[LAU] MOTU AVB discussion from LAC

[Max]

On 30.03.19 04:52, Paul Davis wrote:
> On Fri, Mar 29, 2019 at 4:57 PM Max <abonnements at revolwear.com 
> <mailto:abonnements at revolwear.com>> wrote:
> 
>     Fernando Lopez-Lezcano discussed the pitfalls of using the MOTU AVB
>     external audio interfaces with Linux in his paper [1] and also in the
>     keynote at LAC-19.
>     Let's start another thread around this card.
> 
>     Fernando mentioned that different firmwares expose different issues,
>     but
>     downgrading is possible. Issues are:
>     1. Channels are not persistent and swap around.
>     2. Total number of channels has been reduced in newer firmwares.
>     3. An endless card acquisition loop between Jack and Pulseaudio
>     caused by
>     the long time the card needs to switch sampling rate.
>     4. Seemingly erratic behavior, opening the device fails, fails again,
>     again, then works suddenly.
> 
> issue 4 is not resolvable via firmware changes. it seems part of the 
> device design.

I think 3 and 4 may actually be the same issue.

>     I have a couple of questions and experiences.
> 
>     Is there a table of the firmware versions somewhere (linuxaudio wiki?)
>     which tell me which versions has which features (removed)?
> 
> 
> fernando mentioned a post on "linux musicians" which describes each one.

for reference, I guess the thread is this one:
https://www.linuxmusicians.com/viewtopic.php?f=6&t=18046

It's a rather long thread with some interesting tidbits here and there. 
Someone even wrote a test program, but one has to be signed in to see 
attachments on this forum.

Three takeaways from this thread:
1. AVB might be supported by the kernel at some point, which would allow 
to use the AVB interfaces without the USB connection.
See: https://github.com/L-Acoustics/avdecc for a multiplatform 
implementation.

2. Firmware 1.2.8. is the last one which provides 64 channels via USB 
Class Compliant, downgrade to this version if more then 24 channels are 
needed.

The old firmwares are downloadable here:
http://motu.com/techsupport/technotes/firmwarechangelog/

3. There is no way to change the number of channels through ALSA or via 
the configuration panel of the web-interface. It's static.

>     Are all the Class Compliant models having the same quirks as listed
>     above? For example the
>     MOTU UltraLite AVB versus the MOTU 624 AVB?
> 
>     Is it possible to use the Thunderbolt port to connect the card to a
>     Linux computer?
> 
>     Why can't I tell ALSA to use only the first 2, 4 whatever channels of
>     the device? I can only open the device if all channels are
>     connected. Is
>     this always like that or is that a limitation of MOTU's implementation?
> 
> ALSA doesn't work that way. You open a device. You tell it how many 
> channels you want to use.

That's what I thought, but apparently it only succeeds to open an ALSA 
device if the number of channels match.

>     On my laptop with only one USB bus, If I connected the  MOTU 624 AVB
>     and
>     then another high speed usb device, the computer could not connect the
>     later, because the MOTU reserved all the bandwith for itself.
>     Connecting
>     the other device and then the MOTU worked.
> 
> 
> this is correct, and likely a sensible design choice. others may 
> disagree. i actually threw away a webcam because i mistakenly believed 
> that it was no longer working (because of this issue).

That's hilarious. I only stumbled upon this issue by looking at dmesg 
output because of something unrelated. Thankfully the dmesg output is 
quite verbose and clear about what's going on.