Re: [PATCH 1/2] ALSA: usb-audio: ignore delay calculation for capture stream

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

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>> Shouldn't we add support for delay on the capture path instead? We could
>> use the same frame counter to report the delay and resync when the urb
>> is retired.
>
> Sure, it must be possible like that way.
> Wouldn't you like to volunteer? ;)

here's a suggestion. Since we don't submit multiple URBs as in the 
playback case, we can only track the delay using the frame counter for a 
single URB, and we reset the counters when the URB is retired.
Did a couple of tests, seems to work ok.
I couldn't figure out if the related patches are already in git, I 
didn't find them. Takashi, did you merge them already?
-Pierre



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diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index c659310..65dc53e 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -59,7 +59,12 @@ snd_pcm_uframes_t snd_usb_pcm_delay(struct snd_usb_substream *subs,
 
 	/* 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);
+	est_delay =  frame_diff * rate / 1000;
+	if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK)
+		est_delay = subs->last_delay - est_delay;
+	else
+		est_delay = subs->last_delay + est_delay;
+
 	if (est_delay < 0)
 		est_delay = 0;
 	return est_delay;
@@ -1147,6 +1152,10 @@ static void retire_capture_urb(struct snd_usb_substream *subs,
 	int i, period_elapsed = 0;
 	unsigned long flags;
 	unsigned char *cp;
+	int current_frame_number;
+
+	/* read frame number here, update pointer in critical section */
+	current_frame_number = usb_get_current_frame_number(subs->dev);
 
 	stride = runtime->frame_bits >> 3;
 
@@ -1180,6 +1189,13 @@ static void retire_capture_urb(struct snd_usb_substream *subs,
 			subs->transfer_done -= runtime->period_size;
 			period_elapsed = 1;
 		}
+		/* capture delay is by construction limited to one URB, reset delays here */
+		runtime->delay = subs->last_delay = 0;
+
+		/* realign last_frame_number */
+		subs->last_frame_number = current_frame_number;
+		subs->last_frame_number &= 0xFF; /* keep 8 LSBs */
+
 		spin_unlock_irqrestore(&subs->lock, flags);
 		/* copy a data chunk */
 		if (oldptr + bytes > runtime->buffer_size * stride) {

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