We've got mail#

Sun 26 Mar 2023 11:19:45 AM UTC

As you'll see if you're reading this on www.liminix.org (but not if you're using the RSS feed) we're now on version 0.2 of the web site. I've starting to add a bit more structure beyond the blog entries and would draw your attention particularly to:

• Mailing Lists: one for announcements, one for discussions, and one for techy stuff. Please sign up, it's a bit lonely being the only subscriber.

• Continuous Build service, which has been running for a while and invaluable for most of that while - but only this week have I made it available outside my home network. The web site has some notes on what it tests: for more information read the source :-)

In development news - yes, there was also a little bit of development -

• got "kexecboot" into a usable state, and wrote an explanation of how to use it.

• wrote a proof-of-concept watchdog service. This is only in the arhcive example right now and hasn't made it to Liminix "core"

• removed the second copy of Lua I had accidentally introduced into the image

• unified kernel parameter-passing behaviour. I'm slightly proud of this...

At your command

The problem is this: many devices have hacked-up or restricted versions of U-boot installed which hardcode the kernel command-line parameters instead of letting the user configure them. These hardcoded parameters are most likely wrong for Liminix, but luckily we can configure the kernel to ignore the bootloader command line and use a builtin command line or refer to the embedded DTB.

However, this creates another problem: when we are hacking away on an open device with a serial cable, or when we are booting with kexecboot, or when running emulated Liminix with QEMU, we want to change the bootloader kernel command line.

So we needed one kernel with CONFIG_MIPS_CMDLINE_FROM_BOOTLOADER and one with CONFIG_MIPS_CMDLINE_FROM_DTB. Because kernel configuration is a property of the configuration not of the "output", this meant editing the configuration.nix to comment-out or uncomment the kexecboot or flashable module depending on which output was needed - a reliable source of strange behaviour whenever I forgot to.

Our solution is to add an option for "ignore the bootloader command line unless it contains a magic string", and then ensure that the tooling for tftpboot and kexecboot adds that string. The bootloader hardcoded command line doesn't, so we have the desired behaviour in both use cases.

This does require a small kernel patch but hopefully that's not a huge maintenance burden to carry.

(Fixing vendor U-boot on all devices is most certainly not within scope for Liminix.)

Next week

Next week we are moving on to "Task 2":

NixWRT used a self-designed module system (instead of Nixos modules) to allow us to easily express the configuration of multiple instances of the same module – for example when two differently configured copies of the same service are running on different network interfaces. But it had no validity checking or type checking of module options, making it painfully easy to generate and install a bad image. In this phase we revisit this decision, and figure out how to gain the advantages of the Nixos options/type system while still allowing multiple instances of the same module

See you then.