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
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