[ALSA] usb-audio: use 1 ms URBs when capturing
USB generic driver
When capturing audio data, we do not know beforehand how many samples
the device sends per frame, so we have to use URBs that are as short as
possible to make sure that we can handle period boundaries without any
additional latencies.
Furthermore, the total count of URBs submitted doesn't matter when
capturing, so we can just use the maximum number.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
diff --git a/sound/usb/usbaudio.c b/sound/usb/usbaudio.c
index 8298c46..9e38d3d 100644
--- a/sound/usb/usbaudio.c
+++ b/sound/usb/usbaudio.c
@@ -97,7 +97,7 @@
#define MAX_PACKS 10
#define MAX_PACKS_HS (MAX_PACKS * 8) /* in high speed mode */
-#define MAX_URBS 5 /* max. 20ms long packets */
+#define MAX_URBS 8
#define SYNC_URBS 4 /* always four urbs for sync */
#define MIN_PACKS_URB 1 /* minimum 1 packet per urb */
@@ -920,10 +920,12 @@
else
subs->curpacksize = maxsize;
- if (snd_usb_get_speed(subs->dev) == USB_SPEED_FULL)
+ if (is_playback)
urb_packs = nrpacks;
else
- urb_packs = (nrpacks * 8) >> subs->datainterval;
+ urb_packs = 1;
+ if (snd_usb_get_speed(subs->dev) == USB_SPEED_HIGH)
+ urb_packs = (urb_packs * 8) >> subs->datainterval;
/* allocate a temporary buffer for playback */
if (is_playback) {
@@ -935,9 +937,13 @@
}
/* decide how many packets to be used */
- total_packs = (period_bytes + maxsize - 1) / maxsize;
- if (total_packs < 2 * MIN_PACKS_URB)
- total_packs = 2 * MIN_PACKS_URB;
+ if (is_playback) {
+ total_packs = (period_bytes + maxsize - 1) / maxsize;
+ if (total_packs < 2 * MIN_PACKS_URB)
+ total_packs = 2 * MIN_PACKS_URB;
+ } else {
+ total_packs = MAX_URBS * urb_packs;
+ }
subs->nurbs = (total_packs + urb_packs - 1) / urb_packs;
if (subs->nurbs > MAX_URBS) {
/* too much... */