[alsa-devel] Buffer size for ALSA USB PCM audio
Alan Stern
stern at rowland.harvard.edu
Wed Aug 28 20:46:21 CEST 2013
On Wed, 28 Aug 2013, Clemens Ladisch wrote:
> Sorry, what I said applies more to explicit sync endpoints. When using
> implicit sync, a playback URB is submitted for each completed capture
> URB, with the number of samples per packet identical to the
> corresponding capture packet, so the parameters must be identical.
Got it. Below is an updated patch.
James, the problem you encountered was most likely a result of the
faulty treatment of capture endpoints that Clemens pointed out. It was
quite obvious in the usbmon traces that the unpatched driver used 8
packets per URB whereas the patched driver used 22. This updated patch
should fix that problem.
Alan Stern
Index: usb-3.11/sound/usb/usbaudio.h
===================================================================
--- usb-3.11.orig/sound/usb/usbaudio.h
+++ usb-3.11/sound/usb/usbaudio.h
@@ -55,7 +55,6 @@ struct snd_usb_audio {
struct list_head mixer_list; /* list of mixer interfaces */
int setup; /* from the 'device_setup' module param */
- int nrpacks; /* from the 'nrpacks' module param */
bool autoclock; /* from the 'autoclock' module param */
struct usb_host_interface *ctrl_intf; /* the audio control interface */
Index: usb-3.11/sound/usb/card.c
===================================================================
--- usb-3.11.orig/sound/usb/card.c
+++ usb-3.11/sound/usb/card.c
@@ -79,7 +79,6 @@ static bool enable[SNDRV_CARDS] = SNDRV_
/* Vendor/product IDs for this card */
static int vid[SNDRV_CARDS] = { [0 ... (SNDRV_CARDS-1)] = -1 };
static int pid[SNDRV_CARDS] = { [0 ... (SNDRV_CARDS-1)] = -1 };
-static int nrpacks = 8; /* max. number of packets per urb */
static int device_setup[SNDRV_CARDS]; /* device parameter for this card */
static bool ignore_ctl_error;
static bool autoclock = true;
@@ -94,8 +93,6 @@ module_param_array(vid, int, NULL, 0444)
MODULE_PARM_DESC(vid, "Vendor ID for the USB audio device.");
module_param_array(pid, int, NULL, 0444);
MODULE_PARM_DESC(pid, "Product ID for the USB audio device.");
-module_param(nrpacks, int, 0644);
-MODULE_PARM_DESC(nrpacks, "Max. number of packets per URB.");
module_param_array(device_setup, int, NULL, 0444);
MODULE_PARM_DESC(device_setup, "Specific device setup (if needed).");
module_param(ignore_ctl_error, bool, 0444);
@@ -374,7 +371,6 @@ static int snd_usb_audio_create(struct u
chip->dev = dev;
chip->card = card;
chip->setup = device_setup[idx];
- chip->nrpacks = nrpacks;
chip->autoclock = autoclock;
chip->probing = 1;
@@ -756,10 +752,6 @@ static struct usb_driver usb_audio_drive
static int __init snd_usb_audio_init(void)
{
- if (nrpacks < 1 || nrpacks > MAX_PACKS) {
- printk(KERN_WARNING "invalid nrpacks value.\n");
- return -EINVAL;
- }
return usb_register(&usb_audio_driver);
}
Index: usb-3.11/sound/usb/endpoint.h
===================================================================
--- usb-3.11.orig/sound/usb/endpoint.h
+++ usb-3.11/sound/usb/endpoint.h
@@ -12,6 +12,8 @@ int snd_usb_endpoint_set_params(struct s
snd_pcm_format_t pcm_format,
unsigned int channels,
unsigned int period_bytes,
+ unsigned int period_frames,
+ unsigned int buffer_periods,
unsigned int rate,
struct audioformat *fmt,
struct snd_usb_endpoint *sync_ep);
Index: usb-3.11/sound/usb/card.h
===================================================================
--- usb-3.11.orig/sound/usb/card.h
+++ usb-3.11/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 */
Index: usb-3.11/sound/usb/pcm.c
===================================================================
--- usb-3.11.orig/sound/usb/pcm.c
+++ usb-3.11/sound/usb/pcm.c
@@ -562,6 +562,7 @@ static int configure_sync_endpoint(struc
subs->pcm_format,
subs->channels,
subs->period_bytes,
+ 0, 0,
subs->cur_rate,
subs->cur_audiofmt,
NULL);
@@ -598,6 +599,7 @@ static int configure_sync_endpoint(struc
subs->pcm_format,
sync_fp->channels,
sync_period_bytes,
+ 0, 0,
subs->cur_rate,
sync_fp,
NULL);
@@ -620,6 +622,8 @@ static int configure_endpoint(struct snd
subs->pcm_format,
subs->channels,
subs->period_bytes,
+ subs->period_frames,
+ subs->buffer_periods,
subs->cur_rate,
subs->cur_audiofmt,
subs->sync_endpoint);
@@ -656,6 +660,8 @@ static int snd_usb_hw_params(struct snd_
subs->pcm_format = params_format(hw_params);
subs->period_bytes = params_period_bytes(hw_params);
+ subs->period_frames = params_period_size(hw_params);
+ subs->buffer_periods = params_periods(hw_params);
subs->channels = params_channels(hw_params);
subs->cur_rate = params_rate(hw_params);
@@ -1330,6 +1336,7 @@ static void prepare_playback_urb(struct
frames = 0;
urb->number_of_packets = 0;
spin_lock_irqsave(&subs->lock, flags);
+ subs->frame_limit += ep->max_urb_frames;
for (i = 0; i < ctx->packets; i++) {
if (ctx->packet_size[i])
counts = ctx->packet_size[i];
@@ -1344,6 +1351,7 @@ static void prepare_playback_urb(struct
subs->transfer_done += counts;
if (subs->transfer_done >= runtime->period_size) {
subs->transfer_done -= runtime->period_size;
+ subs->frame_limit = 0;
period_elapsed = 1;
if (subs->fmt_type == UAC_FORMAT_TYPE_II) {
if (subs->transfer_done > 0) {
@@ -1366,8 +1374,10 @@ static void prepare_playback_urb(struct
break;
}
}
- if (period_elapsed &&
- !snd_usb_endpoint_implicit_feedback_sink(subs->data_endpoint)) /* finish at the period boundary */
+ /* finish at the period boundary or after enough frames */
+ if ((period_elapsed ||
+ subs->transfer_done >= subs->frame_limit) &&
+ !snd_usb_endpoint_implicit_feedback_sink(ep))
break;
}
bytes = frames * ep->stride;
Index: usb-3.11/sound/usb/endpoint.c
===================================================================
--- usb-3.11.orig/sound/usb/endpoint.c
+++ usb-3.11/sound/usb/endpoint.c
@@ -571,11 +571,14 @@ static int data_ep_set_params(struct snd
snd_pcm_format_t pcm_format,
unsigned int channels,
unsigned int period_bytes,
+ unsigned int frames_per_period,
+ unsigned int periods_per_buffer,
struct audioformat *fmt,
struct snd_usb_endpoint *sync_ep)
{
- unsigned int maxsize, i, urb_packs, total_packs, packs_per_ms;
- int is_playback = usb_pipeout(ep->pipe);
+ unsigned int maxsize, minsize, packs_per_ms, max_packs_per_urb;
+ unsigned int max_packs_per_period, urbs_per_period, urb_packs;
+ unsigned int max_urbs, i;
int frame_bits = snd_pcm_format_physical_width(pcm_format) * channels;
if (pcm_format == SNDRV_PCM_FORMAT_DSD_U16_LE && fmt->dsd_dop) {
@@ -608,58 +611,67 @@ static int data_ep_set_params(struct snd
else
ep->curpacksize = maxsize;
- if (snd_usb_get_speed(ep->chip->dev) != USB_SPEED_FULL)
+ if (snd_usb_get_speed(ep->chip->dev) != USB_SPEED_FULL) {
packs_per_ms = 8 >> ep->datainterval;
- else
- packs_per_ms = 1;
-
- if (is_playback && !snd_usb_endpoint_implicit_feedback_sink(ep)) {
- urb_packs = max(ep->chip->nrpacks, 1);
- urb_packs = min(urb_packs, (unsigned int) MAX_PACKS);
+ max_packs_per_urb = MAX_PACKS_HS;
} else {
- urb_packs = 1;
+ packs_per_ms = 1;
+ max_packs_per_urb = MAX_PACKS;
}
-
- urb_packs *= packs_per_ms;
-
if (sync_ep && !snd_usb_endpoint_implicit_feedback_sink(ep))
- urb_packs = min(urb_packs, 1U << sync_ep->syncinterval);
+ max_packs_per_urb = min(max_packs_per_urb,
+ 1U << sync_ep->syncinterval);
+ max_packs_per_urb = max(1u, max_packs_per_urb >> ep->datainterval);
+
+ /*
+ * Capture endpoints need to use small URBs because there's no way
+ * to tell in advance where the next period will end, and we don't
+ * want the next URB to complete much after the period ends.
+ *
+ * Playback endpoints with implicit sync much use the same parameters
+ * as their corresponding capture endpoint.
+ */
+ if (usb_pipein(ep->pipe) ||
+ snd_usb_endpoint_implicit_feedback_sink(ep)) {
+
+ /* make capture URBs <= 1 ms and smaller than a period */
+ urb_packs = min(max_packs_per_urb, packs_per_ms);
+ while (urb_packs > 1 && urb_packs * maxsize >= period_bytes)
+ urb_packs >>= 1;
+ ep->nurbs = MAX_URBS;
- /* decide how many packets to be used */
- if (is_playback && !snd_usb_endpoint_implicit_feedback_sink(ep)) {
- unsigned int minsize, maxpacks;
+ /*
+ * Playback endpoints without implicit sync are adjusted so that
+ * a period fits as evenly as possible in the smallest number of
+ * URBs. The total number of URBs is adjusted to the size of the
+ * ALSA buffer, subject to the MAX_URBS and MAX_QUEUE limits.
+ */
+ } else {
/* determine how small a packet can be */
- minsize = (ep->freqn >> (16 - ep->datainterval))
- * (frame_bits >> 3);
+ minsize = (ep->freqn >> (16 - ep->datainterval)) *
+ (frame_bits >> 3);
/* with sync from device, assume it can be 12% lower */
if (sync_ep)
minsize -= minsize >> 3;
minsize = max(minsize, 1u);
- total_packs = (period_bytes + minsize - 1) / minsize;
- /* we need at least two URBs for queueing */
- if (total_packs < 2) {
- total_packs = 2;
- } else {
- /* and we don't want too long a queue either */
- maxpacks = max(MAX_QUEUE * packs_per_ms, urb_packs * 2);
- total_packs = min(total_packs, maxpacks);
- }
- } else {
- while (urb_packs > 1 && urb_packs * maxsize >= period_bytes)
- urb_packs >>= 1;
- total_packs = MAX_URBS * urb_packs;
- }
- ep->nurbs = (total_packs + urb_packs - 1) / urb_packs;
- if (ep->nurbs > MAX_URBS) {
- /* too much... */
- ep->nurbs = MAX_URBS;
- total_packs = MAX_URBS * urb_packs;
- } else if (ep->nurbs < 2) {
- /* too little - we need at least two packets
- * to ensure contiguous playback/capture
- */
- ep->nurbs = 2;
+ /* how many packets will contain an entire ALSA period? */
+ max_packs_per_period = DIV_ROUND_UP(period_bytes, minsize);
+
+ /* how many URBs will contain a period? */
+ urbs_per_period = DIV_ROUND_UP(max_packs_per_period,
+ max_packs_per_urb);
+ /* how many packets are needed in each URB? */
+ urb_packs = DIV_ROUND_UP(max_packs_per_period, urbs_per_period);
+
+ /* limit the number of frames in a single URB */
+ ep->max_urb_frames = DIV_ROUND_UP(frames_per_period,
+ urbs_per_period);
+
+ /* try to use enough URBs to contain an entire ALSA buffer */
+ max_urbs = min((unsigned) MAX_URBS,
+ MAX_QUEUE * packs_per_ms / urb_packs);
+ ep->nurbs = min(max_urbs, urbs_per_period * periods_per_buffer);
}
/* allocate and initialize data urbs */
@@ -667,8 +679,7 @@ static int data_ep_set_params(struct snd
struct snd_urb_ctx *u = &ep->urb[i];
u->index = i;
u->ep = ep;
- u->packets = (i + 1) * total_packs / ep->nurbs
- - i * total_packs / ep->nurbs;
+ u->packets = urb_packs;
u->buffer_size = maxsize * u->packets;
if (fmt->fmt_type == UAC_FORMAT_TYPE_II)
@@ -745,6 +756,8 @@ out_of_memory:
* @pcm_format: the audio fomat.
* @channels: the number of audio channels.
* @period_bytes: the number of bytes in one alsa period.
+ * @period_frames: the number of frames in one alsa period.
+ * @buffer_periods: the number of periods in one alsa buffer.
* @rate: the frame rate.
* @fmt: the USB audio format information
* @sync_ep: the sync endpoint to use, if any
@@ -757,6 +770,8 @@ int snd_usb_endpoint_set_params(struct s
snd_pcm_format_t pcm_format,
unsigned int channels,
unsigned int period_bytes,
+ unsigned int period_frames,
+ unsigned int buffer_periods,
unsigned int rate,
struct audioformat *fmt,
struct snd_usb_endpoint *sync_ep)
@@ -790,7 +805,8 @@ int snd_usb_endpoint_set_params(struct s
switch (ep->type) {
case SND_USB_ENDPOINT_TYPE_DATA:
err = data_ep_set_params(ep, pcm_format, channels,
- period_bytes, fmt, sync_ep);
+ period_bytes, period_frames,
+ buffer_periods, fmt, sync_ep);
break;
case SND_USB_ENDPOINT_TYPE_SYNC:
err = sync_ep_set_params(ep, fmt);
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