Re: ALSA drivers for USB audio devices

["Jeffrey W. Baker" <jwbaker@acm.org>]

On Tue, 2002-03-19 at 14:39, Paul Davis wrote:
> >Recently on this very list, Paul Barton-Davis proposed that one could
>                              ^^^^^^^^^^^^^^^^^
> 			     Paul        Davis (op.net/~pbd/name.html)

My mistake.  Sorry.

> >build a driver for a USB audio device entirely in user space.  I think
> >this is an interesting idea, as I own an Edirol UA-5 USB audio device. 
> 
> Pray that it follows the "tandards" The existing OSS-based driver is
> full of special case code for just about every device so far
> supported.

It supposedly uses some Steinberg driver that supports a number of USB
devices, not all from Roland.  That gives me hope.  However, the ASIO
driver doesn't support 24/96 operation, so this may be a
device-by-device extension.  Duplex operation is limited to 24/48 anyway
because of USB bandwidth.  I won't care as long as it works on my
hardware :)

> there are two approaches:
> 
>       1) a new low-level ALSA driver that handles interactions
>              with a USB audio device, and presents the ALSA "midlevel"
> 	     code with the normal *internal* ALSA API
> 
>       2) a shared library that would be loaded by alsa-lib, based
>              on a description in an asoundrc file. this library 
> 	     would use an existing USB driver that allowed raw
> 	     "USB packet" I/O - it would format the USB packets
> 	     in user space based on requests from alsa-lib,
> 	     and then deliver them to the kernel USB driver.

The latter sounds more workable -- building packets is userspace is not
hard at all, and profiling for performance is more practical.

> the userspace implementation is only a translation layer between the
> ALSA PCM API and the USB audio packet standard. it would be an
> existing USB kernel driver that would take care of streaming the data
> to and from the h/w. i don't think the user space code would have any
> problems with this. the kernel driver might, but if so, its a crock
> anyway, and needs reimplementing. i don't know either way.
> 
> i suspect that it would be faster to write the low-level module, but
> more interesting and potentially more flexible to write the user space
> library. 

Probably less duplication of code as well.

-jwb


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