From: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
For playback usages, the endpoints are started before the prepare
step, and valid audio data will be rendered with a delay that
cannot be recovered.
Worst-case the initial delay due to buffering of empty URBS can
be up to 12ms
Tested with audio_time:
Timestamps without delay taken into account:
test$ ./audio_time -Dhw:1,0 -p
playback: systime: 120093327 nsec, audio time 125000000 nsec, systime delta -4906673
playback: systime: 245090755 nsec, audio time 250000000 nsec, systime delta -4909245
playback: systime: 370034641 nsec, audio time 375000000 nsec, systime delta -4965359
playback: systime: 495089634 nsec, audio time 500000000 nsec, systime delta -4910366
Timestamps with delay taken into account (12ms delay shown)
test$ ./audio_time -Dhw:1,0 -p -d
playback: systime: 120095090 nsec, audio time 108000000 nsec, systime delta 12095090
playback: systime: 245090932 nsec, audio time 232000000 nsec, systime delta 13090932
playback: systime: 370091792 nsec, audio time 357000000 nsec, systime delta 13091792
playback: systime: 495092309 nsec, audio time 482000000 nsec, systime delta 13092309
Suggested-by: Rakesh Ughreja <rakesh.ughreja@xxxxxxxxx>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@xxxxxxxxx>
---
sound/usb/card.h | 1 +
sound/usb/pcm.c | 32 +++++++++++++++++++++++++++++---
2 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/sound/usb/card.h b/sound/usb/card.h
index 71778ca..d128058 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -148,6 +148,7 @@ struct snd_usb_substream {
int last_frame_number; /* stored frame number */
int last_delay; /* stored delay */
+ int start_delay; /* initial delay due to empty frames */
struct {
int marker;
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 5bcc729..2bf7537 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -86,6 +86,7 @@ static snd_pcm_uframes_t snd_usb_pcm_pointer(struct snd_pcm_substream *substream
hwptr_done = subs->hwptr_done;
substream->runtime->delay = snd_usb_pcm_delay(subs,
substream->runtime->rate);
+ substream->runtime->delay += subs->start_delay;
spin_unlock(&subs->lock);
return hwptr_done / (substream->runtime->frame_bits >> 3);
}
@@ -858,9 +859,34 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
/* 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)
+ if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK) {
ret = start_endpoints(subs, true);
+ /*
+ * Since we submit empty URBS, the constant initial delay will
+ * not be recovered and will be added to the dynamic delay
+ * measured with the frame counter. Worst-case the
+ * startup delay can be (MAX_URBS-1) * MAX_PACKS = 12ms
+ */
+ if (ret == 0) {
+ struct snd_usb_endpoint *ep = subs->data_endpoint;
+ int total_packs = 0;
+ int i;
+
+ for (i = 0; i < ep->nurbs - 1; i++) {
+ struct snd_urb_ctx *u = &ep->urb[i];
+
+ total_packs += u->packets;
+ }
+ subs->start_delay = DIV_ROUND_UP(total_packs *
+ subs->cur_rate, 1000);
+ if (subs->start_delay)
+ dev_dbg(&subs->dev->dev,
+ "Initial delay for EP @%p: %d frames\n",
+ ep, subs->start_delay);
+ }
+ }
+
unlock:
snd_usb_unlock_shutdown(subs->stream->chip);
return ret;
@@ -1549,6 +1575,7 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
runtime->delay = subs->last_delay;
runtime->delay += frames;
subs->last_delay = runtime->delay;
+ runtime->delay += subs->start_delay;
/* realign last_frame_number */
subs->last_frame_number = usb_get_current_frame_number(subs->dev);
@@ -1597,8 +1624,7 @@ static void retire_playback_urb(struct snd_usb_substream *subs,
subs->last_delay = 0;
else
subs->last_delay -= processed;
- runtime->delay = subs->last_delay;
-
+ runtime->delay = subs->last_delay + subs->start_delay;
/*
* Report when delay estimate is off by more than 2ms.
* The error should be lower than 2ms since the estimate relies
--
1.9.1
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