Re: [alsa-devel] Buffer size for ALSA USB PCM audio

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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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