[alsa-devel] USB Implicit Feedback

[Daniel Mack]

Hi,

On 09/07/2014 09:06 AM, Pierre C wrote:
> - The Apple app notes seem to say it's the best way to do things, and I'd  
> like it to also work on Macs. But I'm not married to Apple, and it seems  
> Macs also support explicit feedback, so this isn't a very important point.

Explicit feedback is much more common than anything else, and I haven't
yet seen a standard compliant device with implicit feedback in the wild.
And more common means more test coverage :)

> - Implicit feedback would use one less IN endpoint, and that would be  
> useful.

Yes, but it will always cause traffic in both directions, even if only
one is actually used. Unless you're in full-duplex mode, you'll waste
50% of the bandwidth.

>> 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
> 
> I have read this source code before but I'm not sure what the second  
> parameter of "get_endpoint(alts, 1)" really means, does it imply that an
> altsetting has to have 2 endpoints and it takes the second one, but in  
> which order ? order of definition in descriptors, endpoint number ?...

That's the index of the endpoint in the given alt interface.

> But is it bmAttributes in the playback endpoint descriptor, or its buddy  
> capture endpoint ? The USB docs are quite vague and I see several possible  
> interpretations ... Does the playback AudioStreaming descriptor need to  
> have 1 endpoint, or 2 ?... I've tried various combinations with the latest  
> stable linux kernel, none worked.

It's been a while since I looked into this, but when I implemented the
code in the first place, I had no standard compliant device to test
with, so I had to force the driver into implicit mode with a quirk. I'd
recommend adding printk() to the driver and trace the calls and see how
they parse the descriptors.

But as I said - if explicit feedback mode already works for you, I'd
stick with it.


Best regards,
Daniel