[ALSA] usb-audio: schedule high speed URBs with 1 ms alignment

USB generic driver
The EHCI driver doesn't interrupt more than once per millisecond, and
organizes all iso transfers with frame-sized ITDs, so we can (try to)
be more efficient by aligning all URBs on frame boundaries.

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

diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 49075f0..a62d131 100644 (file)
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -893,7 +893,7 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
 {
        unsigned int maxsize, n, i;
        int is_playback = subs->direction == SNDRV_PCM_STREAM_PLAYBACK;
-       unsigned int npacks[MAX_URBS], urb_packs, total_packs;
+       unsigned int npacks[MAX_URBS], urb_packs, total_packs, packs_per_ms;
 
        /* calculate the frequency in 16.16 format */
        if (snd_usb_get_speed(subs->dev) == USB_SPEED_FULL)
@@ -920,14 +920,18 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
        else
                subs->curpacksize = maxsize;
 
+       if (snd_usb_get_speed(subs->dev) == USB_SPEED_HIGH)
+               packs_per_ms = 8 >> subs->datainterval;
+       else
+               packs_per_ms = 1;
+
        if (is_playback) {
                urb_packs = nrpacks;
                urb_packs = max(urb_packs, (unsigned int)MIN_PACKS_URB);
                urb_packs = min(urb_packs, (unsigned int)MAX_PACKS);
        } else
                urb_packs = 1;
-       if (snd_usb_get_speed(subs->dev) == USB_SPEED_HIGH)
-               urb_packs = (urb_packs * 8) >> subs->datainterval;
+       urb_packs *= packs_per_ms;
 
        /* allocate a temporary buffer for playback */
        if (is_playback) {
@@ -949,8 +953,12 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
                        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;
+               /* round up to multiple of packs_per_ms */
+               total_packs = (total_packs + packs_per_ms - 1)
+                               & ~(packs_per_ms - 1);
+               /* 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 {
                total_packs = MAX_URBS * urb_packs;
        }
@@ -972,7 +980,7 @@ static int init_substream_urbs(snd_usb_substream_t *subs, unsigned int period_by
                subs->nurbs = 2;
                npacks[0] = (total_packs + 1) / 2;
                npacks[1] = total_packs - npacks[0];
-       } else if (npacks[subs->nurbs-1] < MIN_PACKS_URB) {
+       } else if (npacks[subs->nurbs-1] < MIN_PACKS_URB * packs_per_ms) {
                /* the last packet is too small.. */
                if (subs->nurbs > 2) {
                        /* merge to the first one */