(1<<2)
+#define    GT_FIFO_IAWRERR			(1<<1)
+#define    GT_FIFO_IARDERR			(1<<0)
+
 #define  GT_FIFO_FREE_ENTRIES			0x120008
 #define    GT_FIFO_NUM_RESERVED_ENTRIES		20
 

drivers/gpu/drm/i915/intel_display.c

@@ -8241,6 +8241,7 @@ void gen6_enable_rps(struct drm_i915_private *dev_priv)
 	u32 rp_state_cap = I915_READ(GEN6_RP_STATE_CAP);
 	u32 gt_perf_status = I915_READ(GEN6_GT_PERF_STATUS);
 	u32 pcu_mbox, rc6_mask = 0;
+	u32 gtfifodbg;
 	int cur_freq, min_freq, max_freq;
 	int i;
 
@@ -8252,6 +8253,13 @@ void gen6_enable_rps(struct drm_i915_private *dev_priv)
 	 */
 	I915_WRITE(GEN6_RC_STATE, 0);
 	mutex_lock(&dev_priv->dev->struct_mutex);
+
+	/* Clear the DBG now so we don't confuse earlier errors */
+	if ((gtfifodbg = I915_READ(GTFIFODBG))) {
+		DRM_ERROR("GT fifo had a previous error %x\n", gtfifodbg);
+		I915_WRITE(GTFIFODBG, gtfifodbg);
+	}
+
 	gen6_gt_force_wake_get(dev_priv);
 
 	/* disable the counters and set deterministic thresholds */