[PATCH v1] drm/i915/gt: Retry RING_HEAD reset until it sticks

[Nitin Gote]

From: Chris Wilson <chris at chris-wilson.co.uk>

On Haswell, in particular, we see an issue where resets fails because
the engine resumes from an incorrect RING_HEAD. Since the RING_HEAD
doesn't point to the remaining requests to re-run, but may instead point
into the uninitialised portion of the ring, the GPU may be then fed
invalid instructions from a privileged context, oft pushing the GPU into
an unrecoverable hang.

If at first the write doesn't succeed, try, try again.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/5432
Testcase: igt/i915_selftest/hangcheck
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Nitin Gote <nitin.r.gote at intel.com>
---
 .../gpu/drm/i915/gt/intel_ring_submission.c   | 44 +++++++++++++------
 drivers/gpu/drm/i915/i915_utils.h             |  9 ++++
 2 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_ring_submission.c b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
index 72277bc8322e..6427f07ed23e 100644
--- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
+++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
@@ -192,6 +192,7 @@ static bool stop_ring(struct intel_engine_cs *engine)
 static int xcs_resume(struct intel_engine_cs *engine)
 {
 	struct intel_ring *ring = engine->legacy.ring;
+	ktime_t kt;
 
 	ENGINE_TRACE(engine, "ring:{HEAD:%04x, TAIL:%04x}\n",
 		     ring->head, ring->tail);
@@ -230,9 +231,20 @@ static int xcs_resume(struct intel_engine_cs *engine)
 	set_pp_dir(engine);
 
 	/* First wake the ring up to an empty/idle ring */
-	ENGINE_WRITE_FW(engine, RING_HEAD, ring->head);
+	until_timeout_ns(kt, 2 * NSEC_PER_MSEC) {
+		ENGINE_WRITE_FW(engine, RING_HEAD, ring->head);
+		if (ENGINE_READ_FW(engine, RING_HEAD) == ring->head)
+			break;
+	}
+
 	ENGINE_WRITE_FW(engine, RING_TAIL, ring->head);
-	ENGINE_POSTING_READ(engine, RING_TAIL);
+	if (ENGINE_READ_FW(engine, RING_HEAD) != ENGINE_READ_FW(engine, RING_TAIL)) {
+		ENGINE_TRACE(engine, "failed to reset empty ring: [%x, %x]: %x\n",
+				ENGINE_READ_FW(engine, RING_HEAD),
+				ENGINE_READ_FW(engine, RING_TAIL),
+				ring->head);
+		goto err;
+	}
 
 	ENGINE_WRITE_FW(engine, RING_CTL,
 			RING_CTL_SIZE(ring->size) | RING_VALID);
@@ -241,12 +253,16 @@ static int xcs_resume(struct intel_engine_cs *engine)
 	if (__intel_wait_for_register_fw(engine->uncore,
 					 RING_CTL(engine->mmio_base),
 					 RING_VALID, RING_VALID,
-					 5000, 0, NULL))
+					 5000, 0, NULL)) {
+		ENGINE_TRACE(engine, "failed to restart\n");
 		goto err;
+	}
 
-	if (GRAPHICS_VER(engine->i915) > 2)
+	if (GRAPHICS_VER(engine->i915) > 2) {
 		ENGINE_WRITE_FW(engine,
 				RING_MI_MODE, _MASKED_BIT_DISABLE(STOP_RING));
+		ENGINE_POSTING_READ(engine, RING_MI_MODE);
+	}
 
 	/* Now awake, let it get started */
 	if (ring->tail != ring->head) {
@@ -259,16 +275,16 @@ static int xcs_resume(struct intel_engine_cs *engine)
 	return 0;
 
 err:
-	drm_err(&engine->i915->drm,
-		"%s initialization failed; "
-		"ctl %08x (valid? %d) head %08x [%08x] tail %08x [%08x] start %08x [expected %08x]\n",
-		engine->name,
-		ENGINE_READ(engine, RING_CTL),
-		ENGINE_READ(engine, RING_CTL) & RING_VALID,
-		ENGINE_READ(engine, RING_HEAD), ring->head,
-		ENGINE_READ(engine, RING_TAIL), ring->tail,
-		ENGINE_READ(engine, RING_START),
-		i915_ggtt_offset(ring->vma));
+	ENGINE_TRACE(engine,
+		     "initialization failed; "
+		     "ctl %08x (valid? %d) head %08x [%08x] tail %08x [%08x] start %08x [expected %08x]\n",
+		     ENGINE_READ(engine, RING_CTL),
+		     ENGINE_READ(engine, RING_CTL) & RING_VALID,
+		     ENGINE_READ(engine, RING_HEAD), ring->head,
+		     ENGINE_READ(engine, RING_TAIL), ring->tail,
+		     ENGINE_READ(engine, RING_START),
+		     i915_ggtt_offset(ring->vma));
+	GEM_TRACE_DUMP();
 	return -EIO;
 }
 
diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
index 609214231ffc..538dfb8fa1d8 100644
--- a/drivers/gpu/drm/i915/i915_utils.h
+++ b/drivers/gpu/drm/i915/i915_utils.h
@@ -232,6 +232,15 @@ wait_remaining_ms_from_jiffies(unsigned long timestamp_jiffies, int to_wait_ms)
 	}
 }
 
+/*
+ * until_timeout_ns - Keep retrying (busy spin) until the duration has passed
+ */
+
+#define until_timeout_ns(end, timeout_ns) \
+	for ((end) = ktime_get() + (timeout_ns); \
+			ktime_before(ktime_get(), (end)); \
+			cpu_relax())
+
 /*
  * __wait_for - magic wait macro
  *
-- 
2.25.1