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authorAlan Stern <stern@rowland.harvard.edu>2013-09-24 23:51:58 +0400
committerTakashi Iwai <tiwai@suse.de>2013-09-26 12:25:31 +0400
commit976b6c064a957445eb0573b270f2d0282630e9b9 (patch)
tree3a80a94273feeb9c30ac6f2e23976d020dc44aaf /sound/usb/card.h
parente8bc99425e8159cd5f56bb76419158857bd358ed (diff)
downloadlinux-976b6c064a957445eb0573b270f2d0282630e9b9.tar.xz
ALSA: improve buffer size computations for USB PCM audio
This patch changes the way URBs are allocated and their sizes are determined for PCM playback in the snd-usb-audio driver. Currently the driver allocates too few URBs for endpoints that don't use implicit sync, making underruns more likely to occur. This may be a holdover from before I/O delays could be measured accurately; in any case, it is no longer necessary. The patch allocates as many URBs as possible, subject to four limitations: The total number of URBs for the endpoint is not allowed to exceed MAX_URBS (which the patch increases from 8 to 12). The total number of packets per URB is not allowed to exceed MAX_PACKS (or MAX_PACKS_HS for high-speed devices), which is decreased from 20 to 6. The total duration of queued data is not allowed to exceed MAX_QUEUE, which is decreased from 24 ms to 18 ms. The total number of ALSA frames in the output queue is not allowed to exceed the ALSA buffer size. The last requirement is the hardest to implement. Currently the number of URBs needed to fill a buffer cannot be determined in advance, because a buffer contains a fixed number of frames whereas the number of frames in an URB varies to match shifts in the device's clock rate. To solve this problem, the patch changes the logic for deciding how many packets an URB should contain. Rather than using as many as possible without exceeding an ALSA period boundary, now the driver uses only as many packets as needed to transfer a predetermined number of frames. As a result, unless the device's clock has an exceedingly variable rate, the number of URBs making up each period (and hence each buffer) will remain constant. The overall effect of the patch is that playback works better in low-latency settings. The user can still specify values for frames/period and periods/buffer that exceed the capabilities of the hardware, of course. But for values that are within those capabilities, the performance will be improved. For example, testing shows that a high-speed device can handle 32 frames/period and 3 periods/buffer at 48 KHz, whereas the current driver starts to get glitchy at 64 frames/period and 2 periods/buffer. A side effect of these changes is that the "nrpacks" module parameter is no longer used. The patch removes it. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> CC: Clemens Ladisch <clemens@ladisch.de> Tested-by: Daniel Mack <zonque@gmail.com> Tested-by: Eldad Zack <eldad@fogrefinery.com> Signed-off-by: Takashi Iwai <tiwai@suse.de>
Diffstat (limited to 'sound/usb/card.h')
-rw-r--r--sound/usb/card.h10
1 files changed, 7 insertions, 3 deletions
diff --git a/sound/usb/card.h b/sound/usb/card.h
index 5ecacaa90b53..ca98a9b915c9 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -2,11 +2,11 @@
#define __USBAUDIO_CARD_H
#define MAX_NR_RATES 1024
-#define MAX_PACKS 20
+#define MAX_PACKS 6 /* per URB */
#define MAX_PACKS_HS (MAX_PACKS * 8) /* in high speed mode */
-#define MAX_URBS 8
+#define MAX_URBS 12
#define SYNC_URBS 4 /* always four urbs for sync */
-#define MAX_QUEUE 24 /* try not to exceed this queue length, in ms */
+#define MAX_QUEUE 18 /* try not to exceed this queue length, in ms */
struct audioformat {
struct list_head list;
@@ -87,6 +87,7 @@ struct snd_usb_endpoint {
unsigned int phase; /* phase accumulator */
unsigned int maxpacksize; /* max packet size in bytes */
unsigned int maxframesize; /* max packet size in frames */
+ unsigned int max_urb_frames; /* max URB size in frames */
unsigned int curpacksize; /* current packet size in bytes (for capture) */
unsigned int curframesize; /* current packet size in frames (for capture) */
unsigned int syncmaxsize; /* sync endpoint packet size */
@@ -116,6 +117,8 @@ struct snd_usb_substream {
unsigned int channels_max; /* max channels in the all audiofmts */
unsigned int cur_rate; /* current rate (for hw_params callback) */
unsigned int period_bytes; /* current period bytes (for hw_params callback) */
+ unsigned int period_frames; /* current frames per period */
+ unsigned int buffer_periods; /* current periods per buffer */
unsigned int altset_idx; /* USB data format: index of alternate setting */
unsigned int txfr_quirk:1; /* allow sub-frame alignment */
unsigned int fmt_type; /* USB audio format type (1-3) */
@@ -125,6 +128,7 @@ struct snd_usb_substream {
unsigned int hwptr_done; /* processed byte position in the buffer */
unsigned int transfer_done; /* processed frames since last period update */
+ unsigned int frame_limit; /* limits number of packets in URB */
/* data and sync endpoints for this stream */
unsigned int ep_num; /* the endpoint number */