drm/i915: Serialize per-engine resets against new requests

We rely on disabling the execlists (by stopping the tasklet) to prevent
new requests from submitting to the engine ELSP before we are ready.
However, we re-enable the engine before we call init_hw which gives
userspace the opportunity to subit a new request which is then
overwritten by init_hw -- but not before the HW may have started
executing. The subsequent out-of-order CSB is detected by our sanity
checks in intel_lrc_irq_handler().

Fixes: a1ef70e144 ("drm/i915: Add support for per engine reset recovery")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721123238.16428-3-chris@chris-wilson.co.uk
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -1951,6 +1951,12 @@ int i915_reset_engine(struct intel_engine_cs *engine)
 	}
 
 	ret = intel_gpu_reset(engine->i915, intel_engine_flag(engine));
+	if (ret) {
+		/* If we fail here, we expect to fallback to a global reset */
+		DRM_DEBUG_DRIVER("Failed to reset %s, ret=%d\n",
+				 engine->name, ret);
+		goto out;
+	}
 
 	/*
 	 * The request that caused the hang is stuck on elsp, we know the
@@ -1959,15 +1965,6 @@ int i915_reset_engine(struct intel_engine_cs *engine)
 	 */
 	i915_gem_reset_engine(engine, active_request);
 
-	i915_gem_reset_finish_engine(engine);
-
-	if (ret) {
-		/* If we fail here, we expect to fallback to a global reset */
-		DRM_DEBUG_DRIVER("Failed to reset %s, ret=%d\n",
-				 engine->name, ret);
-		goto out;
-	}
-
 	/*
 	 * The engine and its registers (and workarounds in case of render)
 	 * have been reset to their default values. Follow the init_ring
@@ -1979,6 +1976,7 @@ int i915_reset_engine(struct intel_engine_cs *engine)
 
 	error->reset_engine_count[engine->id]++;
 out:
+	i915_gem_reset_finish_engine(engine);
 	return ret;
 }