At Tue, 7 Aug 2012 09:32:11 -0500, Andrew Eikum wrote:

Hi folks,

I've got either a bug or a misunderstanding in how ALSA handles ioplugs that provide hw_params restrictions. My understanding is that the snd_pcm_ioplug_set_param_* functions are supposed to restrict which hw_params settings a client may set. This seems to work for at least access and format. But I noticed that it's broken for buffer_bytes and periods at least.

Attached is two source files demonstrating the problem. alsa-example.c is a dead-simple ioplug module. It sets some hw_params restrictions, and does basically nothing else. The important thing to notice is that it sets /only one/ allowable period size, period count, and buffer size, all of which are compatible with each other (256 byte period * 8 periods == 2048 byte buffer size).

There is also test client, alsa-test.c, which attempts to violate those hw_params restrictions, and succeeds! The eventual call to the ioplug's example_hw_params() callback gets an invalid buffer_size parameter (it happens to be 512).

Why is it wrong? 512 frames = 512 * 2 channel * 2 bytes-per-sample = 2048 bytes.

I guess you are confused about the units passed to snd_pcm_hw_params_set_buffer_size_near(). It's in frames, not in bytes.

Takashi

Here is the output when run on my Arch Linux with alsa-lib 1.0.25:

[aeikum@aeikum ioplug_buffer_bytes]$ ./alsa-test ALSA-example: Opening bytes: 512 ALSA-example: hw_params: 512 TEST FAILURE: buffer_size != 8812: 512 ALSA-example: stop ALSA-example: close [aeikum@aeikum ioplug_buffer_bytes]$

Am I misunderstanding the snd_pcm_ioplug_set_param_* methods, or is this a bug? I am seeing this behavior in the wild with a pulseaudio client that ignores my hw_params restrictions.

Thanks for any insight, Andrew [2 alsa-example.c <text/x-csrc; utf-8 (7bit)>] /* build with:

• $ gcc -fPIC -DPIC -c -Wall -Wno-unused-variable -Wno-unused-function alsa-example.c
• $ gcc -Wl,--no-undefined -shared -fPIC -lasound -o libasound_module_pcm_example.so alsa-example.o -lc
• # cp libasound_module_pcm_example.so /usr/lib/alsa-lib/
• create an 'example' PCM in asoundrc of type 'example'
• test with attached alsa-test.c

*/

#include <alsa/asoundlib.h> #include <alsa/pcm_external.h>

#include <stdio.h> #include <errno.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <stdint.h> #include <sys/time.h>

struct example_pcm { snd_pcm_ioplug_t io; };

static int example_start(snd_pcm_ioplug_t *io) { struct example_pcm *sp_pcm = io->private_data; fprintf(stderr, "ALSA-example: start\n"); return 0; }

static int example_stop(snd_pcm_ioplug_t *io) { struct example_pcm *sp_pcm = io->private_data; fprintf(stderr, "ALSA-example: stop\n"); return 0; }

static snd_pcm_sframes_t example_pointer(snd_pcm_ioplug_t *io) { struct example_pcm *sp_pcm = io->private_data; fprintf(stderr, "ALSA-example: pointer\n"); return 0; }

static snd_pcm_sframes_t example_transfer(snd_pcm_ioplug_t *io, const snd_pcm_channel_area_t *areas, snd_pcm_uframes_t offset, snd_pcm_uframes_t size) { struct example_pcm *sp_pcm = io->private_data; fprintf(stderr, "ALSA-example: transfer\n"); return size; }

static int example_close(snd_pcm_ioplug_t *io) { struct example_pcm *sp_pcm = io->private_data; fprintf(stderr, "ALSA-example: close\n"); return 0; }

static int example_hw_params(snd_pcm_ioplug_t *io, snd_pcm_hw_params_t *params) { struct example_pcm *sp_pcm = io->private_data;

fprintf(stderr, "ALSA-example: hw_params\n");

if(io->buffer_size != 8812)
    fprintf(stderr, "TEST FAILURE: buffer_size != 8812: %lu\n", io->buffer_size);
else
    fprintf(stderr, "TEST SUCCESS: buffer_size == 8812: %lu\n", io->buffer_size);

return 0;

}

static const snd_pcm_ioplug_callback_t example_playback_callback = { example_start, /* start, required */ example_stop, /* stop, required */ example_pointer, /* pointer, required */ example_transfer, /* transfer, optional */ example_close, /* close, optional */ example_hw_params, /* hw_params, optional */ NULL, /* hw_free, optional */ NULL, /* sw_params, optional */ NULL, /* prepare, optional */ NULL, /* drain, optional */ NULL, /* pause, optional */ NULL, /* resume, optional */ NULL, /* poll_descriptors_count, optional */ NULL, /* poll_descriptors, optional */ NULL, /* poll_revents, optional */ NULL, /* dump, optional */ NULL/* delay, optional */ };

static int example_set_hw_params(snd_pcm_ioplug_t *io) { static const snd_pcm_access_t access_list[] = { SND_PCM_ACCESS_RW_INTERLEAVED };

static const unsigned int format_list[] = {
    SND_PCM_FORMAT_S16
};

int err;

if((err = snd_pcm_ioplug_set_param_list(io, SND_PCM_IOPLUG_HW_ACCESS,
                sizeof(access_list) / sizeof(*access_list), access_list)) < 0)
    return err;

if((err = snd_pcm_ioplug_set_param_list(io, SND_PCM_IOPLUG_HW_FORMAT,
                sizeof(format_list) / sizeof(*format_list), format_list)) < 0)
    return err;

if((err = snd_pcm_ioplug_set_param_minmax(io, SND_PCM_IOPLUG_HW_RATE,
                44100, 44100)) < 0)
    return err;

if((err = snd_pcm_ioplug_set_param_minmax(io, SND_PCM_IOPLUG_HW_CHANNELS,
                2, 2)) < 0)
    return err;

/* 8 * 256 = 2048 */
if((err = snd_pcm_ioplug_set_param_minmax(io, SND_PCM_IOPLUG_HW_PERIOD_BYTES,
                256,256)) < 0)
    return err;

if((err = snd_pcm_ioplug_set_param_minmax(io, SND_PCM_IOPLUG_HW_PERIODS,
                8, 8)) < 0)
    return err;

if((err = snd_pcm_ioplug_set_param_minmax(io, SND_PCM_IOPLUG_HW_BUFFER_BYTES,
                2048, 2048)) < 0)
    return err;

return 0;

}

int _snd_pcm_example_open(snd_pcm_t **pcmp, const char *name, snd_config_t *root, snd_config_t *conf, snd_pcm_stream_t stream, int mode) { struct example_pcm *sp_pcm; int err;

fprintf(stderr, "ALSA-example: Opening\n");

sp_pcm = malloc(sizeof(*sp_pcm));
if(!sp_pcm)
    return -ENOMEM;
memset(sp_pcm, 0, sizeof(*sp_pcm));

sp_pcm->io.version = SND_PCM_IOPLUG_VERSION;
sp_pcm->io.name = "example Protocol Plugin";
sp_pcm->io.callback = &example_playback_callback;
sp_pcm->io.private_data = sp_pcm;

err = snd_pcm_ioplug_create(&sp_pcm->io, name, stream, mode);
if(err){
    free(sp_pcm);
    return err > 0 ? -err : err;
}

err = example_set_hw_params(&sp_pcm->io);
if(err){
    snd_pcm_ioplug_delete(&sp_pcm->io);
    free(sp_pcm);
    return err;
}

*pcmp = sp_pcm->io.pcm;

return 0;

}

SND_PCM_PLUGIN_SYMBOL(example); [3 alsa-test.c <text/x-csrc; utf-8 (7bit)>] /* to build:

• $ gcc -Wall -o alsa-test alsa-test.c -lasound

*/

#include <alsa/asoundlib.h>

int main(int argc, char **argv) { snd_pcm_t *pcm; snd_pcm_hw_params_t *hw_params; unsigned int rate; snd_pcm_uframes_t bytes; int err;

if((err = snd_pcm_open(&pcm, "example", SND_PCM_STREAM_PLAYBACK, 0) < 0)){
    fprintf(stderr, "snd_pcm_open: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

if((err = snd_pcm_hw_params_malloc(&hw_params)) < 0){
    fprintf(stderr, "snd_pcm_hw_params_malloc: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

if((err = snd_pcm_hw_params_any(pcm, hw_params) < 0)){
    fprintf(stderr, "snd_pcm_hw_params_any: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

if((err = snd_pcm_hw_params_set_access(pcm, hw_params,
                SND_PCM_ACCESS_RW_INTERLEAVED) < 0)){
    fprintf(stderr, "snd_pcm_hw_params_set_access: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

if((err = snd_pcm_hw_params_set_format(pcm, hw_params,
                SND_PCM_FORMAT_S16_LE) < 0)){
    fprintf(stderr, "snd_pcm_hw_params_set_format: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

rate = 44100;
if((err = snd_pcm_hw_params_set_rate_near(pcm, hw_params, &rate, NULL)) < 0){
    fprintf(stderr, "snd_pcm_hw_params_set_rate_near: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

if((err = snd_pcm_hw_params_set_channels(pcm, hw_params, 2)) < 0){
    fprintf(stderr, "snd_pcm_hw_params_set_channels: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

bytes = 512;
if((err = snd_pcm_hw_params_set_buffer_size_near(pcm, hw_params, &bytes)) < 0){
    fprintf(stderr, "snd_pcm_hw_params_set_buffer_size_near: %d (%s)\n", err, snd_strerror(err));
    return 1;
}
fprintf(stderr, "bytes: %lu\n", bytes);

if((err = snd_pcm_hw_params(pcm, hw_params)) < 0){
    fprintf(stderr, "snd_pcm_hw_params: %d (%s)\n", err, snd_strerror(err));
    return 1;
}

snd_pcm_hw_params_free(hw_params);
snd_pcm_close(pcm);

return 0;

} [4 <text/plain; us-ascii (7bit)>] _______________________________________________ Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel