[ALSA] usb-audio: start submitting URBs in the prepared state

Modules: USB generic driver

If we submit all our URBs when a playback stream is started, the first
hwptr_done update for each URB happens at the same time.  This results
in an underrun when there isn't enough PCM data available at this
point for all URBs.

To avoid this, we begin submitting our URBs earlier (when the stream
is prepared), with empy packets.  When the stream is started, the
prepare_playback_urb() call for each URB will be run only when the
respective URB has completed previously, so the first hwptr_done
updates will be done nicely staggered.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usbaudio.c

@@ -41,7 +41,6 @@
 #include <sound/driver.h>
 #include <linux/bitops.h>
 #include <linux/init.h>
-#include <linux/interrupt.h>
 #include <linux/list.h>
 #include <linux/slab.h>
 #include <linux/string.h>
@@ -185,7 +184,6 @@ struct snd_usb_substream {
 	unsigned int num_formats;		/* number of supported audio formats (list) */
 	struct list_head fmt_list;	/* format list */
 	spinlock_t lock;
-	struct tasklet_struct start_period_elapsed;	/* for start trigger */
 
 	struct snd_urb_ops ops;		/* callbacks (must be filled at init) */
 };
@@ -479,6 +477,28 @@ static int retire_playback_sync_urb_hs(snd_usb_substream_t *subs,
 	return 0;
 }
 
+/*
+ * Prepare urb for streaming before playback starts.
+ *
+ * We don't care about (or have) any data, so we just send a transfer delimiter.
+ */
+static int prepare_startup_playback_urb(snd_usb_substream_t *subs,
+					snd_pcm_runtime_t *runtime,
+					struct urb *urb)
+{
+	unsigned int i;
+	snd_urb_ctx_t *ctx = urb->context;
+
+	urb->dev = ctx->subs->dev;
+	urb->number_of_packets = subs->packs_per_ms;
+	for (i = 0; i < subs->packs_per_ms; ++i) {
+		urb->iso_frame_desc[i].offset = 0;
+		urb->iso_frame_desc[i].length = 0;
+	}
+	urb->transfer_buffer_length = 0;
+	return 0;
+}
+
 /*
  * prepare urb for playback data pipe
  *
@@ -568,12 +588,8 @@ static int prepare_playback_urb(snd_usb_substream_t *subs,
 		subs->hwptr_done -= runtime->buffer_size;
 	spin_unlock_irqrestore(&subs->lock, flags);
 	urb->transfer_buffer_length = offs * stride;
-	if (period_elapsed) {
-		if (likely(subs->running))
-			snd_pcm_period_elapsed(subs->pcm_substream);
-		else
-			tasklet_hi_schedule(&subs->start_period_elapsed);
-	}
+	if (period_elapsed)
+		snd_pcm_period_elapsed(subs->pcm_substream);
 	return 0;
 }
 
@@ -588,22 +604,12 @@ static int retire_playback_urb(snd_usb_substream_t *subs,
 	return 0;
 }
 
-/*
- * Delay the snd_pcm_period_elapsed() call until after the start trigger
- * callback so that we're not longer in the substream's lock.
- */
-static void start_period_elapsed(unsigned long data)
-{
-	snd_usb_substream_t *subs = (snd_usb_substream_t *)data;
-	snd_pcm_period_elapsed(subs->pcm_substream);
-}
-
 
 /*
  */
 static struct snd_urb_ops audio_urb_ops[2] = {
 	{
-		.prepare =	prepare_playback_urb,
+		.prepare =	prepare_startup_playback_urb,
 		.retire =	retire_playback_urb,
 		.prepare_sync =	prepare_playback_sync_urb,
 		.retire_sync =	retire_playback_sync_urb,
@@ -618,7 +624,7 @@ static struct snd_urb_ops audio_urb_ops[2] = {
 
 static struct snd_urb_ops audio_urb_ops_high_speed[2] = {
 	{
-		.prepare =	prepare_playback_urb,
+		.prepare =	prepare_startup_playback_urb,
 		.retire =	retire_playback_urb,
 		.prepare_sync =	prepare_playback_sync_urb_hs,
 		.retire_sync =	retire_playback_sync_urb_hs,
@@ -863,25 +869,40 @@ static snd_pcm_uframes_t snd_usb_pcm_pointer(snd_pcm_substream_t *substream)
 
 
 /*
- * start/stop substream
+ * start/stop playback substream
  */
-static int snd_usb_pcm_trigger(snd_pcm_substream_t *substream, int cmd)
+static int snd_usb_pcm_playback_trigger(snd_pcm_substream_t *substream,
+					int cmd)
 {
-	snd_usb_substream_t *subs = (snd_usb_substream_t *)substream->runtime->private_data;
-	int err;
+	snd_usb_substream_t *subs = substream->runtime->private_data;
 
 	switch (cmd) {
 	case SNDRV_PCM_TRIGGER_START:
-		err = start_urbs(subs, substream->runtime);
-		break;
+		subs->ops.prepare = prepare_playback_urb;
+		return 0;
 	case SNDRV_PCM_TRIGGER_STOP:
-		err = deactivate_urbs(subs, 0, 0);
-		break;
+		return deactivate_urbs(subs, 0, 0);
 	default:
-		err = -EINVAL;
-		break;
+		return -EINVAL;
+	}
+}
+
+/*
+ * start/stop capture substream
+ */
+static int snd_usb_pcm_capture_trigger(snd_pcm_substream_t *substream,
+				       int cmd)
+{
+	snd_usb_substream_t *subs = substream->runtime->private_data;
+
+	switch (cmd) {
+	case SNDRV_PCM_TRIGGER_START:
+		return start_urbs(subs, substream->runtime);
+	case SNDRV_PCM_TRIGGER_STOP:
+		return deactivate_urbs(subs, 0, 0);
+	default:
+		return -EINVAL;
 	}
-	return err < 0 ? err : 0;
 }
 
 
@@ -1413,7 +1434,7 @@ static int snd_usb_hw_free(snd_pcm_substream_t *substream)
 static int snd_usb_pcm_prepare(snd_pcm_substream_t *substream)
 {
 	snd_pcm_runtime_t *runtime = substream->runtime;
-	snd_usb_substream_t *subs = (snd_usb_substream_t *)runtime->private_data;
+	snd_usb_substream_t *subs = runtime->private_data;
 
 	if (! subs->cur_audiofmt) {
 		snd_printk(KERN_ERR "usbaudio: no format is specified!\n");
@@ -1433,7 +1454,13 @@ static int snd_usb_pcm_prepare(snd_pcm_substream_t *substream)
 	deactivate_urbs(subs, 0, 1);
 	wait_clear_urbs(subs);
 
-	return 0;
+	/* for playback, submit the URBs now; otherwise, the first hwptr_done
+	 * updates for all URBs would happen at the same time when starting */
+	if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK) {
+		subs->ops.prepare = prepare_startup_playback_urb;
+		return start_urbs(subs, runtime);
+	} else
+		return 0;
 }
 
 static snd_pcm_hardware_t snd_usb_playback =
@@ -1847,7 +1874,7 @@ static snd_pcm_ops_t snd_usb_playback_ops = {
 	.hw_params =	snd_usb_hw_params,
 	.hw_free =	snd_usb_hw_free,
 	.prepare =	snd_usb_pcm_prepare,
-	.trigger =	snd_usb_pcm_trigger,
+	.trigger =	snd_usb_pcm_playback_trigger,
 	.pointer =	snd_usb_pcm_pointer,
 	.page =		snd_pcm_get_vmalloc_page,
 };
@@ -1859,7 +1886,7 @@ static snd_pcm_ops_t snd_usb_capture_ops = {
 	.hw_params =	snd_usb_hw_params,
 	.hw_free =	snd_usb_hw_free,
 	.prepare =	snd_usb_pcm_prepare,
-	.trigger =	snd_usb_pcm_trigger,
+	.trigger =	snd_usb_pcm_capture_trigger,
 	.pointer =	snd_usb_pcm_pointer,
 	.page =		snd_pcm_get_vmalloc_page,
 };
@@ -2078,9 +2105,6 @@ static void init_substream(snd_usb_stream_t *as, int stream, struct audioformat
 
 	INIT_LIST_HEAD(&subs->fmt_list);
 	spin_lock_init(&subs->lock);
-	if (stream == SNDRV_PCM_STREAM_PLAYBACK)
-		tasklet_init(&subs->start_period_elapsed, start_period_elapsed,
-			     (unsigned long)subs);
 
 	subs->stream = as;
 	subs->direction = stream;