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ALSA: usb-audio: Improve frames size computation

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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: Alexander Tsoy <alexander@tsoy.me>
Link: https://lore.kernel.org/r/20200424022449.14972-1-alexander@tsoy.me
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Alexander Tsoy 2020-04-24 05:24:48 +03:00 committed by Takashi Iwai
parent 10635d2d2a
commit f0bd62b640
4 changed files with 45 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
sound/usb/card.h
View File

@ -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 */

43
sound/usb/endpoint.c
View File

@ -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);

1
sound/usb/endpoint.h
View File

@ -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,

2
sound/usb/pcm.c
View File

@ -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);