sound: Implement asynchronous device detach

Hot-unplugging a sound device, such as a USB sound card, whilst being
consumed by an application, results in an infinite loop until either the
application closes the device's file descriptor, or the channel
automatically times out after hw.snd.timeout seconds. In the case of a
detach however, the timeout approach is still not ideal, since we want
all resources to be released immediatelly, without waiting for N seconds
until we can use the bus again.

The timeout mechanism works by calling chn_sleep() in chn_read() and
chn_write() (see pcm/channel.c) in order to send the thread to sleep,
using cv_timedwait_sig(). Since chn_sleep() sets the CHN_F_SLEEPING flag
while waiting for cv_timedwait_sig() to return, we can test this flag in
pcm_unregister() (called during detach) and wakeup the sleeping
thread(s) to immediately kill the channel(s) being consumed.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 months
PR:		194727
Reviewed by:	dev_submerge.ch, bapt, markj
Differential Revision:	https://reviews.freebsd.org/D43545

(cherry picked from commit 44e128f)

share/man/man4/snd_uaudio.4

@@ -27,7 +27,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 29, 2024
+.Dd March 26, 2024
 .Dt SND_UAUDIO 4
 .Os
 .Sh NAME
@@ -156,15 +156,6 @@ and modified for
 by
 .An Hiten Pandya Aq Mt hmp@FreeBSD.org .
 .Sh BUGS
-The PCM framework in
-.Fx
-only supports synchronous device detach.
-That means all mixer and DSP character devices belonging to a given
-USB audio device must be closed when receiving an error on a DSP read,
-a DSP write or a DSP IOCTL request.
-Else the USB audio driver will wait for this to happen, preventing
-enumeration of new devices on the parenting USB controller.
-.Pp
 Some USB audio devices might refuse to work properly unless the sample
 rate is configured the same for both recording and playback, even if
 only simplex is used.

sys/dev/sound/pcm/dsp.c

@@ -274,7 +274,7 @@ dsp_close(void *data)
 
 	d = priv->sc;
 	/* At this point pcm_unregister() will destroy all channels anyway. */
-	if (!PCM_REGISTERED(d))
+	if (PCM_DETACHING(d))
 		goto skip;
 
 	PCM_GIANT_ENTER(d);

sys/dev/sound/pcm/mixer.c

@@ -817,17 +817,6 @@ mixer_uninit(device_t dev)
 	KASSERT(m->type == MIXER_TYPE_PRIMARY,
 	    ("%s(): illegal mixer type=%d", __func__, m->type));
 
-	snd_mtxlock(m->lock);
-
-	if (m->busy) {
-		snd_mtxunlock(m->lock);
-		return EBUSY;
-	}
-
-	/* destroy dev can sleep --hps */
-
-	snd_mtxunlock(m->lock);
-
 	pdev->si_drv1 = NULL;
 	destroy_dev(pdev);
 

sys/dev/sound/pcm/sound.c

@@ -1001,26 +1001,22 @@ pcm_unregister(device_t dev)
 
 	CHN_FOREACH(ch, d, channels.pcm) {
 		CHN_LOCK(ch);
-		if (ch->refcount > 0) {
-			device_printf(dev,
-			    "unregister: channel %s busy (pid %d)\n",
-			    ch->name, ch->pid);
-			CHN_UNLOCK(ch);
-			PCM_RELEASE_QUICK(d);
-			return (EBUSY);
+		if (ch->flags & CHN_F_SLEEPING) {
+			/*
+			 * We are detaching, so do not wait for the timeout in
+			 * chn_read()/chn_write(). Wake up the thread and kill
+			 * the channel immediately.
+			 */
+			CHN_BROADCAST(&ch->intr_cv);
+			ch->flags |= CHN_F_DEAD;
 		}
+		chn_abort(ch);
 		CHN_UNLOCK(ch);
 	}
 
 	dsp_destroy_dev(dev);
 
-	if (mixer_uninit(dev) == EBUSY) {
-		device_printf(dev, "unregister: mixer busy\n");
-		PCM_LOCK(d);
-		PCM_RELEASE(d);
-		PCM_UNLOCK(d);
-		return (EBUSY);
-	}
+	(void)mixer_uninit(dev);
 
 	/* remove /dev/sndstat entry first */
 	sndstat_unregister(dev);

sys/dev/sound/usb/uaudio.c

@@ -1255,20 +1255,13 @@ uaudio_detach_sub(device_t dev)
 	unsigned i = uaudio_get_child_index_by_dev(sc, dev);
 	int error = 0;
 
-repeat:
 	if (sc->sc_child[i].pcm_registered) {
 		error = pcm_unregister(dev);
-	} else {
-		if (sc->sc_child[i].mixer_init)
-			error = mixer_uninit(dev);
+	} else if (sc->sc_child[i].mixer_init) {
+		error = mixer_uninit(dev);
 	}
 
-	if (error) {
-		device_printf(dev, "Waiting for sound application to exit!\n");
-		usb_pause_mtx(NULL, 2 * hz);
-		goto repeat;		/* try again */
-	}
-	return (0);			/* success */
+	return (error);
 }
 
 static int