[alsa-devel] USB Implicit Feedback

[Daniel Mack]

On 09/06/2014 08:53 AM, Pierre C wrote:
> I'm implementing a USB Audio Class 2 device using a Cortex-M4 with high  
> speed USB. So far it works well, the device is recognized by ALSA,  
> playback, capture, sample rate change, and all controls like volume and  
> mute work. I'm using explicit feedback.
> 
> I'd like to use implicit feedback instead,

Why?

> and had some problems...
> 
> With implicit feedback, when playback starts, ALSA should start capture  
> automatically (even if there are no applications requesting capture at the  
> moment) to use the number of capture samples as feedback. But it doesn't.
> 
> So, I'd like to know :
> 
> - Is it actually supported ?

Yes.

> - If it is, what descriptors should I use to make it work ?

The usage mask of bmAttributes of the endpoint should be
USB_ENDPOINT_USAGE_IMPLICIT_FB, along with some other constraints. See
set_sync_endpoint() in sound/usb/pcm.c

Note, however, that we only implemented implicit feedback mode in the
driver because there are devices out there which use such modes. If
you're doing the device-side yourself, I'd actually recommend going for
explicit feedback, especially if that already works for you. Any
particular reason why you want to change that?


Daniel