[Intel-gfx] [PATCH 3/4] drm/i915: Fix missed IRQs on gen6.

[Eric Anholt]

Our wait-on-interrupts path goes:

I915_WRITE(IMR, enabled_val)
POSTING_READ(IMR);
if (!i915_gem_seqno_passed())
   sleep();

However, the IMR write can still be queued in the GT FIFO even after
the POSTING_READ, since the GT FIFO isn't implementing PCI semantics.
Yay.  So, when we're doing a POSTING_READ, we get to check that the
FIFO is empty, and unfortunately to check that the FIFO is empty, it
appears we have to do the FORCEWAKE dance.

Previously, piglit copypixels-sync test had about a 0.5% chance of
triggering a missed IRQ (meaning I've never seen 500 successful runs
without a failure).  With this pair of patches, I'm at 5700 clean runs
as I type this.

Signed-off-by: Eric Anholt <eric at anholt.net>
---
 drivers/gpu/drm/i915/i915_drv.c |   41 +++++++++++++++++++++++++++++++++++++-
 drivers/gpu/drm/i915/i915_drv.h |    3 +-
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 0c46bfc..eabf82c 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -304,7 +304,10 @@ static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 		udelay(10);
 
 	I915_WRITE_NOTRACE(FORCEWAKE, 1);
-	POSTING_READ(FORCEWAKE);
+	/* This is a posting read, but the POSTING_READ macro calls
+	 * this function.
+	 */
+	(void)I915_READ_NOTRACE(FORCEWAKE);
 
 	count = 0;
 	while (count++ < 50 && (I915_READ_NOTRACE(FORCEWAKE_ACK) & 1) == 0)
@@ -332,7 +335,10 @@ void gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 static void __gen6_gt_force_wake_put(struct drm_i915_private *dev_priv)
 {
 	I915_WRITE_NOTRACE(FORCEWAKE, 0);
-	POSTING_READ(FORCEWAKE);
+	/* This is a posting read, but the POSTING_READ macro calls
+	 * this function.
+	 */
+	(void)I915_READ_NOTRACE(FORCEWAKE);
 }
 
 /*
@@ -360,6 +366,37 @@ void __gen6_gt_wait_for_fifo(struct drm_i915_private *dev_priv)
 	}
 }
 
+/* In PCI, writes are not guaranteed to reach the device until a read
+ * from the device has occurred.  The POSTING_READ macro is used to
+ * indicate when we are doing one of those reads to force writes to
+ * the device, so that we can for example test a value in some cached
+ * memory and go to sleep waiting for an interrupt
+ * (i915_wait_request()) or implement some timing requirement.
+ *
+ * However, as of gen6, a mere PCI write posting read doesn't mean
+ * that the writes have reached the device, as they get queued in a
+ * FIFO to hide the wake-from-rc6 latency, even if the chip wasn't in
+ * RC6 at the time.  For gen6, we also have to wait for the fifo to
+ * drain, which means force-waking the GPU so that we can read the
+ * fifo count.
+ */
+void
+intel_posting_read(struct drm_i915_private *dev_priv, uint32_t reg)
+{
+	int ret;
+
+	if (dev_priv->info->gen < 6) {
+		(void)I915_READ_NOTRACE(reg);
+		return;
+	}
+
+	gen6_gt_force_wake_get(dev_priv);
+	ret = wait_for(I915_READ_NOTRACE(GT_FIFO_FREE_ENTRIES) == 0x3f, 500);
+	gen6_gt_force_wake_put(dev_priv);
+
+	WARN_ON_ONCE(ret != 0);
+}
+
 static int i915_drm_freeze(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 56ef324..a064044 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1366,6 +1366,7 @@ extern void intel_display_print_error_state(struct seq_file *m,
 void gen6_gt_force_wake_get(struct drm_i915_private *dev_priv);
 void gen6_gt_force_wake_put(struct drm_i915_private *dev_priv);
 void __gen6_gt_wait_for_fifo(struct drm_i915_private *dev_priv);
+void intel_posting_read(struct drm_i915_private *dev_priv, uint32_t reg);
 
 /* We give fast paths for the really cool registers */
 #define NEEDS_FORCE_WAKE(dev_priv, reg) \
@@ -1423,7 +1424,7 @@ __i915_write(64, q)
 #define I915_WRITE64(reg, val)	i915_write64(dev_priv, (reg), (val))
 #define I915_READ64(reg)	i915_read64(dev_priv, (reg))
 
-#define POSTING_READ(reg)	(void)I915_READ_NOTRACE(reg)
+#define POSTING_READ(reg)	intel_posting_read(dev_priv, reg)
 #define POSTING_READ16(reg)	(void)I915_READ16_NOTRACE(reg)
 
 
-- 
1.7.5.1