drm/i915: Put all non-blocking modesets onto an ordered wq

We have plenty of global registers and whatnot programmed without
any further locking by the modeset code. Currently non-bocking
modesets are allowed to execute in parallel which could corrupt
said registers.

To avoid the problem let's run all non-blocking modesets on an
ordered workqueue. We still put page flips etc. to system_unbound_wq
allowing page flips on one pipe to execute in parallel with page flips
or a modeset on a another pipe (assuming no known state is shared
between them, at which point they would have been added to the same
atomic commit and serialized that way).

Blocking modesets are already serialized with each other by
connection_mutex, and thus are safe. To serialize them with
non-blocking modesets we just flush the workqueue before executing
blocking modesets.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 94f0502 ("drm/i915: nonblocking commit")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113133622.8593-1-ville.syrjala@linux.intel.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -1960,6 +1960,9 @@ struct drm_i915_private {
 	 */
 	struct workqueue_struct *wq;
 
+	/* ordered wq for modesets */
+	struct workqueue_struct *modeset_wq;
+
 	/* Display functions */
 	struct drm_i915_display_funcs display;
 

drivers/gpu/drm/i915/intel_display.c

@@ -12483,11 +12483,15 @@ static int intel_atomic_commit(struct drm_device *dev,
 	INIT_WORK(&state->commit_work, intel_atomic_commit_work);
 
 	i915_sw_fence_commit(&intel_state->commit_ready);
-	if (nonblock)
+	if (nonblock && intel_state->modeset) {
+		queue_work(dev_priv->modeset_wq, &state->commit_work);
+	} else if (nonblock) {
 		queue_work(system_unbound_wq, &state->commit_work);
-	else
+	} else {
+		if (intel_state->modeset)
+			flush_workqueue(dev_priv->modeset_wq);
 		intel_atomic_commit_tail(state);
-
+	}
 
 	return 0;
 }
@@ -14450,6 +14454,8 @@ int intel_modeset_init(struct drm_device *dev)
 	enum pipe pipe;
 	struct intel_crtc *crtc;
 
+	dev_priv->modeset_wq = alloc_ordered_workqueue("i915_modeset", 0);
+
 	drm_mode_config_init(dev);
 
 	dev->mode_config.min_width = 0;
@@ -15254,6 +15260,8 @@ void intel_modeset_cleanup(struct drm_device *dev)
 	intel_cleanup_gt_powersave(dev_priv);
 
 	intel_teardown_gmbus(dev_priv);
+
+	destroy_workqueue(dev_priv->modeset_wq);
 }
 
 void intel_connector_attach_encoder(struct intel_connector *connector,