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>

1 files changed, 7 insertions, 3 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index 5ecacaa90b5..ca98a9b915c 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 */