[alsa-devel] Support for USB-Audio devices with Multiple configurations?

[Cameron Stone]

Clemens Ladisch wrote:
> Cameron Stone wrote:
>> I'm writing some firmware for a USB microphone array with multiple
>> configurations, and I'm wondering if it's possible to tell ALSA which
>> configuration to use in a module option or something (device_setup,
>> perhaps).
>>
>> I've been looking through usbaudio.c in the kernel source
>> (2.6.28-11-generic from the ubuntu Jaunty package), and I can't find any
>> generic device configuration like that.
>>
>> If this is not possible, does anyone have any recommendations on how to
>> implement this? I can imagine doing it using quirks for this particular
>> USB_ID, but this seems like a fairly generic type of capability, so I'd
>> like to make it general if possible.
> 
> While the USB specifications define configurations, there is no generic
> way how a driver could automatically determine which configuration to
> use.

I was thinking in terms of a module parameter that would be set if a
configuration other than the default (first) configuration was desired.
Could that work?

>> If anyone is wondering *why* I want multiple configurations: it's for
>> bandwidth management. I have 8 channels, but I want to be able to sample
>> 1 channel as fast as possible without wasting bandwidth on the other 7.
>> I couldn't figure out a way to do that with a single configurations.
> 
> Bandwidth management is implemented by having multiple alternate
> settings for the audio streaming interface.  (This is why alternate
> setting zero must have no endpoint or a zero-sized endpoint, so that
> in the default case, the interface does not use any bandwidth.)

Is it possible to have alternate settings with different numbers of
channels? I can see how I could easily en/disable the channels in the
firmware, but I can't see how to inform the driver that the number of
channels has changed.

Thanks,
	Cameron.