Existing code only updates the audio delay when URBs were submitted/retired. This can introduce an uncertainty of 8ms on the number of samples played out with the default settings, and a lot more when URBs convey more packets to reduce the interrupt rate and power consumption.

This patch relies on the USB frame counter to reduce the uncertainty to ~1ms. The delay information essentially becomes independent of the URB size and number of packets. This should improve audio/video sync and help applications like PulseAudio which require accurate audio timing.

Signed-off-by: Pierre-Louis Bossart pierre-louis.bossart@linux.intel.com --- sound/usb/card.h | 2 ++ sound/usb/pcm.c | 23 +++++++++++++++++++++++ sound/usb/pcm.h | 3 +++ sound/usb/urb.c | 20 +++++++++++++++++--- 4 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/sound/usb/card.h b/sound/usb/card.h index ae4251d..a39edcc 100644 --- a/sound/usb/card.h +++ b/sound/usb/card.h @@ -94,6 +94,8 @@ struct snd_usb_substream { spinlock_t lock;

struct snd_urb_ops ops; /* callbacks (must be filled at init) */ + int last_frame_number; /* stored frame number */ + int last_delay; /* stored delay */ };

struct snd_usb_stream { diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index b8dcbf4..1696e2b 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -34,6 +34,27 @@ #include "clock.h" #include "power.h"

+/* return the estimated delay based on USB frame counters */ +snd_pcm_uframes_t snd_usb_pcm_delay(struct snd_usb_substream *subs, + unsigned int rate) +{ + int current_frame_number; + int frame_diff; + int est_delay; + + current_frame_number = usb_get_current_frame_number(subs->dev); + frame_diff = current_frame_number-subs->last_frame_number; + if (frame_diff < 0) + frame_diff += 1024; /* handle 10-bit wrap-around */ + + /* Approximation based on number of samples per USB frame (ms), + some truncation for 44.1 but the estimate is good enough */ + est_delay = subs->last_delay-(frame_diff*rate/1000L); + if (est_delay < 0) + est_delay = 0; + return est_delay; +} + /* * return the current pcm pointer. just based on the hwptr_done value. */ @@ -45,6 +66,8 @@ static snd_pcm_uframes_t snd_usb_pcm_pointer(struct snd_pcm_substream *substream subs = (struct snd_usb_substream *)substream->runtime->private_data; spin_lock(&subs->lock); hwptr_done = subs->hwptr_done; + substream->runtime->delay = snd_usb_pcm_delay(subs, + substream->runtime->rate); spin_unlock(&subs->lock); return hwptr_done / (substream->runtime->frame_bits >> 3); } diff --git a/sound/usb/pcm.h b/sound/usb/pcm.h index ed3e283..df7a003 100644 --- a/sound/usb/pcm.h +++ b/sound/usb/pcm.h @@ -1,6 +1,9 @@ #ifndef __USBAUDIO_PCM_H #define __USBAUDIO_PCM_H

+snd_pcm_uframes_t snd_usb_pcm_delay(struct snd_usb_substream *subs, + unsigned int rate); + void snd_usb_set_pcm_ops(struct snd_pcm *pcm, int stream);

int snd_usb_init_pitch(struct snd_usb_audio *chip, int iface, diff --git a/sound/usb/urb.c b/sound/usb/urb.c index e184349..ccf77d1 100644 --- a/sound/usb/urb.c +++ b/sound/usb/urb.c @@ -718,7 +718,15 @@ static int prepare_playback_urb(struct snd_usb_substream *subs, subs->hwptr_done += bytes; if (subs->hwptr_done >= runtime->buffer_size * stride) subs->hwptr_done -= runtime->buffer_size * stride; + + /* update delay with exact number of samples queued */ + runtime->delay = subs->last_delay; runtime->delay += frames; + subs->last_delay = runtime->delay; + + /* realign last_frame_number */ + subs->last_frame_number = usb_get_current_frame_number(subs->dev); + spin_unlock_irqrestore(&subs->lock, flags); urb->transfer_buffer_length = bytes; if (period_elapsed) @@ -737,12 +745,18 @@ static int retire_playback_urb(struct snd_usb_substream *subs, unsigned long flags; int stride = runtime->frame_bits >> 3; int processed = urb->transfer_buffer_length / stride; + int est_delay;

spin_lock_irqsave(&subs->lock, flags); - if (processed > runtime->delay) - runtime->delay = 0; + + est_delay = snd_usb_pcm_delay(subs, runtime->rate); + /* update delay with exact number of samples played */ + if (processed > subs->last_delay) + subs->last_delay = 0; else - runtime->delay -= processed; + subs->last_delay -= processed; + runtime->delay = subs->last_delay; + spin_unlock_irqrestore(&subs->lock, flags); return 0; }