[LAU] Problems connecting Sound Devices usbpre2 to JACK

[Chris Caudle]

On 2021-04-23 16:57, robertlazarski wrote:
> I am stuck on this part. jltcdump won't connect after starting jackd 
> from qjackctl.
> 
> [linux-fesf(robert)]
> /home/robert> sudo /usr/local/bin/jltcdump

Seeing "sudo" is a big red flag.  Why are you using sudo to start 
jltcdump?  Do you start all of your audio applications using sudo?  By 
using sudo to start jltcdump you are running it as root, but if you did 
not start qjackctl using sudo then jackd and jltcdump will be running 
under different user accounts and cannot access each other.

More to the point, it is recommended to never run jackd or jack 
applications as root.  Usually when someone is using sudo to start audio 
applications it indicates that the security configuration of their 
machine is incorrect and not allowing users to run applications with 
realtime permissions.
You should have a directory /etc/security/limits.d/ with files for 
various settings.  Depending on your distribution the settings may be in 
/etc/security/limits.conf or in separate files in /etc/security/limits.d 
or a mix of some settings in limits.conf and some settings in the 
individual files in limits.d/.
For example this is what I have in my 
/etc/security/limits.d/95-jack.conf file:

# Modified limits for users of jack-audio-connection-kit

## Automatically appended by jack-audio-connection-kit
## Other documents recommend rtprio of 60 or 70 -- Chris
@jackuser - rtprio 95

@jackuser - memlock 16777216

## Automatically appended by jack-audio-connection-kit
# This seemed to override the value of 90 I set above, so commenting out
# @pulse-rt - rtprio 80
@pulse-rt - nice -20


I am running Fedora linux on my machine, which creates the group 
jackuser for user accounts which will be using jackd.  I think the 
current default Fedora file sets the rtprio limit to 70, but I had used 
95 as a limit going way back based on recommendations I saw in some 
jackd FAQ.  As a practical matter it won't really matter as long as you 
also make sure anything else which tries to set priorities, like the 
rtirq script, is coordinated to use the same maximum value.

The last section that I commented out was part of the file from Fedora, 
but I think since my user account is a member of both jackuser and 
pulse-rt groups, the later rtprio limit of 80 for the pulse-rt group was 
overriding the limit of 95 set earlier for the jackuser group.

After correcting any problems in your conf files you will have to log 
out and log back in for the changes to take effect.   You can verify the 
limit in use with the ulimit program:
~]$ ulimit -r
95

After verifying your user account can set realtime permissions, start 
your applications again but do not use sudo.  I suspect that is the 
cause of your problems.

-- 
Chris Caudle