[ALSA] usb-audio: actually schedule playback URBs at frame boundaries

USB generic driver
Change prepare_playback_urb() not to stop preparing packets before a
frame boundary has been reached.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

1 files changed, 7 insertions, 1 deletions

diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 2b4f916a0a9a..3f7930c0b616 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -164,6 +164,7 @@ struct snd_usb_substream {
         unsigned int curframesize;      /* current packet size in frames (for capture) */
         unsigned int fill_max: 1;       /* fill max packet size always */
         unsigned int fmt_type;          /* USB audio format type (1-3) */
+        unsigned int packs_per_ms;      /* packets per millisecond (for playback) */
 
         unsigned int running: 1;        /* running status */
 
@@ -537,9 +538,13 @@ static int prepare_playback_urb(snd_usb_substream_t *subs,
                                         urb->iso_frame_desc[i].length = 0;
                                         urb->number_of_packets++;
                                 }
+                                break;
                         }
-                        break;
                 }
+                /* finish at the frame boundary at/after the period boundary */
+                if (period_elapsed &&
+                    (i & (subs->packs_per_ms - 1)) == subs->packs_per_ms - 1)
+                        break;
         }
         if (subs->hwptr_done + offs > runtime->buffer_size) {
                 /* err, the transferred area goes over buffer boundary. */
@@ -907,6 +912,7 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
                 packs_per_ms = 8 >> subs->datainterval;
         else
                 packs_per_ms = 1;
+        subs->packs_per_ms = packs_per_ms;
 
         if (is_playback) {
                 urb_packs = nrpacks;