Support for USB-Audio devices with Multiple configurations?

Support for USB-Audio devices with Multiple configurations?

[Cameron Stone]

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.

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.
Please let me know if you know one. ;-)

Thanks,
	Cameron.

Re: Support for USB-Audio devices with Multiple configurations?

[Clemens Ladisch]

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.

> 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.)


Best regards,
Clemens

Re: 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.

Re: Support for USB-Audio devices with Multiple configurations?

[Clemens Ladisch]

Cameron Stone wrote:
> Is it possible to have alternate settings with different numbers of
> channels?

Yes.  Each alternate setting has its own set of descriptors; you can
change _any_ parameter in those descriptors.


Best regards,
Clemens

Re: Support for USB-Audio devices with Multiple configurations?

[Cameron Stone]

Clemens Ladisch wrote:
> Cameron Stone wrote:
>> Is it possible to have alternate settings with different numbers of
>> channels?
> 
> Yes.  Each alternate setting has its own set of descriptors; you can
> change _any_ parameter in those descriptors.

Great, I've got this working now.

However, how do I ask arecord to use a particular alternate setting? I'd
like to have two alternate settings with the same number of channels,
but different microphones enabled. Is that possible? I'm using the
following command line (my USB mic array is card 1):

$ arecord -Dplughw:1,0 test.wav -d 1 -c 4 -r 30

I have two alternate settings with 4 channels, and arecord seems to
always choose the last one (the one with the highest alternate setting
number. This could also be the alsa driver making this choice.

amixer can display and control the different interfaces using the
second, but I can't find a command line option for arecord.

Thanks,	
	Cameron.

FYI arecord -L returns:

front:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    Front speakers
surround40:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    4.0 Surround output to Front and Rear speakers
surround41:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=NVidia,DEV=0
    HDA NVidia, ALC662 Analog
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
null
    Discard all samples (playback) or generate zero samples (capture)
front:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    Front speakers
surround40:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    4.0 Surround output to Front and Rear speakers
surround41:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=Adaptor,DEV=0
    8-Mic AVR Adaptor, USB Audio
    IEC958 (S/PDIF) Digital Audio Output

Re: Support for USB-Audio devices with Multiple configurations?

[Clemens Ladisch]

Cameron Stone wrote:
> Clemens Ladisch wrote:
> > Each alternate setting has its own set of descriptors; you can
> > change _any_ parameter in those descriptors.
> 
> Great, I've got this working now.
> 
> However, how do I ask arecord to use a particular alternate setting? I'd
> like to have two alternate settings with the same number of channels,
> but different microphones enabled. Is that possible?

This difference does not show up in the altsettings' descriptors, so the
driver would not know which one to choose.

This needs to be a mixer control (a selector unit).


HTH
Clemens