drm/i915: Prevent leaking of -EIO from i915_wait_request()

Reporting -EIO from i915_wait_request() has proven very troublematic
over the years, with numerous hard-to-reproduce bugs cropping up in the
corner case of where a reset occurs and the code wasn't expecting such
an error.

If the we reset the GPU or have detected a hang and wish to reset the
GPU, the request is forcibly complete and the wait broken. Currently, we
report either -EAGAIN or -EIO in order for the caller to retreat and
restart the wait (if appropriate) after dropping and then reacquiring
the struct_mutex (essential to allow the GPU reset to proceed). However,
if we take the view that the request is complete (no further work will
be done on it by the GPU because it is dead and soon to be reset), then
we can proceed with the task at hand and then drop the struct_mutex
allowing the reset to occur. This transfers the burden of checking
whether it is safe to proceed to the caller, which in all but one
instance it is safe - completely eliminating the source of all spurious
-EIO.

Of note, we only have two API entry points where we expect that
userspace can observe an EIO. First is when submitting an execbuf, if
the GPU is terminally wedged, then the operation cannot succeed and an
-EIO is reported. Secondly, existing userspace uses the throttle ioctl
to detect an already wedged GPU before starting using HW acceleration
(or to confirm that the GPU is wedged after an error condition). So if
the GPU is wedged when the user calls throttle, also report -EIO.

v2: Split more carefully the change to i915_wait_request() and assorted
ABI from the reset handling.
v3: Add a couple of WARN_ON(EIO) to the interruptible modesetting code
so that we don't start to leak EIO there in future (and break our hang
resistant modesetting).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-9-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3088,8 +3088,6 @@ i915_gem_find_active_request(struct intel_engine_cs *engine);
 
 bool i915_gem_retire_requests(struct drm_device *dev);
 void i915_gem_retire_requests_ring(struct intel_engine_cs *engine);
-int __must_check i915_gem_check_wedge(struct i915_gpu_error *error,
-				      bool interruptible);
 
 static inline u32 i915_reset_counter(struct i915_gpu_error *error)
 {

drivers/gpu/drm/i915/i915_gem.c

@@ -206,11 +206,10 @@ i915_gem_object_put_pages_phys(struct drm_i915_gem_object *obj)
 	BUG_ON(obj->madv == __I915_MADV_PURGED);
 
 	ret = i915_gem_object_set_to_cpu_domain(obj, true);
-	if (ret) {
+	if (WARN_ON(ret)) {
 		/* In the event of a disaster, abandon all caches and
 		 * hope for the best.
 		 */
-		WARN_ON(ret != -EIO);
 		obj->base.read_domains = obj->base.write_domain = I915_GEM_DOMAIN_CPU;
 	}
 
@@ -1105,15 +1104,13 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 	return ret;
 }
 
-int
-i915_gem_check_wedge(struct i915_gpu_error *error,
-		     bool interruptible)
+static int
+i915_gem_check_wedge(unsigned reset_counter, bool interruptible)
 {
-	if (i915_reset_in_progress_or_wedged(error)) {
-		/* Recovery complete, but the reset failed ... */
-		if (i915_terminally_wedged(error))
-			return -EIO;
+	if (__i915_terminally_wedged(reset_counter))
+		return -EIO;
 
+	if (__i915_reset_in_progress(reset_counter)) {
 		/* Non-interruptible callers can't handle -EAGAIN, hence return
 		 * -EIO unconditionally for these. */
 		if (!interruptible)
@@ -1287,13 +1284,14 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
 		prepare_to_wait(&engine->irq_queue, &wait, state);
 
 		/* We need to check whether any gpu reset happened in between
-		 * the caller grabbing the seqno and now ... */
+		 * the request being submitted and now. If a reset has occurred,
+		 * the request is effectively complete (we either are in the
+		 * process of or have discarded the rendering and completely
+		 * reset the GPU. The results of the request are lost and we
+		 * are free to continue on with the original operation.
+		 */
 		if (req->reset_counter != i915_reset_counter(&dev_priv->gpu_error)) {
-			/* ... but upgrade the -EAGAIN to an -EIO if the gpu
-			 * is truely gone. */
-			ret = i915_gem_check_wedge(&dev_priv->gpu_error, interruptible);
-			if (ret == 0)
-				ret = -EAGAIN;
+			ret = 0;
 			break;
 		}
 
@@ -2154,11 +2152,10 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj)
 	BUG_ON(obj->madv == __I915_MADV_PURGED);
 
 	ret = i915_gem_object_set_to_cpu_domain(obj, true);
-	if (ret) {
+	if (WARN_ON(ret)) {
 		/* In the event of a disaster, abandon all caches and
 		 * hope for the best.
 		 */
-		WARN_ON(ret != -EIO);
 		i915_gem_clflush_object(obj, true);
 		obj->base.read_domains = obj->base.write_domain = I915_GEM_DOMAIN_CPU;
 	}
@@ -2729,8 +2726,11 @@ __i915_gem_request_alloc(struct intel_engine_cs *engine,
 
 	*req_out = NULL;
 
-	ret = i915_gem_check_wedge(&dev_priv->gpu_error,
-				   dev_priv->mm.interruptible);
+	/* ABI: Before userspace accesses the GPU (e.g. execbuffer), report
+	 * EIO if the GPU is already wedged, or EAGAIN to drop the struct_mutex
+	 * and restart.
+	 */
+	ret = i915_gem_check_wedge(reset_counter, dev_priv->mm.interruptible);
 	if (ret)
 		return ret;
 
@@ -4165,9 +4165,9 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
 	if (ret)
 		return ret;
 
-	ret = i915_gem_check_wedge(&dev_priv->gpu_error, false);
-	if (ret)
-		return ret;
+	/* ABI: return -EIO if already wedged */
+	if (i915_terminally_wedged(&dev_priv->gpu_error))
+		return -EIO;
 
 	spin_lock(&file_priv->mm.lock);
 	list_for_each_entry(request, &file_priv->mm.request_list, client_list) {

drivers/gpu/drm/i915/i915_gem_userptr.c

@@ -112,10 +112,8 @@ static void cancel_userptr(struct work_struct *work)
 		was_interruptible = dev_priv->mm.interruptible;
 		dev_priv->mm.interruptible = false;
 
-		list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link) {
-			int ret = i915_vma_unbind(vma);
-			WARN_ON(ret && ret != -EIO);
-		}
+		list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link)
+			WARN_ON(i915_vma_unbind(vma));
 		WARN_ON(i915_gem_object_put_pages(obj));
 
 		dev_priv->mm.interruptible = was_interruptible;

drivers/gpu/drm/i915/intel_display.c

@@ -13436,11 +13436,9 @@ static int intel_atomic_prepare_commit(struct drm_device *dev,
 
 			ret = __i915_wait_request(intel_plane_state->wait_req,
 						  true, NULL, NULL);
-
-			/* Swallow -EIO errors to allow updates during hw lockup. */
-			if (ret == -EIO)
-				ret = 0;
 			if (ret) {
+				/* Any hang should be swallowed by the wait */
+				WARN_ON(ret == -EIO);
 				mutex_lock(&dev->struct_mutex);
 				drm_atomic_helper_cleanup_planes(dev, state);
 				mutex_unlock(&dev->struct_mutex);
@@ -13792,10 +13790,11 @@ intel_prepare_plane_fb(struct drm_plane *plane,
 		 */
 		if (needs_modeset(crtc_state))
 			ret = i915_gem_object_wait_rendering(old_obj, true);
-
-		/* Swallow -EIO errors to allow updates during hw lockup. */
-		if (ret && ret != -EIO)
+		if (ret) {
+			/* GPU hangs should have been swallowed by the wait */
+			WARN_ON(ret == -EIO);
 			return ret;
+		}
 	}
 
 	/* For framebuffer backed by dmabuf, wait for fence */

drivers/gpu/drm/i915/intel_lrc.c

@@ -1048,7 +1048,7 @@ void intel_logical_ring_stop(struct intel_engine_cs *engine)
 		return;
 
 	ret = intel_engine_idle(engine);
-	if (ret && !i915_reset_in_progress_or_wedged(&dev_priv->gpu_error))
+	if (ret)
 		DRM_ERROR("failed to quiesce %s whilst cleaning up: %d\n",
 			  engine->name, ret);
 

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -3184,8 +3184,7 @@ intel_stop_engine(struct intel_engine_cs *engine)
 		return;
 
 	ret = intel_engine_idle(engine);
-	if (ret &&
-	    !i915_reset_in_progress_or_wedged(&to_i915(engine->dev)->gpu_error))
+	if (ret)
 		DRM_ERROR("failed to quiesce %s whilst cleaning up: %d\n",
 			  engine->name, ret);