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

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

Hi,

On Fri, May 01, 2026 at 03:22:58PM -0300, Cássio Gabriel wrote:
> 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 tail_pos, marking the whole contiguous segment as consumed. Any events
> between the copied portion and tail_pos are lost.
> 
> Advance pull_pos by the number of entries actually copied instead. 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@vger.kernel.org
> Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
> ---
>  sound/firewire/tascam/tascam-hwdep.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Thank you for catching the issue.

I agree that this is indeed a bug. While your approach makes sense, I
wonder if it could be simplified by updating tail_pos instead.

Would you mind trying the following change?

======== 8< --------
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))
======== 8< --------


Thanks

Takashi Sakamoto