[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>
diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 9e38d3d..d28106e 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -938,7 +938,15 @@
/* decide how many packets to be used */
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)
total_packs = 2 * MIN_PACKS_URB;
} else {
