[PATCH v2] ALSA: firewire-tascam: Do not drop unread control events

[Cássio Gabriel]

tscm_hwdep_read_queue() copies as many queued control events as fit in
the userspace buffer. When the buffer is smaller than the current
contiguous queue segment, length is rounded down to the number of bytes
that can be copied.

However, after copying that shortened length, the code advances pull_pos
to the original tail_pos, marking the whole contiguous segment as
consumed. Any events between the copied portion and tail_pos are lost.

Limit tail_pos to the position after the entries actually copied before
updating pull_pos. When the whole segment fits, this is equivalent to the
old tail_pos update; when the buffer is smaller, the remaining events
stay queued for the next read.

Fixes: a8c0d13267a4 ("ALSA: firewire-tascam: notify events of change of state for userspace applications")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>
---
Changes in v2:
- Recompute tail_pos after shortening length instead of adding a separate
  entries_copied variable, as suggested.
- Add Suggested-by tag.
- Link to v1: https://patch.msgid.link/20260501-alsa-firewire-tascam-read-queue-v1-1-7baa4ba1a4de@xxxxxxxxx
---
 sound/firewire/tascam/tascam-hwdep.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sound/firewire/tascam/tascam-hwdep.c b/sound/firewire/tascam/tascam-hwdep.c
index 867b4ea1096e..6270263e7bf4 100644
--- a/sound/firewire/tascam/tascam-hwdep.c
+++ b/sound/firewire/tascam/tascam-hwdep.c
@@ -73,6 +73,7 @@ static long tscm_hwdep_read_queue(struct snd_tscm *tscm, char __user *buf,
 			length = rounddown(remained, sizeof(*entries));
 		if (length == 0)
 			break;
+		tail_pos = head_pos + length / sizeof(*entries);
 
 		spin_unlock_irq(&tscm->lock);
 		if (copy_to_user(pos, &entries[head_pos], length))

---
base-commit: 9e8d6ddd7ecf2ad42d614243f86e50fcf0183b9e
change-id: 20260501-alsa-firewire-tascam-read-queue-7eaef1060adb

Best regards,
--  
Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>