ALSA: Add simple refcount helper functions

There are many open-code to manage the same pattern for refcount +
wakeup sync at closing.  Let's provide the common helper functions to
replace the open-code.

- The recount is kept in struct snd_refcount, where it's initialized
by snd_refcount_init().
- The user can simply reference or unreference via snd_refcount_get()
and snd_refcount_put() functions
- The user can wait for the all usages gone by snd_refcount_sync()

Note that here we use atomic_t instead of refcount_t since the current
users allow reusing the refcount after sync again.  The design of
refcount_t prevents exactly this behavior, so it doesn't fit.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260610154538.51076-2-tiwai@suse.de

1 files changed, 21 insertions, 0 deletions

diff --git a/include/sound/core.h b/include/sound/core.h
index 8b2ca95d13f7..a3f10294e834 100644
--- a/include/sound/core.h
+++ b/include/sound/core.h
@@ -75,6 +75,27 @@ struct snd_device {
 
 #define snd_device(n) list_entry(n, struct snd_device, list)
 
+/*
+ * A simple reference counter with a wait queue;
+ * typically used for usage counts, and you can synchronize at finishing
+ * via snd_refcount_sync(), which is woken up when the refcount reaches to
+ * zero again.
+ */
+struct snd_refcount {
+	atomic_t count;
+	wait_queue_head_t waiter;
+};
+
+void snd_refcount_init(struct snd_refcount *ref);
+
+static inline void snd_refcount_get(struct snd_refcount *ref)
+{
+	atomic_inc(&ref->count);
+}
+
+void snd_refcount_put(struct snd_refcount *ref);
+void snd_refcount_sync(struct snd_refcount *ref);
+
 /* main structure for soundcard */
 
 struct snd_card {