Re: [alsa-devel] How to use implicit feedback with full duplex?

Hi Clemens,

On Tue, 5 Feb 2013, Clemens Ladisch wrote:

Hi,

I thought I'd try to use implicit feedback with my simple audio device:

Interface Descriptor:
  bInterfaceNumber        1
  bAlternateSetting       0
Interface Descriptor:
  bInterfaceNumber        1
  bAlternateSetting       1
  AudioStreaming Interface Descriptor:
    ...
  Endpoint Descriptor:
    bEndpointAddress     0x01  EP 1 OUT
Interface Descriptor:
  bInterfaceNumber        2
  bAlternateSetting       0
Interface Descriptor:
  bInterfaceNumber        2
  bAlternateSetting       1
  AudioStreaming Interface Descriptor:
    ...
  Endpoint Descriptor:
    bEndpointAddress     0x82  EP 2 IN

So I wrote a patch to configure it like those M-Audio devices:

--- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -391,6 +391,17 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt) attr = fmt->ep_attr & USB_ENDPOINT_SYNCTYPE;

switch (subs->stream->chip->usb_id) {

• case USB_ID(0x0582, 0x0018):

• if (is_playback) {
  

• 	implicit_fb = 1;
  

• 	ep = 0x82;
  

• 	iface = usb_ifnum_to_if(dev, 2);
  

• 	if (!iface || iface->num_altsetting < 2)
  

• 		return -EINVAL;
  

• 	alts = &iface->altsetting[1];
  

• 	goto add_sync_ep;
  

• }
  

• break;
  

Looks good to me.

case USB_ID(0x0763, 0x2030): /* M-Audio Fast Track C400 */ if (is_playback) { implicit_fb = 1;

This works fine when playing something:

...

But when I then try to record at the same time, the driver refuses to configure the input endpoint (to the only format, which is already set):

kernel: setting usb interface 2:1 kernel: Re-using EP 82 in iface 2,1 @ffff8801fd2ac000 kernel: Unable to change format on ep #82: already in use

And despite that "alreay in use" check, the input endpoint is affected so much that playback breaks.

Is full duplex supposed to work? Does it work with other devices?

This is probably a "yes, but" :) I use my device mostly full duplex, but with jack opening both playback and capture at the same time.

I assume you are opening two different streams, one for playback and one for capture. In that case, the capture EP is of course already in use, as a source for timing. If I try opening 2 different streams I get the same here - but playback doesn't break for me. I wonder why that happens.

Can you try using jackd -d alsa -d hw:x with the device and see if that works for you?

Cheers, Eldad