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Commit 4d788e04 authored by Clemens Ladisch's avatar Clemens Ladisch Committed by Takashi Iwai
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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: default avatarClemens Ladisch <clemens@ladisch.de>
Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent b7eb4a06

sound/usb/usbaudio.c

+8 −1
Original line number Diff line number Diff line
@@ -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;

	}