>I think the big concern is that there are road blocks to RTAI on newer kernel 
>versions.

RTAI mainline only supports kernel 4.1 (at the time of this email) but 
modifying a few bits of the tree and grabbing code from here:

http://git.xenomai.org/ipipe.git/log/?h=ipipe-4.4.y

4.4 kernel support could be added to RTAI. The easiest way (IMHO) is updating 
my fork of the RTAI tree and adding in the 4.4 specific bits as my tree was 
specifically designed for LinuxCNC, and to be as developer friendly as 
possible, with a much smaller code base, and a completely revamped autotools 
build system. If you're interested:

https://github.com/NTULINUX/RTAI

Maybe some review of this commit will assist in the port of 4.4 to RTAI:

http://git.xenomai.org/xenomai-3.git/commit/?h=next&id=eddbb24f651ee2b2594715099b638ae8cddf485d

I'd keep an eye on this tree as well for kernel 4.4 updates, as Xenomai 2.6 
branch is much more closer related to the RTAI tree than the new version 3 
rewrite:

http://git.xenomai.org/xenomai-2.6.git/log/

4.4 is still in the development phases in Xenomai, but once it's done, updates 
can be pretty seamless with some basic C knowledge and kernel hacking skill, 
I've done updates many times before, 4.4 is nothing special, never is.

One thing that will really, and I mean dramatically, ease this process is the 
new dovetail/symboite development branch, which is essentially the good-ole 
IPIPE that we all know and love, but with a completely redesigned 
infrastructure that follows the way upstream kernel patches _should_ be 
written, PREEMPT_RT for example. It's code base does not touch and patch things 
that it doesn't need to, rather has it's own files with only the minimal amount 
of changes to the upstream code so the kernel compiles as usual if PREEMPT_RT 
in Kconfig is disabled. Currently, and this is has been the case for a very 
long while; disabling IPIPE via Kconfig using the IPIPE/hal-linux/adeos patches 
does not simply just do that, some behaviors are actually still being changed, 
however the vast majority of IPIPE is disabled (and toggled as such) but those 
changes are still being applied to the wide kernel tree due to improper 
implementation and ugly wide-spread changes. Symboite (now called dovetail I 
assum
 e) is a much cleaner design with a code-base that adds in it's own files 
separate from the vanilla tree, therefor minimizes merge conflicts between 
kernel releases (major revisions / PATCHLEVEL) and easing the development 
process.

Dovetail:

http://git.xenomai.org/dovetail.git/

http://elinux.org/images/7/76/Kiszka.pdf (pages 24, 26, and 28, pages as in 
page written in PDF file, not page specified by the viewer / browser)

The differences between hal-linux patches in RTAI vs the ones in Xenomai 
(IPIPE) / official Adeos patches are pretty minimal, using "diff -Naur" against 
the same version from each project will show this. A few extra exported 
symbols, added definitions, nothing dramatic.

Cheers!

Alec Ari

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