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

[Eldad Zack]

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