[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 (file)
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -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 */
        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 {