From 4d788e040b72d2a46ea3ba726b7fa0b65de06c88 Mon Sep 17 00:00:00 2001 From: Clemens Ladisch Date: Mon, 26 Jan 2009 08:09:28 +0100 Subject: 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 Signed-off-by: Takashi Iwai --- sound/usb/usbaudio.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'sound/usb/usbaudio.c') 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; } -- cgit v1.2.3