[media] v4l2-ctrls: eliminate lockdep false alarms for struct v4l2_ctrl_handler.lock

When calling v4l2_ctrl_add_handler(), lockdep would detect a potential
recursive locking problem on a situation that is by design intended and
not a recursive lock.  This happened because all struct
v4l2_ctrl_handler.lock mutexes were created as members of the same lock
class in v4l2_ctrl_handler_init(), and v4l2_ctrl_add_handler() takes the
hdl->lock on two different v4l2_ctrl_handler instances.
This change breaks the large lockdep lock class for struct
v4l2_ctrl_handler.lock and breaks it into v4l2_ctrl_handler
instantiation specific lock classes with meaningful class names.
This will validly eliminate lockdep alarms for v4l2_ctrl_handler locking
validation, as long as the relationships between drivers adding v4l2
controls to their own handler from other v4l2 drivers' control handlers
remains straightforward.
struct v4l2_ctrl_handler.lock lock classes are created with names such
that the output of cat /proc/lockdep indicates where in the v4l2 driver
code v4l2_ctrl_handle_init() is being called on instantiations:
ffffffffa045f490 FD:   10 BD:    8 +.+...: cx2341x:1534:(hdl)->lock
ffffffffa0497d20 FD:   12 BD:    2 +.+.+.: saa7115:1581:(hdl)->lock
ffffffffa04ac660 FD:   14 BD:    2 +.+.+.: msp3400_driver:756:(hdl)->lock
ffffffffa0484b90 FD:   12 BD:    1 +.+.+.: ivtv_gpio:366:(&itv->hdl_gpio)->lock
ffffffffa04eb530 FD:   11 BD:    2 +.+.+.: cx25840_core:1982:(&state->hdl)->lock
ffffffffa04fbc80 FD:   11 BD:    3 +.+.+.: wm8775:246:(&state->hdl)->lock
Some lock chains, that were previously causing the recursion alarms, are
now visible in the output of cat /proc/lockdep_chains:
irq_context: 0
[ffffffffa0497d20] saa7115:1581:(hdl)->lock
[ffffffffa045f490] cx2341x:1534:(hdl)->lock
irq_context: 0
[ffffffffa04ac660] msp3400_driver:756:(hdl)->lock
[ffffffffa045f490] cx2341x:1534:(hdl)->lock
irq_context: 0
[ffffffffa0484b90] ivtv_gpio:366:(&itv->hdl_gpio)->lock
[ffffffffa045f490] cx2341x:1534:(hdl)->lock
irq_context: 0
[ffffffffa04eb530] cx25840_core:1982:(&state->hdl)->lock
[ffffffffa045f490] cx2341x:1534:(hdl)->lock
irq_context: 0
[ffffffffa04fbc80] wm8775:246:(&state->hdl)->lock
[ffffffffa045f490] cx2341x:1534:(hdl)->lock
Signed-off-by: Andy Walls <awalls@md.metrocast.net>
[hans.verkuil@cisco.com: keep mutex_init in v4l2_ctrl_handler_init_class]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/v4l2-ctrls.c

@@ -1362,11 +1362,13 @@ static inline int handler_set_err(struct v4l2_ctrl_handler *hdl, int err)
 }
 
 /* Initialize the handler */
-int v4l2_ctrl_handler_init(struct v4l2_ctrl_handler *hdl,
-			   unsigned nr_of_controls_hint)
+int v4l2_ctrl_handler_init_class(struct v4l2_ctrl_handler *hdl,
+				 unsigned nr_of_controls_hint,
+				 struct lock_class_key *key, const char *name)
 {
 	hdl->lock = &hdl->_lock;
 	mutex_init(hdl->lock);
+	lockdep_set_class_and_name(hdl->lock, key, name);
 	INIT_LIST_HEAD(&hdl->ctrls);
 	INIT_LIST_HEAD(&hdl->ctrl_refs);
 	hdl->nr_of_buckets = 1 + nr_of_controls_hint / 8;
@@ -1375,7 +1377,7 @@ int v4l2_ctrl_handler_init(struct v4l2_ctrl_handler *hdl,
 	hdl->error = hdl->buckets ? 0 : -ENOMEM;
 	return hdl->error;
 }
-EXPORT_SYMBOL(v4l2_ctrl_handler_init);
+EXPORT_SYMBOL(v4l2_ctrl_handler_init_class);
 
 /* Free all controls and control refs */
 void v4l2_ctrl_handler_free(struct v4l2_ctrl_handler *hdl)

include/media/v4l2-ctrls.h

@@ -259,7 +259,7 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
 		    s32 *min, s32 *max, s32 *step, s32 *def, u32 *flags);
 
 
-/** v4l2_ctrl_handler_init() - Initialize the control handler.
+/** v4l2_ctrl_handler_init_class() - Initialize the control handler.
   * @hdl:	The control handler.
   * @nr_of_controls_hint: A hint of how many controls this handler is
   *		expected to refer to. This is the total number, so including
@@ -268,12 +268,35 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
   *		are allocated) or the control lookup becomes slower (not enough
   *		buckets are allocated, so there are more slow list lookups).
   *		It will always work, though.
+  * @key:	Used by the lock validator if CONFIG_LOCKDEP is set.
+  * @name:	Used by the lock validator if CONFIG_LOCKDEP is set.
   *
   * Returns an error if the buckets could not be allocated. This error will
   * also be stored in @hdl->error.
+  *
+  * Never use this call directly, always use the v4l2_ctrl_handler_init
+  * macro that hides the @key and @name arguments.
   */
-int v4l2_ctrl_handler_init(struct v4l2_ctrl_handler *hdl,
-			   unsigned nr_of_controls_hint);
+int v4l2_ctrl_handler_init_class(struct v4l2_ctrl_handler *hdl,
+				 unsigned nr_of_controls_hint,
+				 struct lock_class_key *key, const char *name);
+
+#ifdef CONFIG_LOCKDEP
+#define v4l2_ctrl_handler_init(hdl, nr_of_controls_hint)		\
+(									\
+	({								\
+		static struct lock_class_key _key;			\
+		v4l2_ctrl_handler_init_class(hdl, nr_of_controls_hint,	\
+					&_key,				\
+					KBUILD_BASENAME ":"		\
+					__stringify(__LINE__) ":"	\
+					"(" #hdl ")->_lock");		\
+	})								\
+)
+#else
+#define v4l2_ctrl_handler_init(hdl, nr_of_controls_hint)		\
+	v4l2_ctrl_handler_init_class(hdl, nr_of_controls_hint, NULL, NULL)
+#endif
 
 /** v4l2_ctrl_handler_free() - Free all controls owned by the handler and free
   * the control list.