drm/i915: catch gtfifo errors on forcewake_put

This is similar to a patch I wrote several months ago. It's been updated
for the new FORCEWAKE_MT. As recommended by Chris Wilson, use WARN()
instead of DRM_ERROR, so we can get a backtrace.

This shouldn't impact performance too much as the extra register read
can replace the POSTING_READ we had previously.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -385,16 +385,27 @@ void gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 	spin_unlock_irqrestore(&dev_priv->gt_lock, irqflags);
 }
 
+static void gen6_gt_check_fifodbg(struct drm_i915_private *dev_priv)
+{
+	u32 gtfifodbg;
+	gtfifodbg = I915_READ_NOTRACE(GTFIFODBG);
+	if (WARN(gtfifodbg & GT_FIFO_CPU_ERROR_MASK,
+	     "MMIO read or write has been dropped %x\n", gtfifodbg))
+		I915_WRITE_NOTRACE(GTFIFODBG, GT_FIFO_CPU_ERROR_MASK);
+}
+
 void __gen6_gt_force_wake_put(struct drm_i915_private *dev_priv)
 {
 	I915_WRITE_NOTRACE(FORCEWAKE, 0);
-	POSTING_READ(FORCEWAKE);
+	/* The below doubles as a POSTING_READ */
+	gen6_gt_check_fifodbg(dev_priv);
 }
 
 void __gen6_gt_force_wake_mt_put(struct drm_i915_private *dev_priv)
 {
 	I915_WRITE_NOTRACE(FORCEWAKE_MT, (1<<16) | 0);
-	POSTING_READ(FORCEWAKE_MT);
+	/* The below doubles as a POSTING_READ */
+	gen6_gt_check_fifodbg(dev_priv);
 }
 
 /*