[arch-proaudio] Introducing: dedicated realtime group (not only for jack/jack2)

[David Runge]

On 2018-07-29 18:52:36 (-0400), bill-auger wrote:
> can you explain why a new group was needed? why remove functionality
> from the audio group - that is the commonly expected purpose of the
> audio group on every distro i have used that caters to A/V use-cases -
> conversely, what purpose would the audio group now serve?
The audio group is installed via syusers.d with the systemd package
(defined in /usr/lib/sysusers.d/basic.conf).
It's one of the system's default groups, which grants access to
/dev/audio and /dev/snd/* [1] (/dev/rtc0, mentioned last, got dropped in
by jack/jack2 a few years back). The accompanying udev rule can be found
in /usr/lib/udev/rules.d/50-udev-default.rules (also part of the systemd
package). Traditionally, other software installing own rules, relies on
this group and its settings (e.g. libffado, as found in
/usr/lib/udev/rules.d/60-ffado.rules).

The compromise introduced a long time ago for the jack and jack2
packages (as described upstream on the realtime topic [2]) also relies
on the audio group.
However, acquiring realtime privileges and accessing audio hardware are
two separate things. Note, that while acquiring realtime privileges can
be benificial for pro-audio applications, it is not the only use-case!

To make things more clear:
audio - access specific audio hardware
realtime - acquire realtime privileges (which you can use for pro-audio)

> most such distros (including the arch derivative i use) also have a
> package which is usually named similarly to 'proaudio-settings', that
> depends on JACK and the kernel-rt and provides optimizations for
> real-time audio use - but the JACK package should not require the
> kernel-rt nor any privileges or optimizations; simply because JACK
> itself does not require those and is intended to be fully usable
> without them
That's exactly why the realtime group will exist and realtime-privileges
will become an optional dependency for jack and jack2.

It doesn't make any sense to have realtime-privileges depend on linux-rt
and or jack/jack2, as realtime-privileges is meant to extend the
capabilities of what a certain user group is allowed to achieve with the
system (and the system's hardware) and therefore is an extension of what
said group can achieve with linux-rt and/or jack/jack2.

> i proposed before the idea of maintaining a consistent
> 'proaudio-settings' optimizations package in the arch upstream - does
> this 'realtime-privilege' package do nothing more than create
> 'realtime' group, with no other optimizations? - if it also has other
> general optimizations for real-time audio, then i suggest it be
> renamed to a commonly recognizable name like: 'proaudio-settings' - if
> not, then i still propose such a 'proaudio-settings' optimization
> package - such a package would necessarily depend on JACK and the
> 'realtime-privilege' package - but that would make it clear that it is
> the optimization package that provides realtime privileges and not the
> JACK package
The "optimizations" (which are more capabilities given to the realtime
group) are now all in the realtime-privileges package, instead of
redundantly packaged in jack/jack2 and being bound to the audio group
(which is there for accessing audio hardware, not for acquiring realtime
privileges).

Again: Acquiring realtime is not a pro-audio only thing.
jack/jack2 can work without acquiring realtime (mind [2]). However, one
can extend reliability of some software by acquiring realtime
privileges. Therefore it makes no sense to have separate settings for
pro-audio, as realtime-privileges is a more general approach to whatever
one wants to achieve with that.
In other words: You wouldn't want to have to install jack/jack2 just to
acquire realtime privileges for another piece of software, that is non
pro-audio related, but you can now optionally extend jack/jack2's
capabilities by using realtime-privileges (with a more clear
distinction as to what the groups you're in actually stand for).

I hope this answers your questions.

Best,
David

[1] https://wiki.archlinux.org/index.php/Users_and_groups#Pre-systemd_groups
[2] http://jackaudio.github.io/faq/linux_rt_config.html

-- 
https://sleepmap.de
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