Commit f0bd62b6 authored by Alexander Tsoy's avatar Alexander Tsoy Committed by Takashi Iwai
Browse files

ALSA: usb-audio: Improve frames size computation 



For computation of the the next frame size current value of fs/fps and
accumulated fractional parts of fs/fps are used, where values are stored
in Q16.16 format. This is quite natural for computing frame size for
asynchronous endpoints driven by explicit feedback, since in this case
fs/fps is a value provided by the feedback endpoint and it's already in
the Q format. If an error is accumulated over time, the device can
adjust fs/fps value to prevent buffer overruns/underruns.

But for synchronous endpoints the accuracy provided by these computations
is not enough. Due to accumulated error the driver periodically produces
frames with incorrect size (+/- 1 audio sample).

This patch fixes this issue by implementing a different algorithm for
frame size computation. It is based on accumulating of the remainders
from division fs/fps and it doesn't accumulate errors over time. This
new method is enabled for synchronous and adaptive playback endpoints.

Signed-off-by: default avatarAlexander Tsoy <alexander@tsoy.me>
Link: https://lore.kernel.org/r/20200424022449.14972-1-alexander@tsoy.me


Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent 10635d2d

sound/usb/card.h

+4 −0
Original line number Diff line number Diff line
@@ -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];	/* small/large frame sizes in samples */

	unsigned int sample_rem;	/* remainder from division fs/fps */

	unsigned int sample_accum;	/* sample 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 */

sound/usb/endpoint.c

+38 −5
Original line number Diff line number Diff line
@@ -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 +146,29 @@ 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)

{

	int ret;



	if (ep->fill_max)

		return ep->maxframesize;



	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];

	}



	return ret;

}



static void retire_outbound_urb(struct snd_usb_endpoint *ep,

				struct snd_urb_ctx *urb_ctx)

{
@@ -190,6 +213,8 @@ 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 +899,17 @@ 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;
@@ -936,6 +968,7 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep) 
	ep->active_mask = 0;

	ep->unlink_mask = 0;

	ep->phase = 0;

	ep->sample_accum = 0;



	snd_usb_endpoint_start_quirk(ep);

sound/usb/endpoint.h

+1 −0
Original line number Diff line number Diff line
@@ -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,

sound/usb/pcm.c

+2 −0
Original line number Diff line number Diff line
@@ -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);

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