First, reMarkable should be congratulated for a cool product and not locking down the device. It is easy to get root SSH access and they have not locked down the debugging interface. This makes it possible to boot your own boot loader (u-boot) and Linux kernel. They also use OpenEmbedded to build the rootFS which is great, although it would be much better if they contributed their changes back and released their source code.
They also release their u-boot and Linux source code, although they do legally have to. Here they could do a better job of upstreaming their changes, this improves the user experience (as it's easier to update kernels) and helps developers.
A new mainline Linux kernel release happens about every 2-3 months. These releases are the major and minor versions, for example 4.14 and 4.15. Some of these releases are marked as long term, the 4.14 release is a long term release. Long term releases continue to receive point releases afterwards. At this time of this writing the latest 4.14 point release is 4.14.218.
It is perfectly acceptable to use a long term release (for example 4.14) it is critical that the point releases are regularly updated. These releases fix critical security vulnerabilities as well as other important bugs. To get an idea of how many CVEs (security vulnerabilities) have been fixed for the 4.14 kernel, have a look here: https://www.linuxkernelcves.com/streams/4.14
If you are used to Linux on your desktop you might use long term releases then. For example Ubuntu 20.04 shipped with the 5.4 kernel. Ubuntu 20.04 will continue to use the 5.4 release. The key difference here is that Ubuntu continues to update the point releases of their kernel.
The rM2 shipped and as of this writing continues to use the 4.14.78 kernel. The 4.14 kernel was released in 2017 and the 4.14.78 update was shipped in 2018. The 4.14.78 kernel is unpatched and unsupported, when you purchase the product the software is already out of date.
The rM2 uses a kernel that was forked from this branch: https://github.com/Freescale/linux-fslc/tree/4.14-1.0.x-imx. It appears there have not been any updates to a newer release (either 4.14.x or later) and it also seems like there has been no effort to upstream the changes.
It should be pretty easy to rebase the rM2 zero-sugar tree on the latest from the https://github.com/Freescale/linux-fslc/tree/4.14-1.0.x-imx branch to at least get an updated 4.14 kernel with fixes.
There is on going work to update to a 5.4 kernel. Currently the work is to rebase it on the i.MX branch from the FSL sources. That is, to rebase the remarkable patches on top of this branch: https://github.com/Freescale/linux-fslc/tree/5.4-2.2.x-imx
The latest 5.4 rM2 kernel is available here: https://github.com/alistair23/linux/tree/5.4-rM2-2.2.x-imx
Currently all features work with the 5.4 kernel!
There is also on going work to run a mainline (official) kernel on the rM2. That effort can be seen here: https://github.com/alistair23/linux/tree/zero-sugar-mainline
This is a work in progress, so only things that work are listed below
- Full boot - UART