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 (file)
--- 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;
        }