drm/i915: clear up wedged transitions

We have two important transitions of the wedged state in the current
code:

- 0 -> 1: This means a hang has been detected, and signals to everyone
  that they please get of any locks, so that the reset work item can
  do its job.

- 1 -> 0: The reset handler has completed.

Now the last transition mixes up two states: "Reset completed and
successful" and "Reset failed". To distinguish these two we do some
tricks with the reset completion, but I simply could not convince
myself that this doesn't race under odd circumstances.

Hence split this up, and add a new terminal state indicating that the
hw is gone for good.

Also add explicit #defines for both states, update comments.

v2: Split out the reset handling bugfix for the throttle ioctl.

v3: s/tmp/wedged/ sugested by Chris Wilson. Also fixup up a rebase
error which prevented this patch from actually compiling.

v4: To unify the wedged state with the reset counter, keep the
reset-in-progress state just as a flag. The terminally-wedged state is
now denoted with a big number.

v5: Add a comment to the reset_counter special values explaining that
WEDGED & RESET_IN_PROGRESS needs to be true for the code to be
correct.

v6: Fixup logic errors introduced with the wedged+reset_counter
unification. Since WEDGED implies reset-in-progress (in a way we're
terminally stuck in the dead-but-reset-not-completed state), we need
ensure that we check for this everywhere. The specific bug was in
wait_for_error, which would simply have timed out.

v7: Extract an inline i915_reset_in_progress helper to make the code
more readable. Also annote the reset-in-progress case with an
unlikely, to help the compiler optimize the fastpath. Do the same for
the terminally wedged case with i915_terminally_wedged.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1672,7 +1672,7 @@ i915_wedged_read(struct file *filp,
 
 	len = snprintf(buf, sizeof(buf),
 		       "wedged :  %d\n",
-		       atomic_read(&dev_priv->gpu_error.wedged));
+		       atomic_read(&dev_priv->gpu_error.reset_counter));
 
 	if (len > sizeof(buf))
 		len = sizeof(buf);

drivers/gpu/drm/i915/i915_drv.h

@@ -771,11 +771,37 @@ struct i915_gpu_error {
 	/* Protected by the above dev->gpu_error.lock. */
 	struct drm_i915_error_state *first_error;
 	struct work_struct work;
-	struct completion completion;
 
 	unsigned long last_reset;
 
-	atomic_t wedged;
+	/**
+	 * State variable controlling the reset flow
+	 *
+	 * Upper bits are for the reset counter.
+	 *
+	 * Lowest bit controls the reset state machine: Set means a reset is in
+	 * progress. This state will (presuming we don't have any bugs) decay
+	 * into either unset (successful reset) or the special WEDGED value (hw
+	 * terminally sour). All waiters on the reset_queue will be woken when
+	 * that happens.
+	 */
+	atomic_t reset_counter;
+
+	/**
+	 * Special values/flags for reset_counter
+	 *
+	 * Note that the code relies on
+	 * 	I915_WEDGED & I915_RESET_IN_PROGRESS_FLAG
+	 * being true.
+	 */
+#define I915_RESET_IN_PROGRESS_FLAG	1
+#define I915_WEDGED			0xffffffff
+
+	/**
+	 * Waitqueue to signal when the reset has completed. Used by clients
+	 * that wait for dev_priv->mm.wedged to settle.
+	 */
+	wait_queue_head_t reset_queue;
 
 	/* For gpu hang simulation. */
 	unsigned int stop_rings;
@@ -1543,6 +1569,16 @@ void i915_gem_retire_requests(struct drm_device *dev);
 void i915_gem_retire_requests_ring(struct intel_ring_buffer *ring);
 int __must_check i915_gem_check_wedge(struct i915_gpu_error *error,
 				      bool interruptible);
+static inline bool i915_reset_in_progress(struct i915_gpu_error *error)
+{
+	return unlikely(atomic_read(&error->reset_counter)
+			& I915_RESET_IN_PROGRESS_FLAG);
+}
+
+static inline bool i915_terminally_wedged(struct i915_gpu_error *error)
+{
+	return atomic_read(&error->reset_counter) == I915_WEDGED;
+}
 
 void i915_gem_reset(struct drm_device *dev);
 void i915_gem_clflush_object(struct drm_i915_gem_object *obj);

drivers/gpu/drm/i915/i915_gem.c

@@ -89,36 +89,32 @@ static void i915_gem_info_remove_obj(struct drm_i915_private *dev_priv,
 static int
 i915_gem_wait_for_error(struct i915_gpu_error *error)
 {
-	struct completion *x = &error->completion;
-	unsigned long flags;
 	int ret;
 
-	if (!atomic_read(&error->wedged))
+#define EXIT_COND (!i915_reset_in_progress(error))
+	if (EXIT_COND)
 		return 0;
 
+	/* GPU is already declared terminally dead, give up. */
+	if (i915_terminally_wedged(error))
+		return -EIO;
+
 	/*
 	 * Only wait 10 seconds for the gpu reset to complete to avoid hanging
 	 * userspace. If it takes that long something really bad is going on and
 	 * we should simply try to bail out and fail as gracefully as possible.
 	 */
-	ret = wait_for_completion_interruptible_timeout(x, 10*HZ);
+	ret = wait_event_interruptible_timeout(error->reset_queue,
+					       EXIT_COND,
+					       10*HZ);
 	if (ret == 0) {
 		DRM_ERROR("Timed out waiting for the gpu reset to complete\n");
 		return -EIO;
 	} else if (ret < 0) {
 		return ret;
 	}
+#undef EXIT_COND
 
-	if (atomic_read(&error->wedged)) {
-		/* GPU is hung, bump the completion count to account for
-		 * the token we just consumed so that we never hit zero and
-		 * end up waiting upon a subsequent completion event that
-		 * will never happen.
-		 */
-		spin_lock_irqsave(&x->wait.lock, flags);
-		x->done++;
-		spin_unlock_irqrestore(&x->wait.lock, flags);
-	}
 	return 0;
 }
 
@@ -942,23 +938,14 @@ int
 i915_gem_check_wedge(struct i915_gpu_error *error,
 		     bool interruptible)
 {
-	if (atomic_read(&error->wedged)) {
-		struct completion *x = &error->completion;
-		bool recovery_complete;
-		unsigned long flags;
-
-		/* Give the error handler a chance to run. */
-		spin_lock_irqsave(&x->wait.lock, flags);
-		recovery_complete = x->done > 0;
-		spin_unlock_irqrestore(&x->wait.lock, flags);
-
+	if (i915_reset_in_progress(error)) {
 		/* Non-interruptible callers can't handle -EAGAIN, hence return
 		 * -EIO unconditionally for these. */
 		if (!interruptible)
 			return -EIO;
 
-		/* Recovery complete, but still wedged means reset failure. */
-		if (recovery_complete)
+		/* Recovery complete, but the reset failed ... */
+		if (i915_terminally_wedged(error))
 			return -EIO;
 
 		return -EAGAIN;
@@ -1025,7 +1012,7 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
 
 #define EXIT_COND \
 	(i915_seqno_passed(ring->get_seqno(ring, false), seqno) || \
-	atomic_read(&dev_priv->gpu_error.wedged))
+	i915_reset_in_progress(&dev_priv->gpu_error))
 	do {
 		if (interruptible)
 			end = wait_event_interruptible_timeout(ring->irq_queue,
@@ -1379,7 +1366,7 @@ out:
 		/* If this -EIO is due to a gpu hang, give the reset code a
 		 * chance to clean up the mess. Otherwise return the proper
 		 * SIGBUS. */
-		if (!atomic_read(&dev_priv->gpu_error.wedged))
+		if (i915_terminally_wedged(&dev_priv->gpu_error))
 			return VM_FAULT_SIGBUS;
 	case -EAGAIN:
 		/* Give the error handler a chance to run and move the
@@ -3983,9 +3970,9 @@ i915_gem_entervt_ioctl(struct drm_device *dev, void *data,
 	if (drm_core_check_feature(dev, DRIVER_MODESET))
 		return 0;
 
-	if (atomic_read(&dev_priv->gpu_error.wedged)) {
+	if (i915_reset_in_progress(&dev_priv->gpu_error)) {
 		DRM_ERROR("Reenabling wedged hardware, good luck\n");
-		atomic_set(&dev_priv->gpu_error.wedged, 0);
+		atomic_set(&dev_priv->gpu_error.reset_counter, 0);
 	}
 
 	mutex_lock(&dev->struct_mutex);
@@ -4069,7 +4056,7 @@ i915_gem_load(struct drm_device *dev)
 		INIT_LIST_HEAD(&dev_priv->fence_regs[i].lru_list);
 	INIT_DELAYED_WORK(&dev_priv->mm.retire_work,
 			  i915_gem_retire_work_handler);
-	init_completion(&dev_priv->gpu_error.completion);
+	init_waitqueue_head(&dev_priv->gpu_error.reset_queue);
 
 	/* On GEN3 we really need to make sure the ARB C3 LP bit is set */
 	if (IS_GEN3(dev)) {

drivers/gpu/drm/i915/i915_irq.c

@@ -862,8 +862,10 @@ done:
  */
 static void i915_error_work_func(struct work_struct *work)
 {
-	drm_i915_private_t *dev_priv = container_of(work, drm_i915_private_t,
-						    gpu_error.work);
+	struct i915_gpu_error *error = container_of(work, struct i915_gpu_error,
+						    work);
+	drm_i915_private_t *dev_priv = container_of(error, drm_i915_private_t,
+						    gpu_error);
 	struct drm_device *dev = dev_priv->dev;
 	char *error_event[] = { "ERROR=1", NULL };
 	char *reset_event[] = { "RESET=1", NULL };
@@ -871,14 +873,18 @@ static void i915_error_work_func(struct work_struct *work)
 
 	kobject_uevent_env(&dev->primary->kdev.kobj, KOBJ_CHANGE, error_event);
 
-	if (atomic_read(&dev_priv->gpu_error.wedged)) {
+	if (i915_reset_in_progress(error)) {
 		DRM_DEBUG_DRIVER("resetting chip\n");
 		kobject_uevent_env(&dev->primary->kdev.kobj, KOBJ_CHANGE, reset_event);
+
 		if (!i915_reset(dev)) {
-			atomic_set(&dev_priv->gpu_error.wedged, 0);
+			atomic_set(&error->reset_counter, 0);
 			kobject_uevent_env(&dev->primary->kdev.kobj, KOBJ_CHANGE, reset_done_event);
+		} else {
+			atomic_set(&error->reset_counter, I915_WEDGED);
 		}
-		complete_all(&dev_priv->gpu_error.completion);
+
+		wake_up_all(&dev_priv->gpu_error.reset_queue);
 	}
 }
 
@@ -1482,11 +1488,12 @@ void i915_handle_error(struct drm_device *dev, bool wedged)
 	i915_report_and_clear_eir(dev);
 
 	if (wedged) {
-		INIT_COMPLETION(dev_priv->gpu_error.completion);
-		atomic_set(&dev_priv->gpu_error.wedged, 1);
+		atomic_set(&dev_priv->gpu_error.reset_counter,
+			   I915_RESET_IN_PROGRESS_FLAG);
 
 		/*
-		 * Wakeup waiting processes so they don't hang
+		 * Wakeup waiting processes so that the reset work item
+		 * doesn't deadlock trying to grab various locks.
 		 */
 		for_each_ring(ring, dev_priv, i)
 			wake_up_all(&ring->irq_queue);

drivers/gpu/drm/i915/intel_display.c

@@ -2223,7 +2223,7 @@ intel_finish_fb(struct drm_framebuffer *old_fb)
 	WARN_ON(waitqueue_active(&dev_priv->pending_flip_queue));
 
 	wait_event(dev_priv->pending_flip_queue,
-		   atomic_read(&dev_priv->gpu_error.wedged) ||
+		   i915_reset_in_progress(&dev_priv->gpu_error) ||
 		   atomic_read(&obj->pending_flip) == 0);
 
 	/* Big Hammer, we also need to ensure that any pending
@@ -2871,7 +2871,7 @@ static bool intel_crtc_has_pending_flip(struct drm_crtc *crtc)
 	unsigned long flags;
 	bool pending;
 
-	if (atomic_read(&dev_priv->gpu_error.wedged))
+	if (i915_reset_in_progress(&dev_priv->gpu_error))
 		return false;
 
 	spin_lock_irqsave(&dev->event_lock, flags);