[Intel-gfx] [PATCH 2/2] drm/i915: Don't count semaphore waits towards a stuck ring

[Chris Wilson]

If we detect a ring is in a valid wait for another, just let it be.
Eventually it will either begin to progress again, or the entire system
will come grinding to a halt and then hangcheck will fire as soon as the
deadlock is detected.

This error was foretold by Ben in
commit 05407ff889ceebe383aa5907219f86582ef96b72
Author: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Date:   Thu May 30 09:04:29 2013 +0300

    drm/i915: detect hang using per ring hangcheck_score

"If ring B is waiting on ring A via semaphore, and ring A is making
progress, albeit slowly - the hangcheck will fire. The check will
determine that A is moving, however ring B will appear hung because
the ACTHD doesn't move. I honestly can't say if that's actually a
realistic problem to hit it probably implies the timeout value is too
low."

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65394
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_irq.c         |   66 +++++++++++++++++++++++--------
 drivers/gpu/drm/i915/intel_ringbuffer.h |    1 +
 2 files changed, 51 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 2d1890d..948eb9f 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -2331,21 +2331,21 @@ ring_idle(struct intel_ring_buffer *ring, u32 seqno)
 		i915_seqno_passed(seqno, ring_last_seqno(ring)));
 }
 
-static bool semaphore_passed(struct intel_ring_buffer *ring)
+static struct intel_ring_buffer *
+semaphore_waits_for(struct intel_ring_buffer *ring, u32 *seqno)
 {
 	struct drm_i915_private *dev_priv = ring->dev->dev_private;
-	u32 acthd = intel_ring_get_active_head(ring) & HEAD_ADDR;
-	struct intel_ring_buffer *signaller;
-	u32 cmd, ipehr, acthd_min;
+	u32 cmd, ipehr, acthd, acthd_min;
 
 	ipehr = I915_READ(RING_IPEHR(ring->mmio_base));
 	if ((ipehr & ~(0x3 << 16)) !=
 	    (MI_SEMAPHORE_MBOX | MI_SEMAPHORE_COMPARE | MI_SEMAPHORE_REGISTER))
-		return false;
+		return NULL;
 
 	/* ACTHD is likely pointing to the dword after the actual command,
 	 * so scan backwards until we find the MBOX.
 	 */
+	acthd = intel_ring_get_active_head(ring) & HEAD_ADDR;
 	acthd_min = max((int)acthd - 3 * 4, 0);
 	do {
 		cmd = ioread32(ring->virtual_start + acthd);
@@ -2354,12 +2354,40 @@ static bool semaphore_passed(struct intel_ring_buffer *ring)
 
 		acthd -= 4;
 		if (acthd < acthd_min)
-			return false;
+			return NULL;
 	} while (1);
 
-	signaller = &dev_priv->ring[(ring->id + (((ipehr >> 17) & 1) + 1)) % 3];
-	return i915_seqno_passed(signaller->get_seqno(signaller, false),
-				 ioread32(ring->virtual_start+acthd+4)+1);
+	*seqno = ioread32(ring->virtual_start+acthd+4)+1;
+	return &dev_priv->ring[(ring->id + (((ipehr >> 17) & 1) + 1)) % 3];
+}
+
+static int semaphore_passed(struct intel_ring_buffer *ring)
+{
+	struct drm_i915_private *dev_priv = ring->dev->dev_private;
+	struct intel_ring_buffer *signaller;
+	u32 seqno, ctl;
+
+	ring->hangcheck.deadlock = true;
+
+	signaller = semaphore_waits_for(ring, &seqno);
+	if (signaller == NULL || signaller->hangcheck.deadlock)
+		return -1;
+
+	/* cursory check for an unkickable deadlock */
+	ctl = I915_READ_CTL(signaller);
+	if (ctl & RING_WAIT_SEMAPHORE && semaphore_passed(signaller) < 0)
+		return -1;
+
+	return i915_seqno_passed(signaller->get_seqno(signaller, false), seqno);
+}
+
+static void semaphore_clear_deadlocks(struct drm_i915_private *dev_priv)
+{
+	struct intel_ring_buffer *ring;
+	int i;
+
+	for_each_ring(ring, dev_priv, i)
+		ring->hangcheck.deadlock = false;
 }
 
 static bool ring_hung(struct intel_ring_buffer *ring)
@@ -2384,13 +2412,17 @@ static bool ring_hung(struct intel_ring_buffer *ring)
 		return false;
 	}
 
-	if (INTEL_INFO(dev)->gen >= 6 &&
-	    tmp & RING_WAIT_SEMAPHORE &&
-	    semaphore_passed(ring)) {
-		DRM_ERROR("Kicking stuck semaphore on %s\n",
-			  ring->name);
-		I915_WRITE_CTL(ring, tmp);
-		return false;
+	if (INTEL_INFO(dev)->gen >= 6 && tmp & RING_WAIT_SEMAPHORE) {
+		switch (semaphore_passed(ring)) {
+		default:
+			return true;
+		case 1:
+			DRM_ERROR("Kicking stuck semaphore on %s\n",
+				  ring->name);
+			I915_WRITE_CTL(ring, tmp);
+		case 0:
+			return false;
+		}
 	}
 
 	return true;
@@ -2422,6 +2454,8 @@ void i915_hangcheck_elapsed(unsigned long data)
 	for_each_ring(ring, dev_priv, i) {
 		u32 seqno, acthd;
 
+		semaphore_clear_deadlocks(dev_priv);
+
 		seqno = ring->get_seqno(ring, false);
 		acthd = intel_ring_get_active_head(ring);
 
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index efc403d..a3e9610 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -38,6 +38,7 @@ struct  intel_hw_status_page {
 #define I915_READ_SYNC_1(ring) I915_READ(RING_SYNC_1((ring)->mmio_base))
 
 struct intel_ring_hangcheck {
+	bool deadlock;
 	u32 seqno;
 	u32 acthd;
 	int score;
-- 
1.7.10.4