[alsa-devel] USB hardware that supports implicit feedback?

Eldad Zack eldad at fogrefinery.com
Fri Nov 16 03:08:12 CET 2012

Hi Daniel,

On Fri, 16 Nov 2012, Daniel Mack wrote:
> On 16.11.2012 05:32, Daniel Griscom wrote:
> > At 10:02 PM +0100 11/15/12, Eldad Zack wrote:
> >> I'm not an ALSA dev, but I recently posted a series of patches for the
> >> M-Audio Fast Track C400, which uses the implicit feedback code that was
> >> already in the tree for other devices.
> Yes, sorry for not responding. I've seen this, but didn't have time yet
> to respond. Thanks for doing this.

Thank you for doing the heavy lifting there! After I noticed the 
clock drift (and read about the implicit feedback mechanism) with the 
first version I posted, I was able to get my hardware to work properly
thanks to your work.

BTW, I took some measurements with a slightly modified jack_delay
(to collect per-period statistics and more decimal precision output) and 
I see +/- 1 usec variation in real-world latency, so I'm very happy with 
it so far.

> Note that I never tested the automatic discovery of that mode. The
> device I wrote the code for doesn't announce it that way, and so a quirk
> was needed in pcm.c to make it happen. Ideally though, what you did is
> the right thing to do, just just need to make sure the driver really
> obeys that setting :)

I think that Autodiscovery should fail with this amtel device because 
the number of endpoints is 1 for the playback ep (this is what I encountered 
with the C400).
Maybe a info message could be added in case the attributes do say 
implicit feedback, but bNumEndpoints < 2?


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