That leaves several missing parts for a good LinuxCNC experience actually running hardware:
As far as UI performance, Mesa with LLVMPipe has made great strides since the first time I tried AXIS on a BeagleBone, and CPU improvements don't hurt either; performance on boards from Pi on up is pretty good now. You can also choose a previewless UI like touchy or gscreen. And, of course, remote NML works again in version 2.7, so you can run your UI on a different machine than your realtime (though this still has plenty of rough corners / papercuts).
Hardware drivers are beginning to trickle in. 2.7 has hm2_spi, which works on any preempt-rt with good /dev/spidev hardware drivers. Unfortunately, out of 3 systems I've tried (U3, XU4, and PI3), none had good spidev hardware drivers out of the box. I have a kernel git with patched U3 and XU4 drivers, which give good performance on U3 and low to adequate performance on the XU4. Our master branch has a userspace spi driver for PI-family CPUs (hm2_rpspi), and a GPIO-only driver for BBB (bb_gpio). One LinuxCNC developer is working on a BBB PRU firmware which provides a hostmot2 register-compatible interface, but that's not available yet. Based on my experience porting MachineKit's BBB GPIO driver to LinuxCNC, the difficulty of doing so is low, so if MK has a hardware driver you'd like to see in LinuxCNC, it's not the worst place you could start with contributing to LinuxCNC. Please remember to credit MachineKit authors by name in your LinuxCNC pull request, and also verify license compatibility.
For almost all of these boards, you can find various realtime patch sets, github repositories, debian packages, and so on. In my experience, they don't always work, which makes for a frustrating experience. If you want to use LinuxCNC 2.7, you need to find a PREEMPT-RT kernel; for our master branch, you can use PREEMPT-RT, Xenomai, or RTAI, depending whether your hardware driver needs a syscall interface in realtime (hm2_spi does, for example; hm2_rpspi and bb_gpio do not)--if you do, then PREEMPT-RT is your only choice.
The Machinekit fork of LinuxCNC offers pre-built realtime kernels and prefabricated OS images, and in my experience it is easy to to start with their image, remove machinekit and install LinuxCNC from source. Unfortunately, I was unable to find where they publish their kernel source or the scripts they use to generate the kernel packages and images. This means that the LinuxCNC project can't build on their packaging and distro-building work. I'll update this space if any little birds tell me where these resources can be found, because it would be a big help for a volunteer who wanted to get these kernel packages or OS images distributed from linuxcnc.org with LinuxCNC preinstalled.
That brings me to the last point: linuxcnc.org doesn't compensate anyone for their work on LinuxCNC. Current developers have all their time and motivation eaten up by supporting existing platforms, so without new volunteers, we simply don't have the time and motivation to make these first-rate, like we do for standard x86 systems. Join us and help make the LinuxCNC you want to use.
Update, 2017/3: A little bird has told me that the machinekit images use
these scripts to build
their OS images, but the kernel source remains elusive. It looks like
they come from http://repos.rcn-ee.com/debian but while there
are debian source packages for e.g., lxqt-sudo there aren't for e.g.,
Entry first conceived on 17 September 2016, 16:08 UTC, last modified on 6 July 2019, 21:49 UTC