drm/i915: Give each sw_fence its own lockclass

Localise the static struct lock_class_key to the caller of
i915_sw_fence_init() so that we create a lock_class instance for each
unique sw_fence rather than all sw_fences sharing the same
lock_class. This eliminate some lockdep false positive when using fences
from within fence callbacks.

For the relatively small number of fences currently in use [2], this adds
160 bytes of unused text/code when lockdep is disabled. This seems
quite high, but fully reducing it via ifdeffery is also quite ugly.
Removing the #fence strings saves 72 bytes with just a single #ifdef.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114204105.29171-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_sw_fence.c

@@ -116,11 +116,14 @@ static void i915_sw_fence_await(struct i915_sw_fence *fence)
 	WARN_ON(atomic_inc_return(&fence->pending) <= 1);
 }
 
-void i915_sw_fence_init(struct i915_sw_fence *fence, i915_sw_fence_notify_t fn)
+void __i915_sw_fence_init(struct i915_sw_fence *fence,
+			  i915_sw_fence_notify_t fn,
+			  const char *name,
+			  struct lock_class_key *key)
 {
 	BUG_ON((unsigned long)fn & ~I915_SW_FENCE_MASK);
 
-	init_waitqueue_head(&fence->wait);
+	__init_waitqueue_head(&fence->wait, name, key);
 	kref_init(&fence->kref);
 	atomic_set(&fence->pending, 1);
 	fence->flags = (unsigned long)fn;

drivers/gpu/drm/i915/i915_sw_fence.h

@@ -40,7 +40,22 @@ typedef int (*i915_sw_fence_notify_t)(struct i915_sw_fence *,
 				      enum i915_sw_fence_notify state);
 #define __i915_sw_fence_call __aligned(4)
 
-void i915_sw_fence_init(struct i915_sw_fence *fence, i915_sw_fence_notify_t fn);
+void __i915_sw_fence_init(struct i915_sw_fence *fence,
+			  i915_sw_fence_notify_t fn,
+			  const char *name,
+			  struct lock_class_key *key);
+#ifdef CONFIG_LOCKDEP
+#define i915_sw_fence_init(fence, fn)				\
+do {								\
+	static struct lock_class_key __key;			\
+								\
+	__i915_sw_fence_init((fence), (fn), #fence, &__key);	\
+} while (0)
+#else
+#define i915_sw_fence_init(fence, fn)				\
+	__i915_sw_fence_init((fence), (fn), NULL, NULL)
+#endif
+
 void i915_sw_fence_commit(struct i915_sw_fence *fence);
 
 int i915_sw_fence_await_sw_fence(struct i915_sw_fence *fence,