Use SATA disks during boot on RockPi 4C under NixOS

In previous posts I got nixos working on my RockPI 4C. One thing that did not sit right was that it required everything on the system to be on either the eMMC or SD card. I have a ssd connected to thing after all.

Anyway, the reason for this is that the system packaged into initramfs (that's the stuff nixos puts in the ESP) needs to be able to mount certain paths on the system.¹

This should work automatically, but in my case, the system needs some extra kernel modules to be able to reach the SSD. I have my ssd installed in the Penta SATA HAT, but I'd expect the same modules to be needed for other options as well. The magic incantation to make it work is:

  boot.initrd.availableKernelModules = [
    "phy_rockchip_pcie"
    "pcie_rockchip_host"
  ];

How do you get this list you might ask? Well, since I know that the disks are working on my running system, I know the list of suspected kernel modules. Just run the lsmod command. This outputs a list of 115 modules.

I can narrow this down a little by excluding modules that are a dependency of other modules as that will be included by the dependant module (see the Used by column). This gets us down to 57 modules. Now we can test which modules are needed and which are not. What I did was input the list of all modules first and confirm that it boots.

After I was in known-working state, I deleted some modules and tried rebooting. Good thing here is that if it did not work, I could just select older generation and I was back in working state again. When a deletion caused the system to no longer boot, I knew that I removed important module. In reality since it was more convenient to test with working boot, I prefered modules which I suspected are not needed first, but in principle it could be done even without that.

And that's it, I now have a working nixos-based rockpi4c including ssd support. I can't really imagine how painful it'd be with a distro which does not offer declarative configuration which could be deployed remotely.