[LAU] Working MADI interfaces for Linux? Was Motu M64 Madi interface (not) class compliant?

[Fernando Lopez-Lezcano]

On 3/12/21 3:00 PM, David Runge wrote:
> On 2021-03-12 00:08:53 (+0100), Thibaud Keller wrote:
>> Thanks Roger for the suggestion,
>> we are actually running at 48k.
>> These limitations seem to be built in as you can see on page 88 of the
>> manual
>> https://www.rme-audio.de/downloads/mface_pro_e.pdf
> 
> This might be due the fact, that the class compliant mode is not aimed
> at Linux and/or general computing, but there to support iPads (that's at
> least the case in the Fireface UFX series).

Ha, interesting... I was actually looking at the usb spec a couple of 
days ago (due to this thread) and trying to find a reference to limits 
on channel count - a quick browse/search did not find anything but I may 
have missed it. Maybe this is in some other document.

> I received this fun piece of information from an RME employee at
> superbooth two years ago. The channel limitations in class compliant
> mode in e.g. the RME Fireface UFX (II, +, etc.) are hardcoded and
> matched to the capabilities of some iPad version that was supposed to be
> supported back then (and is by now long EOL).

Motu's original firmware for the AVB interfaces was upgraded early on to 
be able to do up to 64 channels on usb2 (which I have been using 
extensively). It was latter downgraded to just 24. I started a ticket 
and was told that they were having (unspecified) problems with it.

There is a hack that makes it possible to connect the linux usb driver 
to the proprietary usb endpoints of the audio interface and thus run at 
up to 64 channels (for 44.1/48KHz sampling rate, of course). This 
involves kernel patches and also manually switching the interface to the 
proprietary mode through an hhtp/json command (otherwise it just uses 
the class compliant endpoints[*] after you plug it in). This very long 
thread has information about this:

https://linuxmusicians.com/viewtopic.php?f=6&t=18046&sid=3deb30fa5d04d5240741640f329aa2ea

(many options explored but no real solution that works all the time)
While the hack makes it possible to use 64 channels on newer hardware 
that cannot be downgraded to the older Motu firmware, its behavior is 
not 100% reliable - it might be better behaved in 5.11+ but I have not 
tried yet.

-- Fernando

[*] I imagine there is some initial handshake that tells the card that 
the motu driver is accessing it, and that is obviously missing in linux