Re: [PATCH] ALSA: usb-audio: Apply async workaround for Scarlett 2i4 2nd gen

[Alexander Tsoy <alexander@tsoy.me>]

В Ср, 22/04/2020 в 22:28 +0200, Takashi Iwai пишет:
> On Wed, 22 Apr 2020 21:26:23 +0200,
> Alexander Tsoy wrote:
> > В Ср, 22/04/2020 в 20:55 +0200, Gregor Pintar пишет:
> > > On Wed, 22 Apr 2020 Alexander Tsoy wrote:
> > > > В Вт, 21/04/2020 в 21:31 +0200, Takashi Iwai пишет:
> > > > > I wonder, though, whether we can correct the rounding error
> > > > > in
> > > > > the
> > > > > driver code, too.
> > > > 
> > > > I'm not sure if it's possible with currently used Q16.16
> > > > arithmetic.
> > > 
> > > Maybe calculate fixed correction shifts (like it would be
> > > feedback)?
> > > Something like leap year.
> > > 
> > > In endpoint.c:
> > > static inline unsigned get_usb_high_speed_rate(unsigned int rate)
> > > {
> > > 	return ((rate << 10) + 62) / 125;
> > > }
> > > I guess 62 tries to round it, but exact number is needed. So
> > > exact
> > > value for
> > > 44100 should be 361267.2. For 48000 it is 360448.
> > > If only we can deliver that 0.2 by shifting rate somehow?
> > > 
> > > At least maybe it would be better to disable sample rates that do
> > > not
> > > divide
> > > by 1000 on SYNC playback endpoints, if there are others sample
> > > rates.
> > > 
> > > But I'm not familar with the code or USB.
> > 
> > I think instead of accumulating the fractional part of fs/fps in
> > Q16.16
> > format we should accumulating remainder of division fs/fps.
> > 
> > So for 44100 Hz and High Speed USB the calculations would be:
> > 
> > fs = 44100
> > fps = 8000
> > rem = 44100 % 8000 = 4100
> > accum = 0
> > packet_size_min = 44100 / 8000 = 5
> > packet_size_max = 44100 + (8000 - 1) / 8000 = 6
> > 
> > 
> > 1. accum += rem = 4100
> >    accum < fps => packet_size = packet_size_min = 5
> > 
> > 2. accum += rem = 8200
> >    accum >= fps => {
> >        packet_size = packet_size_max = 6
> >        accum -= fps = 200
> >    }
> > 
> > 3. accum += rem = 4300
> >    accum < fps => packet_size = packet_size_min = 5
> > 
> > ...
> > 
> > 80. accum += rem = 8000
> >     accum >= fps => {
> >         packet_size = packet_size_max = 6
> >         accum -= fps = 0
> >     }
> > ...
> 
> Yeah, something like that is what I had in my mind.
> It'd be greatly appreciated if someone can experiment it.
> Unfortunately I have no proper USB-audio device now at hands...

OK, here is a quick hacky patch, that seems to work for me:


diff --git a/sound/usb/card.h b/sound/usb/card.h
index 395403a2d33f..fc5e3e391219 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -84,6 +84,10 @@ struct snd_usb_endpoint {
 	dma_addr_t sync_dma;		/* DMA address of syncbuf */
 
 	unsigned int pipe;		/* the data i/o pipe */
+	unsigned int framesize[2];	/* min/max frame size in samples */
+	unsigned int sample_rem;	/* remainder of division fs/fps */
+	unsigned int sample_accum;	/* fs % fps accumulator */
+	unsigned int fps;		/* frames per second */
 	unsigned int freqn;		/* nominal sampling rate in fs/fps in Q16.16 format */
 	unsigned int freqm;		/* momentary sampling rate in fs/fps in Q16.16 format */
 	int	   freqshift;		/* how much to shift the feedback value to get Q16.16 */
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 4a9a2f6ef5a4..e0e4dc5cfe0e 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -124,12 +124,12 @@ int snd_usb_endpoint_implicit_feedback_sink(struct snd_usb_endpoint *ep)
 
 /*
  * For streaming based on information derived from sync endpoints,
- * prepare_outbound_urb_sizes() will call next_packet_size() to
+ * prepare_outbound_urb_sizes() will call slave_next_packet_size() to
  * determine the number of samples to be sent in the next packet.
  *
- * For implicit feedback, next_packet_size() is unused.
+ * For implicit feedback, slave_next_packet_size() is unused.
  */
-int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep)
+int snd_usb_endpoint_slave_next_packet_size(struct snd_usb_endpoint *ep)
 {
 	unsigned long flags;
 	int ret;
@@ -146,6 +146,32 @@ int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep)
 	return ret;
 }
 
+/*
+ * For adaptive and synchronous endpoints, prepare_outbound_urb_sizes()
+ * will call next_packet_size() to determine the number of samples to be
+ * sent in the next packet.
+ */
+int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep)
+{
+	unsigned long flags;
+	int ret;
+
+	if (ep->fill_max)
+		return ep->maxframesize;
+
+	spin_lock_irqsave(&ep->lock, flags);
+	ep->sample_accum += ep->sample_rem;
+	if (ep->sample_accum >= ep->fps) {
+		ep->sample_accum -= ep->fps;
+		ret = ep->framesize[1];
+	} else {
+		ret = ep->framesize[0];
+	}
+	spin_unlock_irqrestore(&ep->lock, flags);
+
+	return ret;
+}
+
 static void retire_outbound_urb(struct snd_usb_endpoint *ep,
 				struct snd_urb_ctx *urb_ctx)
 {
@@ -190,6 +216,8 @@ static void prepare_silent_urb(struct snd_usb_endpoint *ep,
 
 		if (ctx->packet_size[i])
 			counts = ctx->packet_size[i];
+		else if (ep->sync_master)
+			counts = snd_usb_endpoint_slave_next_packet_size(ep);
 		else
 			counts = snd_usb_endpoint_next_packet_size(ep);
 
@@ -874,10 +902,17 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
 	ep->maxpacksize = fmt->maxpacksize;
 	ep->fill_max = !!(fmt->attributes & UAC_EP_CS_ATTR_FILL_MAX);
 
-	if (snd_usb_get_speed(ep->chip->dev) == USB_SPEED_FULL)
+	if (snd_usb_get_speed(ep->chip->dev) == USB_SPEED_FULL) {
 		ep->freqn = get_usb_full_speed_rate(rate);
-	else
+		ep->fps = 1000;
+	} else {
 		ep->freqn = get_usb_high_speed_rate(rate);
+		ep->fps = 8000;
+	}
+
+	ep->sample_rem = rate % ep->fps;
+	ep->framesize[0] = rate / ep->fps;
+	ep->framesize[1] = (rate + (ep->fps - 1)) / ep->fps;
 
 	/* calculate the frequency in 16.16 format */
 	ep->freqm = ep->freqn;
diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h
index 63a39d4fa8d8..d23fa0a8c11b 100644
--- a/sound/usb/endpoint.h
+++ b/sound/usb/endpoint.h
@@ -28,6 +28,7 @@ void snd_usb_endpoint_release(struct snd_usb_endpoint *ep);
 void snd_usb_endpoint_free(struct snd_usb_endpoint *ep);
 
 int snd_usb_endpoint_implicit_feedback_sink(struct snd_usb_endpoint *ep);
+int snd_usb_endpoint_slave_next_packet_size(struct snd_usb_endpoint *ep);
 int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep);
 
 void snd_usb_handle_sync_urb(struct snd_usb_endpoint *ep,
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index a4e4064f9aee..b50965ab3b3a 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1579,6 +1579,8 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
 	for (i = 0; i < ctx->packets; i++) {
 		if (ctx->packet_size[i])
 			counts = ctx->packet_size[i];
+		else if (ep->sync_master)
+			counts = snd_usb_endpoint_slave_next_packet_size(ep);
 		else
 			counts = snd_usb_endpoint_next_packet_size(ep);
 
-- 
2.25.3