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[ALSA] usb-audio: fix packets per URB calculation for playback 

USB generic driver
When determining how many packets are needed for one period, we cannot
assume that all packets have their maximum size -- we always use the
nominal sample rate when sending data, and could use an even lower rate
when the endpoint uses frequency feedback.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
This commit is contained in:
Clemens Ladisch 2005-08-12 08:28:27 +02:00 committed by Jaroslav Kysela
parent 15a24c0778
commit d6db392e92
1 changed files with 9 additions and 1 deletions
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10
sound/usb/usbaudio.c
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@ -938,7 +938,15 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
/* decide how many packets to be used */ /* decide how many packets to be used */
if (is_playback) { if (is_playback) {
total_packs = (period_bytes + maxsize - 1) / maxsize; unsigned int minsize;
/* determine how small a packet can be */
minsize = (subs->freqn >> (16 - subs->datainterval))
* (frame_bits >> 3);
/* with sync from device, assume it can be 25% lower */
if (subs->syncpipe)
minsize -= minsize >> 2;
minsize = max(minsize, 1u);
total_packs = (period_bytes + minsize - 1) / minsize;
if (total_packs < 2 * MIN_PACKS_URB) if (total_packs < 2 * MIN_PACKS_URB)
total_packs = 2 * MIN_PACKS_URB; total_packs = 2 * MIN_PACKS_URB;
} else { } else {