4 files changed, 60 insertions, 3 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index ae4251d5abf7..a39edcc32a93 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 b8dcbf407bbb..0b699ca1957e 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -34,6 +34,30 @@
 #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);
+        /*
+         * HCD implementations use different widths, use lower 8 bits.
+         * The delay will be managed up to 256ms, which is more than
+         * enough
+         */
+        frame_diff = (current_frame_number - subs->last_frame_number) & 0xff;
+
+        /* 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 / 1000);
+        if (est_delay < 0)
+                est_delay = 0;
+        return est_delay;
+}
+
 /*
  * return the current pcm pointer.  just based on the hwptr_done value.
  */
@@ -45,6 +69,8 @@ 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);
 }
@@ -417,6 +443,8 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
         subs->hwptr_done = 0;
         subs->transfer_done = 0;
         subs->phase = 0;
+        subs->last_delay = 0;
+        subs->last_frame_number = 0;
         runtime->delay = 0;
 
         return snd_usb_substream_prepare(subs, runtime);
diff --git a/sound/usb/pcm.h b/sound/usb/pcm.h
index ed3e283f618d..df7a003682ad 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 e184349aee83..b4dcccc237dc 100644
--- a/sound/usb/urb.c
+++ b/sound/usb/urb.c
@@ -718,7 +718,16 @@ 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);
+        subs->last_frame_number &= 0xFF; /* keep 8 LSBs */
+
         spin_unlock_irqrestore(&subs->lock, flags);
         urb->transfer_buffer_length = bytes;
         if (period_elapsed)
@@ -737,12 +746,27 @@ 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;
+
+        /*
+         * Report when delay estimate is off by more than 2ms.
+         * The error should be lower than 2ms since the estimate relies
+         * on two reads of a counter updated every ms.
+         */
+        if (abs(est_delay - subs->last_delay) * 1000 > runtime->rate * 2)
+                snd_printk(KERN_DEBUG "delay: estimated %d, actual %d\n",
+                        est_delay, subs->last_delay);
+
         spin_unlock_irqrestore(&subs->lock, flags);
         return 0;
 }