[LAU] USB card oddness

Monty Montgomery xiphmont at gmail.com
Wed Dec 11 01:35:34 UTC 2013


It doesn't have to be; what matters is that's it being scheduled into
a space that fragments the bandwidth the audio device needs.

Even if the audio device only needs 3% of the bandwidth and the mouse
.0001%, the mouse drops that USB frame into low-speed compat mode and
blocks all higher speed devices from using any bandwidth during that
timing period.  If the audio device requires a tiny, uninterrupted
piece of scheduling every period, it's just lost any chance of
successful scheduling.

On top of this, the Linux host schedulers are not particularly
effective about avoiding these conflicts.

[sorry for some ambiguity above; I don't remember if 1.0/1.1 drops a
full frame to low speed or just the microframe].

Monty


On Tue, Dec 10, 2013 at 8:28 PM, Bob van der Poel <bob at mellowood.ca> wrote:
> On Tue, Dec 10, 2013 at 6:04 PM, Gabriel M. Beddingfield
> <gabrbedd at gmail.com> wrote:
>> On 12/10/2013 02:12 PM, Bob van der Poel wrote:
>>>
>>> Figured it out after a few hours :) We'd only recently plugged her
>>> mouse into the same multiport. If the "telephone" is plugged into a
>>> different port, it works. If the mouse is not plugged into the
>>> multiport, it works.
>>
>>
>> Yeah, that's one of the hidden pull-your-hair-out gotchas of USB Audio.  I
>> ran into it once where an audio device was on the same hub as a webcam --
>> and that didn't work well.
>>
>> The deal is that the audio device reserves a certain amount of bandwidth
>> over USB.  If it can't get -- it won't even try to work.  So if some other
>> device has reserved the bandwidth you need (which they can), then there's no
>> room left over for yours.
>>
>> -gabe
>
> Makes sense. But all that's on the port is a mouse and the telephone.
> I don't see either one using much bandwidth. She uses the same port to
> read pics from a camera, but that was not plugged. And, I rebooted to
> make sure things were cleared out. Guess I've got a greedy mouse :)
>
>
> --
> **** Listen to my FREE CD at http://www.mellowood.ca/music/cedars ****
> Bob van der Poel ** Wynndel, British Columbia, CANADA **
> EMAIL: bob at mellowood.ca
> WWW:   http://www.mellowood.ca
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