[Jack-Devel] Firewire drivers in JACK: background info (was: Re: JACK on mobile)

On Thu, Oct 17, 2013 at 10:52:55AM +0100, Pedro Alves wrote:
> E.g., jack bypassing ALSA for firewire always stroke me as odd and
> indicative of possible ALSA limitations, though I'm far from an expert. 

FYI there were a number of reasons why FFADO implemented its audio streaming
system outside of the kernel (and ALSA generally).  Many of these have gone
away over time (especially with the introduction of the new firewire stack),
and I don't believe any of them were due to particular limits of ALSA at the
time(*).  In hindsight it is generally accepted by FFADO developers that
doing this was a bad idea since the timing requirements of the interfaces
are much harder to make in userspace.  This is why we (the FFADO team) have
a long-term goal to move the audio streaming components of FFADO into ALSA's
PCM system, and there are already a few proof of concept implementations
doing the rounds.

I should emphasise that the decision to run with the present structure did
not come from JACK - to my knowledge the JACK developers themselves were not
involved in the initial decisions.  FFADO developers (and those of its
precursor freebob) implemented the drivers independently in a userspace
library and saw a JACK backend as the easiest way to make the firewire
interfaces available to users - so one was written.  The consequence whereby
ALSA applications could not make use of the interfaces was not viewed as a
major problem at the time because the programs one would typically use with
multichannel firewire interfaces were (and generally still are) exclusively
JACK-based for all the usual reasons.

The mixer and control side of things is separate (and is outside the scope
of JACK and the present discussion).  Our current plan is to retain these
components as they are since it will be awkward to accommodate the needs of
these interfaces through the ALSA mixer API.  The control and management of
these interface features is quite complex and therefore better suited to
userspace rather than kernel space.  Of course time will tell.

Regards
  jonathan

(*) Since I was not involved in Freebob/FFADO when these initial decisions
    were made I could be wrong here, and if so I stand corrected.

1382008404.11915_0.ltw:2, <20131017111259.GA9004 at marvin dot atrad dot com dot au>