[Intel-gfx] [PATCH] drm/i915: Restart all engines atomically

[Chris Wilson]

When we restart the engines, and we have active requests, a request on
the first engine may complete and queue a request to the second engine
before we try to restart the second engine. That queueing of the
request may itself cause the engine to restart, and so the subsequent
handling by engine->init_hw() will corrupt the current state.

If we restart all the engines under stop_machine(), we will not process
any interrupts as we restart the engines, and all the engines will
appear to start simultaneously from the snapshot of state.

Fixes: 821ed7df6e2a ("drm/i915: Update reset path to fix incomplete requests")
Testcase: igt/gem_exec_fence/await-hang
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 8a510c7f6828..65651a889813 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4369,11 +4369,30 @@ static void init_unused_rings(struct drm_i915_private *dev_priv)
 	}
 }
 
-int
-i915_gem_init_hw(struct drm_i915_private *dev_priv)
+static int __i915_gem_restart_engines(void *data)
 {
+	struct drm_i915_private *i915 = data;
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
+	int err;
+
+	/* We want all the engines to restart from the same snapshot, the
+	 * restart has to be appear simultaneous (i.e. atomic). If one
+	 * request on the first engine completes and queues a request for
+	 * a secodn engine, *before* we call init_hw on that second engine,
+	 * we may corrupt state.
+	 */
+	for_each_engine(engine, i915, id) {
+		err = engine->init_hw(engine);
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
+int i915_gem_init_hw(struct drm_i915_private *dev_priv)
+{
 	int ret;
 
 	dev_priv->gt.last_init_time = ktime_get();
@@ -4419,11 +4438,12 @@ i915_gem_init_hw(struct drm_i915_private *dev_priv)
 	}
 
 	/* Need to do basic initialisation of all rings first: */
-	for_each_engine(engine, dev_priv, id) {
-		ret = engine->init_hw(engine);
-		if (ret)
-			goto out;
-	}
+	if (dev_priv->gt.active_requests)
+		ret = stop_machine(__i915_gem_restart_engines, dev_priv, NULL);
+	else
+		ret = __i915_gem_restart_engines(dev_priv);
+	if (ret)
+		goto out;
 
 	intel_mocs_init_l3cc_table(dev_priv);
 
-- 
2.11.0