[Intel-gfx] [PATCH 2/2] drm/i915: Implement w/a for sporadic read failures on waking from rc6

[Chris Wilson]

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.

References: https://bugs.freedesktop.org/show_bug.cgi?id=50243
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.c |   19 +++++++++++++++++++
 drivers/gpu/drm/i915/i915_reg.h |    3 +++
 2 files changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 2058316..ba3f1a9 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -424,6 +424,19 @@ 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)
+{
+	unsigned long timeout;
+
+	/* w/a for a sporadic read returning 0 by waiting for the GT
+	 * thread to wake up.
+	 */
+	timeout = jiffies + msecs_to_jiffies(1);
+	while (I915_READ_NOTRACE(GEN6_GT_THREAD_STATUS_REG) & GEN6_GT_THREAD_STATUS_CORE_MASK &&
+	       time_before(jiffies, timeout))
+		;
+}
+
 static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 {
 	unsigned long timeout;
@@ -439,6 +452,8 @@ static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 	while ((I915_READ_NOTRACE(FORCEWAKE_ACK) & 1) == 0 &&
 	       time_before(jiffies, timeout))
 		;
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
@@ -456,6 +471,8 @@ static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
 	while ((I915_READ_NOTRACE(FORCEWAKE_MT_ACK) & 1) == 0 &&
 	       time_before(jiffies, timeout))
 		;
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 /*
@@ -546,6 +563,8 @@ static void vlv_force_wake_get(struct drm_i915_private *dev_priv)
 	count = 0;
 	while (count++ < 50 && (I915_READ_NOTRACE(FORCEWAKE_ACK_VLV) & 1) == 0)
 		udelay(10);
+
+	__gen6_gt_wait_for_thread_c0(dev_priv);
 }
 
 static void vlv_force_wake_put(struct drm_i915_private *dev_priv)
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 23f8954..b51414e 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -1443,6 +1443,9 @@
 #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_PERF_STATUS	0x145948
 #define GEN6_RP_STATE_LIMITS	0x145994
 #define GEN6_RP_STATE_CAP	0x145998
-- 
1.7.10