On 2021/10/2 12:45, Geraldo Nascimento wrote:
On Fri, Oct 01, 2021 at 10:01:34PM -0300, Geraldo Nascimento wrote:
On Fri, Oct 01, 2021 at 08:33:23PM -0300, Geraldo Nascimento wrote:
On Sat, Oct 02, 2021 at 01:31:12AM +0800, Zhang Boyang wrote:
Hello,
Hello, Zhang!
I'm using ALSA to develop an audio-streaming application. I try to
use start_threshold and stop_threshold in combination with small
buffers. However, I think I probably found a bug about this.
I'm setting start_threshold=100 and stop_threshold=50. I'm also using
a buffer of 44 frames. When I call
snd_pcm_writei(the_small_44_frames_buffer), pcm state came to XRUN from
PREPARED directly. I think this is a bug because the stream hasn't
started. It's hard to say a xrun occurred while stream not started.
I'm wondering if this is a ALSA-bug or a misuse of ALSA. A simple bug
test program is attached.
No, I don't think it's a bug. You're bound to run into problems with a
period size of only 44 frames.
Moreover, working with the code you provided, I was able to get a RUNNING
state without XRUNs with a period size of 4410 frames (100 milliseconds of
audio) but I had to comment out snd_pcm_sw_params_set_stop_threshold() for
it to work or I'd have those instant XRUNs.
That's how snd_pcm_sw_params_set_stop_threshold() is supposed to work by
the way. It creates a XRUN once the threshold is hit.
Oh, and snd_pcm_sw_params_set_stop_threshold() is a little
counter-intuitive. The threshold you give the function is compared to
the available free space in the ring buffer, not the filled space.
That's why lowering the stop threshold makes XRUNs more likely...
Thanks,
Geraldo Nascimento
Zhang,
I was able to make your code work in the end, even with 44 frames
writes, which is unadvisable since it eats too much CPU specially if
they are low-end like the ARM64 box I'm writing you from.
The trick was to use high values such as 500000. Lower values such as
250000 were enough to trigger XRUNs in my machine. Of course you can set
it to -1 and it will ignore XRUNs but that's a bit like setting it to
16777216 or 4096 * 4096 which is a hardcoded limit in alsa-lib, I
believe.
And plainly ignoring XRUNs may sound bad under load, believe me.
Thank you,
Geraldo Nascimento
Hi,
Thank you for your reply.
I realized my mistake about the meaning of stop_threshold. The
stop_threshold should be (bufsz-50), where (bufsz = alsa buffer size). I
fixed my code and new code is attached.
However, the problem still exists. I'm trying to minimize audio latency,
so I must use a small buffer. In fact, the problem is not related to the
absoulte size of buffer, but the relationship between the size of
start_threshold, stop_threshold and the buffer. The problem still exists
even using manual start mode (i.e. start_threshold>bufsz).
Currently, I'm using a workaround. After PREPARED or XRUN'd, I manually
buffer incoming audio frames, and when (n_buffered_frames >=
start_threshold && n_buffered_frames >= bufsz-stop_threshold), I write
them in a single write() call. Note that the write must in whole, any
short write may result in immediate XRUN.
Anyway, Thank you for your advice. Have a nice day. :)
Zhang Boyang
#define PCM_DEVICE "hw:0,0"
/*
gcc -Wall -o bug start_stop_threshold_bug.c -lasound
./bug
*/
/*
* Simple sound playback using ALSA API and libasound.
*
* Compile:
* $ cc -o play sound_playback.c -lasound
*
* Usage:
* $ ./play <sample_rate> <channels> <seconds> < <file>
*
* Examples:
* $ ./play 44100 2 5 < /dev/urandom
* $ ./play 22050 1 8 < /path/to/file.wav
*
* Copyright (C) 2009 Alessandro Ghedini <al3xbio@xxxxxxxxx>
* --------------------------------------------------------------
* "THE BEER-WARE LICENSE" (Revision 42):
* Alessandro Ghedini wrote this file. As long as you retain this
* notice you can do whatever you want with this stuff. If we
* meet some day, and you think this stuff is worth it, you can
* buy me a beer in return.
* --------------------------------------------------------------
*/
#include <alsa/asoundlib.h>
#include <stdio.h>
int main(int argc, char **argv) {
unsigned int pcm, tmp;
unsigned rate;
int channels, seconds;
snd_pcm_t *pcm_handle;
snd_pcm_hw_params_t *params;
snd_pcm_uframes_t frames, bufsz;
char *buff;
int buff_size, loops;
rate = 44100;
channels = 2;
seconds = 10;
/* Open the PCM device in playback mode */
if ((pcm = snd_pcm_open(&pcm_handle, PCM_DEVICE,
SND_PCM_STREAM_PLAYBACK, 0) < 0)) {
printf("ERROR: Can't open \"%s\" PCM device. %s\n",
PCM_DEVICE, snd_strerror(pcm));
abort();
}
/* Allocate parameters object and fill it with default values*/
snd_pcm_hw_params_alloca(¶ms);
snd_pcm_hw_params_any(pcm_handle, params);
/* Set parameters */
snd_pcm_hw_params_set_rate_resample(pcm_handle, params, 0);
if ((pcm = snd_pcm_hw_params_set_access(pcm_handle, params,
SND_PCM_ACCESS_RW_INTERLEAVED) < 0)) {
printf("ERROR: Can't set interleaved mode. %s\n", snd_strerror(pcm));
abort();
}
if ((pcm = snd_pcm_hw_params_set_format(pcm_handle, params,
SND_PCM_FORMAT_S16_LE) < 0)) {
printf("ERROR: Can't set format. %s\n", snd_strerror(pcm));
abort();
}
if ((pcm = snd_pcm_hw_params_set_channels(pcm_handle, params, channels) < 0)) {
printf("ERROR: Can't set channels number. %s\n", snd_strerror(pcm));
abort();
}
if ((pcm = snd_pcm_hw_params_set_rate_near(pcm_handle, params, &rate, 0) < 0)) {
printf("ERROR: Can't set rate. %s\n", snd_strerror(pcm));
abort();
}
/* Write parameters */
if ((pcm = snd_pcm_hw_params(pcm_handle, params) < 0)) {
printf("ERROR: Can't set harware parameters. %s\n", snd_strerror(pcm));
abort();
}
if ((pcm = snd_pcm_hw_params_get_buffer_size(params, &bufsz) < 0)) {
printf("ERROR: Can't get buffer size. %s\n", snd_strerror(pcm));
abort();
}
int start_threshold = 100;
int stop_threshold = bufsz - 50;
snd_pcm_sw_params_t *swparams;
snd_pcm_sw_params_alloca(&swparams);
pcm = snd_pcm_sw_params_current(pcm_handle, swparams);
if (pcm < 0) {
printf("Unable to determine current swparams: %s", snd_strerror(pcm));
abort();
}
pcm = snd_pcm_sw_params_set_start_threshold(pcm_handle, swparams, start_threshold);
if (pcm < 0) {
printf("Unable to set start threshold mode: %s", snd_strerror(pcm));
abort();
}
pcm = snd_pcm_sw_params_set_stop_threshold(pcm_handle, swparams, stop_threshold);
if (pcm < 0) {
printf("Unable to set stop threshold mode: %s", snd_strerror(pcm));
abort();
}
pcm = snd_pcm_sw_params(pcm_handle, swparams);
if (pcm < 0) {
printf("Unable to set sw params: %s", snd_strerror(pcm));
abort();
}
/* Resume information */
printf("PCM name: '%s'\n", snd_pcm_name(pcm_handle));
printf("PCM state: %s\n", snd_pcm_state_name(snd_pcm_state(pcm_handle)));
snd_pcm_hw_params_get_channels(params, &tmp);
printf("channels: %i ", tmp);
if (tmp == 1)
printf("(mono)\n");
else if (tmp == 2)
printf("(stereo)\n");
snd_pcm_hw_params_get_rate(params, &tmp, 0);
printf("rate: %d\n", tmp);
printf("seconds: %d\n", seconds);
printf("bufsz: %ld\n", bufsz);
printf("start_threshold: %d\n", start_threshold);
printf("stop_threshold: %d\n", stop_threshold);
frames = 44;
buff_size = frames * channels * 2 /* 2 -> sample size */;
buff = (char *) malloc(buff_size);
memset(buff, 0, buff_size);
snd_pcm_hw_params_get_period_time(params, &tmp, NULL);
for (loops = (seconds * 1000000) / tmp; loops > 0; loops--) {
printf("==== loop ====\n");
printf("(before write) PCM state: %s\n", snd_pcm_state_name(snd_pcm_state(pcm_handle)));
pcm = snd_pcm_writei(pcm_handle, buff, frames);
printf("written %d\n", pcm);
printf("(after write) PCM state: %s\n", snd_pcm_state_name(snd_pcm_state(pcm_handle)));
if (pcm == -EPIPE) {
printf("XRUN.\n");
snd_pcm_prepare(pcm_handle);
abort();
} else if (pcm < 0) {
printf("ERROR. Can't write to PCM device. %s\n", snd_strerror(pcm));
}
}
snd_pcm_drain(pcm_handle);
snd_pcm_close(pcm_handle);
free(buff);
return 0;
}
