sound: usb-audio: limit playback queue length

Once our URBs contain enough packets, queueing more URBs does not give
us any additional underrun protection (as we use double-buffering) but
just increases latency unnecessarily.  Therefore, we try to limit the
queue length to some reasonable value.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 8 insertions, 1 deletions

diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 417d557ed641..f3d4de23fedf 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -108,6 +108,7 @@ MODULE_PARM_DESC(ignore_ctl_error,
 #define MAX_URBS        8
 #define SYNC_URBS       4       /* always four urbs for sync */
 #define MIN_PACKS_URB   1       /* minimum 1 packet per urb */
+#define MAX_QUEUE       24      /* try not to exceed this queue length, in ms */
 
 struct audioformat {
         struct list_head list;
@@ -1079,7 +1080,7 @@ static int init_substream_urbs(struct snd_usb_substream *subs, unsigned int peri
 
         /* decide how many packets to be used */
         if (is_playback) {
-                unsigned int minsize;
+                unsigned int minsize, maxpacks;
                 /* determine how small a packet can be */
                 minsize = (subs->freqn >> (16 - subs->datainterval))
                           * (frame_bits >> 3);
@@ -1094,6 +1095,12 @@ static int init_substream_urbs(struct snd_usb_substream *subs, unsigned int peri
                 /* we need at least two URBs for queueing */
                 if (total_packs < 2 * MIN_PACKS_URB * packs_per_ms)
                         total_packs = 2 * MIN_PACKS_URB * packs_per_ms;
+                else {
+                        /* and we don't want too long a queue either */
+                        maxpacks = max((unsigned int)MAX_QUEUE, urb_packs * 2);
+                        if (total_packs > maxpacks * packs_per_ms)
+                                total_packs = maxpacks * packs_per_ms;
+                }
         } else {
                 total_packs = MAX_URBS * urb_packs;
         }