drm/i915: Implement w/a for sporadic read failures on waking from rc6

As a w/a to prevent reads sporadically returning 0, we need to wait for
the GT thread to return to TC0 before proceeding to read the registers.

v2: adapt for Haswell changes (Eugeni).

v3: use wait_for_atomic_us for thread status polling.

v3: *really* use wait_for_atomic for polling.

References: https://bugs.freedesktop.org/show_bug.cgi?id=50243
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -433,6 +433,22 @@ bool i915_semaphore_is_enabled(struct drm_device *dev)
 	return 1;
 }
 
+static void __gen6_gt_wait_for_thread_c0(struct drm_i915_private *dev_priv)
+{
+	u32 gt_thread_status_mask;
+
+	if (IS_HASWELL(dev_priv->dev))
+		gt_thread_status_mask = GEN6_GT_THREAD_STATUS_CORE_MASK_HSW;
+	else
+		gt_thread_status_mask = GEN6_GT_THREAD_STATUS_CORE_MASK;
+
+	/* w/a for a sporadic read returning 0 by waiting for the GT
+	 * thread to wake up.
+	 */
+	if (wait_for_atomic_us((I915_READ_NOTRACE(GEN6_GT_THREAD_STATUS_REG) & gt_thread_status_mask) == 0, 500))
+		DRM_ERROR("GT thread status wait timed out\n");
+}
+
 static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 {
 	if (wait_for_atomic_us((I915_READ_NOTRACE(FORCEWAKE_ACK) & 1) == 0, 500))
@@ -442,6 +458,8 @@ static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 
 	if (wait_for_atomic_us((I915_READ_NOTRACE(FORCEWAKE_ACK) & 1), 500))
 		DRM_ERROR("Force wake wait timed out\n");
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
@@ -453,6 +471,8 @@ static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
 
 	if (wait_for_atomic_us((I915_READ_NOTRACE(FORCEWAKE_MT_ACK) & 1), 500))
 		DRM_ERROR("Force wake wait timed out\n");
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 /*
@@ -538,6 +558,8 @@ static void vlv_force_wake_get(struct drm_i915_private *dev_priv)
 
 	if (wait_for_atomic_us((I915_READ_NOTRACE(FORCEWAKE_ACK_VLV) & 1), 500))
 		DRM_ERROR("Force wake wait timed out\n");
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 static void vlv_force_wake_put(struct drm_i915_private *dev_priv)

drivers/gpu/drm/i915/i915_reg.h

@@ -1458,6 +1458,10 @@
 #define DDRMPLL1		0X12c20
 #define PEG_BAND_GAP_DATA	0x14d68
 
+#define GEN6_GT_THREAD_STATUS_REG 0x13805c
+#define GEN6_GT_THREAD_STATUS_CORE_MASK 0x7
+#define GEN6_GT_THREAD_STATUS_CORE_MASK_HSW (0x7 | (0x07 << 16))
+
 #define GEN6_GT_PERF_STATUS	0x145948
 #define GEN6_RP_STATE_LIMITS	0x145994
 #define GEN6_RP_STATE_CAP	0x145998