[PATCH] ALSA: usb-audio: Apply async workaround for Scarlett 2i4 2nd gen
Takashi Iwai
tiwai at suse.de
Thu Apr 23 19:35:13 CEST 2020
On Thu, 23 Apr 2020 19:29:08 +0200,
Takashi Iwai wrote:
>
> On Thu, 23 Apr 2020 18:57:34 +0200,
> Alexander Tsoy wrote:
> >
> > And some further notes:
> >
> > - I removed locking from snd_usb_endpoint_next_packet_size() and this
> > seems completely fixed an issue with large URBs I reported here:
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=199327#c28
> >
> > So playing at 96 kHz, driver packs 48 frames per URB and no more audio
> > discontinuities.
>
> Hmm, that's weird.
>
> If removing the lock from snd_usb_endpoint_next_packet_size() really
> fixes the problem, it implies the lock contention. But as far as I
> see the code performed in this lock isn't conflicting so much. The
> URB processing shouldn't happen in parallel for the same EP.
BTW, one potential racy code I found while looking at the code is the
list management in queue_pending_output_urbs(). The fix patch is
below.
Takashi
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -321,17 +321,17 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
ep->next_packet_read_pos %= MAX_URBS;
/* take URB out of FIFO */
- if (!list_empty(&ep->ready_playback_urbs))
+ if (!list_empty(&ep->ready_playback_urbs)) {
ctx = list_first_entry(&ep->ready_playback_urbs,
struct snd_urb_ctx, ready_list);
+ list_del_init(&ctx->ready_list);
+ }
}
spin_unlock_irqrestore(&ep->lock, flags);
if (ctx == NULL)
return;
- list_del_init(&ctx->ready_list);
-
/* copy over the length information */
for (i = 0; i < packet->packets; i++)
ctx->packet_size[i] = packet->packet_size[i];
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