On Do, 2016-08-25 at 20:27 +0200, Jörg Krause wrote:

Hi,

I am looking for a way to toggle a GPIO if ALSA is about to play or has stopped to turn an external amplifier on/off.

Can it be done using libasound or can I use a plugin or write one?

After having a long night digging through ALSA's sourced I got it working writing a PCM hook library. Basically, it does the following:

static int snd_pcm_hook_gpio_hook_close(snd_pcm_hook_t *hook) { int fd;

/* * Set GPIO LOW */

return 0; }

int _snd_pcm_hook_gpio_hook_install(snd_pcm_t *pcm, snd_config_t *conf) { int fd; int err;

snd_pcm_hook_t *h_close = NULL;

err = snd_pcm_hook_add(&h_close, pcm, SND_PCM_HOOK_TYPE_CLOSE,        snd_pcm_hook_gpio_hook_close, NULL); if (err < 0) goto _err;

/* Init and set GPIO HIGH          */

return 0;

_err: if (h_close) snd_pcm_hook_remove(h_close);

return err; } SND_DLSYM_BUILD_VERSION(_snd_pcm_hook_gpio_hook_install, SND_CONFIG_DLSYM_VERSION_HOOK);

For reference my asound.conf:

pcm.softvol {     type softvol     slave {         pcm "gpio_hook"     }     control {         name "Master"         card 0     } } pcm_hook_type.gpio_hook {     lib "libasound_gpio_hook.so" } pcm.softvol_hook {     type hooks     slave {         pcm "hw:0"     }     hooks.0 {         type "gpio_hook"     } }

Jörg