[alsa-devel] proper use of snd_mixer_selem_set_capture_switch ?

[Aaron J. Grier]

does anyone have a good example or walkthrough how to change capture
sources?  I'm having a heck of a time.

my calls go like this:

/* setup */
snd_mixer_selem_id_alloca
snd_mixer_open
snd_mixer_attach
snd_mixer_selem_register
snd_mixer_load

/* set up PCM element, unused in this example */
snd_mixer_selem_id_set_index
snd_mixer_selem_id_set_name
snd_mixer_find_selem

/* set up Line element (linecapture) */
snd_mixer_selem_id_set_index
snd_mixer_selem_id_set_name
snd_mixer_find_selem

/* set up Mic element (miccapture) */
snd_mixer_selem_id_set_index
snd_mixer_selem_id_set_name
snd_mixer_find_selem

/* here's that part that throws me... I'm trying to sit in a loop and
 * trade capture back and forth between Line and Mic.  code reproduced
 * here verbatim... */

      do
      {
         snd_mixer_selem_channel_id_t chn;
         for (chn = 0; chn <= SND_MIXER_SCHN_LAST; chn++)
         {
            int junk;

            if (snd_mixer_selem_has_capture_channel(linecapture, chn))
            {
               status = snd_mixer_selem_get_capture_switch(linecapture,
                  chn, &junk);
               assert(0 == status);
               printf("line, chn %d, junk %d, ", chn, junk);
               status = snd_mixer_selem_set_capture_switch(linecapture,
                 chn, lineflag);
               printf("set %d\n", lineflag);
               assert(0 == status);
            }

            if (snd_mixer_selem_has_capture_channel(miccapture, chn))
            {
               status = snd_mixer_selem_get_capture_switch(miccapture,
                  chn, &junk);
               printf("mic, chn %d, junk %d, ", chn, junk);
               assert(0 == status);
               status = snd_mixer_selem_set_capture_switch(miccapture,
                  chn, micflag);
               printf("set %d\n", micflag);
               assert(0 == status);
            }
         }

         for (chn = 0; chn <= SND_MIXER_SCHN_LAST; chn++)
         {
            if (snd_mixer_selem_has_capture_channel(linecapture, chn))
            {
               status = snd_mixer_selem_get_capture_switch(linecapture,
                  chn, &lineflag);
               printf("read line, chn %d: %d\n", chn, lineflag);
               assert(0 == status);
            }

            if (snd_mixer_selem_has_capture_channel(miccapture, chn))
            {
               status = snd_mixer_selem_get_capture_switch(miccapture,
                  chn, &micflag);
               printf("read mic, chn %d: %d\n", chn, micflag);
               assert(0 == status);
            }
         }

         if (lineflag)
         {
            lineflag = 0;
            micflag = 1;
         }
         else
         {  
            lineflag = 1;
            micflag = 0;
         }

         getchar();

      } while (1);

I get output like this:

line, chn 0, junk 0, set 1
mic, chn 0, junk 1, set 0
line, chn 1, junk 1, set 1
mic, chn 1, junk 0, set 0
read line, chn 0: 1
read mic, chn 0: 0
read line, chn 1: 1
read mic, chn 1: 0

	[all is well, alsamixer shows LR capture on Line input, smash
	return and go for another loop]

line, chn 0, junk 1, set 0
mic, chn 0, junk 0, set 1
line, chn 1, junk 1, set 0
mic, chn 1, junk 0, set 1
read line, chn 0: 1
read mic, chn 0: 1
read line, chn 1: 1
read mic, chn 1: 1

	[here's where it goes funny.  the "junk" is what I set last
	time, as expected, but the readbacks after the
	set_capture_switch indicate both line and mic are being
	captured?  that's not right.  alsamixer shows LR capture on Mic
	input.  smack return again to see what happens.]

line, chn 0, junk 1, set 0
mic, chn 0, junk 1, set 1
line, chn 1, junk 1, set 0
mic, chn 1, junk 1, set 1
read line, chn 0: 0
read mic, chn 0: 1
read line, chn 1: 1
read mic, chn 1: 1

	[this gets really weird.  we make the same write calls we did
	last time, and now we get back something different?  alsamixer
	shows R Line and L Capture.  screwey.]

I feel I must be missing something obvious, like a flush operation, or
something very simple.  who knew changing a mux could be such an
involved procedure?  (=

-- 
  Aaron J. Grier | "Not your ordinary poofy goof." | agrier at poofygoof.com