1 files changed, 26 insertions, 3 deletions

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 1546577ae458..5ceb8f1d63fb 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1091,7 +1091,16 @@ static void 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)
@@ -1109,12 +1118,26 @@ static void retire_playback_urb(struct snd_usb_substream *subs,
         struct snd_pcm_runtime *runtime = subs->pcm_substream->runtime;
         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);
 }