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>
diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 417d557..f3d4de2 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -108,6 +108,7 @@
#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 @@
/* 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 @@
/* 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;
}
