[PATCH 2/3 v3] drm/i915: catch gtfifo errors on forcewake_put

[Ben Widawsky <ben@bwidawsk.net>]

This is similar to a patch I wrote several months ago. It's been updated
for the new FORCEWAKE_MT, and it also no longer clears the debug
register as it may be helpful to get that for the error state.  Also
recommended by Chris Wilson, use WARN() instead of DRM_ERROR, so we can
get a backtrace.

v2: Replace POSTING_READ with I915_READ_NOTRACE(GTFIFODBG). Recommended
by Chris Wilson

v3: Pull the fifo check out into a helper function to reduce code
duplication

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_drv.c |   15 +++++++++++++--
 1 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 1658cfd..a7858a1 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -379,16 +379,27 @@ void gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 		dev_priv->display.force_wake_get(dev_priv);
 }
 
+static void gen6_gt_check_fifodbg(struct drm_i915_private *dev_priv)
+{
+	u32 gtfifodbg;
+	gtfifodbg = I915_READ_NOTRACE(GTFIFODBG);
+	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);
 }
 
 /*
-- 
1.7.9