coresight: Remove atomic type from refcnt

Refcnt is only ever accessed from either inside the coresight_mutex, or
the device's spinlock, making the atomic type and atomic_dec_return()
calls confusing and unnecessary. The only point of synchronisation
outside of these two types of locks is already done with a compare and
swap on 'mode', which a comment has been added for.

There was one instance of refcnt being used outside of a lock in TPIU,
but that can easily be fixed by making it the same as all the other
devices and adding a spinlock. Potentially in the future all the
refcounting and locking can be moved up into the core code, and all the
mostly duplicate code from the individual devices can be removed.

Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20240129154050.569566-8-james.clark@arm.com
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/include/linux/coresight.h b/include/linux/coresight.h
index 365b28022c5b..74bcec526aa9 100644
--- a/include/linux/coresight.h
+++ b/include/linux/coresight.h
@@ -230,8 +230,15 @@ struct coresight_sysfs_link {
  *		actually an 'enum cs_mode', but is stored in an atomic type.
  *		This is always accessed through local_read() and local_set(),
  *		but wherever it's done from within the Coresight device's lock,
- *		a non-atomic read would also work.
- * @refcnt:	keep track of what is in use.
+ *		a non-atomic read would also work. This is the main point of
+ *		synchronisation between code happening inside the sysfs mode's
+ *		coresight_mutex and outside when running in Perf mode. A compare
+ *		and exchange swap is done to atomically claim one mode or the
+ *		other.
+ * @refcnt:	keep track of what is in use. Only access this outside of the
+ *		device's spinlock when the coresight_mutex held and mode ==
+ *		CS_MODE_SYSFS. Otherwise it must be accessed from inside the
+ *		spinlock.
  * @orphan:	true if the component has connections that haven't been linked.
  * @sysfs_sink_activated: 'true' when a sink has been selected for use via sysfs
  *		by writing a 1 to the 'enable_sink' file.  A sink can be
@@ -257,7 +264,7 @@ struct coresight_device {
 	struct csdev_access access;
 	struct device dev;
 	local_t	mode;
-	atomic_t refcnt;
+	int refcnt;
 	bool orphan;
 	/* sink specific fields */
 	bool sysfs_sink_activated;