git: 24576c39789c - main - sound: track kqueue low watermark per-knote for mmaped channels

The branch main has been updated by christos:

URL: https://cgit.FreeBSD.org/src/commit/?id=24576c39789ca13846b9450155923ee0e377aeb4

commit 24576c39789ca13846b9450155923ee0e377aeb4
Author:     Goran Mekić <meka@tilda.center>
AuthorDate: 2026-07-06 12:06:17 +0000
Commit:     Christos Margiolis <christos@FreeBSD.org>
CommitDate: 2026-07-06 12:07:41 +0000

    sound: track kqueue low watermark per-knote for mmaped channels
    
    Use kn->kn_sdata to track the last bs->total value for each knote
    attached to an mmaped channel. An event is delivered only when the total
    byte counter has advanced by at least c->lw since the last delivery.
    After delivery kn_sdata is updated to the current total.
    
    Each knote tracks its own watermark independently, so multiple knotes
    attached to the same mmaped channel all receive events correctly.
    
    Non-mmap channels keep the existing level-triggered behavior via
    chn_polltrigger().
    
    MFC after:      1 week
    Reviewed by:    christos
    Differential Revision:  https://reviews.freebsd.org/D57833
---
 sys/dev/sound/pcm/channel.c | 14 +++++++++-----
 sys/dev/sound/pcm/channel.h |  2 +-
 sys/dev/sound/pcm/dsp.c     | 14 +++++++++++++-
 3 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/sys/dev/sound/pcm/channel.c b/sys/dev/sound/pcm/channel.c
index a0ee16a14386..40a08d48fb21 100644
--- a/sys/dev/sound/pcm/channel.c
+++ b/sys/dev/sound/pcm/channel.c
@@ -270,11 +270,15 @@ chn_lockdestroy(struct pcm_channel *c)
 /**
  * @brief Determine channel is ready for I/O
  *
+ * @param ref_total	for mmaped channels, the previous byte counter
+ *	 *		snapshot from which to compute the delta to
+ *	 *		bufsoft->total; ignored for non-mmaped channels.
+ *
  * @retval 1 = ready for I/O
  * @retval 0 = not ready for I/O
  */
 int
-chn_polltrigger(struct pcm_channel *c)
+chn_polltrigger(struct pcm_channel *c, u_int64_t ref_total)
 {
 	struct snd_dbuf *bs = c->bufsoft;
 	u_int delta;
@@ -282,10 +286,10 @@ chn_polltrigger(struct pcm_channel *c)
 	CHN_LOCKASSERT(c);
 
 	if (c->flags & CHN_F_MMAP) {
-		if (bs->prev_total < c->lw)
+		if (ref_total < c->lw)
 			delta = c->lw;
 		else
-			delta = bs->total - bs->prev_total;
+			delta = bs->total - ref_total;
 	} else {
 		if (c->direction == PCMDIR_PLAY)
 			delta = sndbuf_getfree(bs);
@@ -316,7 +320,7 @@ chn_wakeup(struct pcm_channel *c)
 
 	if (CHN_EMPTY(c, children.busy)) {
 		KNOTE_LOCKED(&bs->sel.si_note, 0);
-		if (SEL_WAITING(&bs->sel) && chn_polltrigger(c))
+		if (SEL_WAITING(&bs->sel) && chn_polltrigger(c, bs->prev_total))
 			selwakeuppri(&bs->sel, PRIBIO);
 		CHN_BROADCAST(&c->intr_cv);
 	} else {
@@ -830,7 +834,7 @@ chn_poll(struct pcm_channel *c, int ev, struct thread *td)
 	}
 
 	ret = 0;
-	if (chn_polltrigger(c)) {
+	if (chn_polltrigger(c, bs->prev_total)) {
 		chn_pollreset(c);
 		ret = ev;
 	} else
diff --git a/sys/dev/sound/pcm/channel.h b/sys/dev/sound/pcm/channel.h
index 6fa4338dce4a..c7f5bf93b8e5 100644
--- a/sys/dev/sound/pcm/channel.h
+++ b/sys/dev/sound/pcm/channel.h
@@ -260,7 +260,7 @@ int chn_read(struct pcm_channel *c, struct uio *buf);
 u_int32_t chn_start(struct pcm_channel *c, int force);
 int chn_sync(struct pcm_channel *c, int threshold);
 int chn_flush(struct pcm_channel *c);
-int chn_polltrigger(struct pcm_channel *c);
+int chn_polltrigger(struct pcm_channel *c, u_int64_t ref_total);
 int chn_poll(struct pcm_channel *c, int ev, struct thread *td);
 
 char *chn_mkname(char *buf, size_t len, struct pcm_channel *c);
diff --git a/sys/dev/sound/pcm/dsp.c b/sys/dev/sound/pcm/dsp.c
index 8bb07d87fac9..da8bf485bc9b 100644
--- a/sys/dev/sound/pcm/dsp.c
+++ b/sys/dev/sound/pcm/dsp.c
@@ -3010,7 +3010,13 @@ dsp_kqevent(struct knote *kn, long hint)
 		return (1);
 	}
 	kn->kn_data = 0;
-	if (chn_polltrigger(ch)) {
+	/*
+	 * For mmaped channels pass the knote's own reference point so the
+	 * low watermark is tracked per-knote.  Non-mmaped channels ignore
+	 * the reference and fire based on the current amount of ready/free
+	 * data in the buffer, so all knotes see the same live state.
+	 */
+	if (chn_polltrigger(ch, (u_int64_t)kn->kn_sdata)) {
 		if (kn->kn_filter == EVFILT_READ) {
 			kn->kn_data = sndbuf_getready(ch->bufsoft);
 			if (ch->flags & CHN_F_MMAP)
@@ -3025,6 +3031,7 @@ dsp_kqevent(struct knote *kn, long hint)
 				kn->kn_kevent.ext[0] = sndbuf_getready(ch->bufsoft) / ch->bufsoft->align;
 		}
 		kn->kn_kevent.ext[1] = ch->xruns;
+		kn->kn_sdata = ch->bufsoft->total;
 	}
 
 	return (kn->kn_data > 0);
@@ -3070,6 +3077,11 @@ dsp_kqfilter(struct cdev *dev, struct knote *kn)
 		knlist_add(&ch->bufsoft->sel.si_note, kn, 1);
 		CHN_UNLOCK(ch);
 		kn->kn_hook = ch;
+		/*
+		 * Start tracking from the current position so the first event
+		 * fires after c->lw additional bytes have been transferred.
+		 */
+		kn->kn_sdata = ch->bufsoft->prev_total;
 	} else
 		err = EINVAL;
 	PCM_GIANT_LEAVE(d);