drm/i915/gt: Give engine->kernel_context distinct timeline lock classes

Assign a separate lockclass to the perma-pinned timelines of the
kernel_context, such that we can use them from within the user timelines
should we ever need to inject GPU operations to fixup faults during
request construction.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008185941.15228-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -736,6 +736,7 @@ intel_engine_init_active(struct intel_engine_cs *engine, unsigned int subclass)
 static struct intel_context *
 create_kernel_context(struct intel_engine_cs *engine)
 {
+	static struct lock_class_key kernel;
 	struct intel_context *ce;
 	int err;
 
@@ -751,6 +752,14 @@ create_kernel_context(struct intel_engine_cs *engine)
 		return ERR_PTR(err);
 	}
 
+	/*
+	 * Give our perma-pinned kernel timelines a separate lockdep class,
+	 * so that we can use them from within the normal user timelines
+	 * should we need to inject GPU operations during their request
+	 * construction.
+	 */
+	lockdep_set_class(&ce->timeline->mutex, &kernel);
+
 	return ce;
 }