ALSA: usb-audio: Properly refcounting clock rate

We fixed the bug introduced by the patch for managing the shared
clocks at the commit 809f44a0 ("ALSA: usb-audio: Clear fixed clock
rate at closing EP"), but it was merely a workaround.  By this change,
the clock reference rate is cleared at each EP close, hence the still
remaining EP may need a re-setup of rate unnecessarily.

This patch introduces the proper refcounting for the clock reference
object so that the clock setup is done only when needed.

Fixes: 809f44a0 ("ALSA: usb-audio: Clear fixed clock rate at closing EP")
Fixes: c11117b6 ("ALSA: usb-audio: Refcount multiple accesses on the single clock")
Link: https://lore.kernel.org/r/20220920181126.4912-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -39,6 +39,7 @@ struct snd_usb_iface_ref {
 struct snd_usb_clock_ref {
 	unsigned char clock;
 	atomic_t locked;
+	int opened;
 	int rate;
 	bool need_setup;
 	struct list_head list;
@@ -803,6 +804,7 @@ snd_usb_endpoint_open(struct snd_usb_audio *chip,
 				ep = NULL;
 				goto unlock;
 			}
+			ep->clock_ref->opened++;
 		}
 
 		ep->cur_audiofmt = fp;
@@ -926,8 +928,10 @@ void snd_usb_endpoint_close(struct snd_usb_audio *chip,
 		endpoint_set_interface(chip, ep, false);
 
 	if (!--ep->opened) {
-		if (ep->clock_ref && !atomic_read(&ep->clock_ref->locked))
-			ep->clock_ref->rate = 0;
+		if (ep->clock_ref) {
+			if (!--ep->clock_ref->opened)
+				ep->clock_ref->rate = 0;
+		}
 		ep->iface = 0;
 		ep->altsetting = 0;
 		ep->cur_audiofmt = NULL;
@@ -1649,8 +1653,7 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep, bool keep_pending)
 			WRITE_ONCE(ep->sync_source->sync_sink, NULL);
 		stop_urbs(ep, false, keep_pending);
 		if (ep->clock_ref)
-			if (!atomic_dec_return(&ep->clock_ref->locked))
-				ep->clock_ref->rate = 0;
+			atomic_dec(&ep->clock_ref->locked);
 	}
 }