Jonathan Fung

Latest Pages

Setting Kernel Parameters in NixOS

3 min read

A Problem

I recently updated my NixOS channels

I had an issue with my laptop where the display would just lock up for no apparent reason. I would have to force shut down, but one time I was able to jump into a tty and get a glimpse of the error logs.

I find a few Arch1 and Framework2 support threads that eventually pointed to drm/amd #3647. Users on the threads and issue tracker said to add ‘amdgpu.dcdebugmask=0x10’ to the “kernel parameters”, or that they were “testing with [it]”, or to”boot with [it]”, or to “add options amdgpu dcdebugmask=0x10 to the modprobe config”. Setting this parameter would disable something called “Panel Self-Refresh”, which I believe is connected to adaptive refresh rate. (For my sanity, this was enough to satiate my curiosity)

I’ve only heard of kernel parameters by name before, but kind of just thought they were flags to set when compiling the kernel from source. So I had to find out what “booting with” and modprobe actually meant. Additionally, because NixOS takes a fundamentally different approach to configuration than most other Linux distros, it’s always a bit of a journey to figure out how to apply these lower-level tweaks.

And a Solution

I found a couple NixOS attributes as interfaces for setting kernel parameters: boot.kernelParams, boot.kernel.sysctl, and boot.extraModprobeConfig. To actually test these options I also had to figure out how to check if this parameter value was actually set.

NixOS attribute options
# Attribute 1
boot.kernelParams = ["amdgpu.dcdebugmask=0x10"];
 
# Attribute 2
boot.kernel.sysctl = {"amdgpu.dcdebugmask" = "0x10";};
 
# Attribute 3
boot.extraModprobeConfig = ''
  options amdgpu dcdebugmask=0x10
'';

Initially, I found two methods for introspection:

  • Directory /proc/sys which is usually accessed by the sysctl <mymodule> wrapper. The man page states: “sysctl is used to modify kernel parameters at run time.”
  • A file with path resembling /sys/module/<mymodule>/parameters/<paramname> which seems to have more modules than the previous

(Later on I found modprobe --showconfig | grep <mymodule>, which dumps the effective configuration of a module.)

Introspection options
ls /proc/sys/amdgpu                           # Introspection 1
cat /sys/module/amdgpu/parameters/dcdebugmask # Introspection 2

Aside

This cursory search planted a seed of inquiry: “What is a kernel parameter?“. My future self ended up falling into this rabbit hole.

Here is a table that shows if the introspection option correctly showed the configured value for the respective attribute option. I also rebooted my system each time.

/proc/sys/sys/.../<paramname>
boot.kernelParamsNoYes
boot.kernel.sysctlNoNo
boot.extraModprobeConfigNoYes

In conclusion, I have no idea what the difference is between all these different options. I ended up going with boot.kernelParams because it seemed the most canonical.

Footnotes

  1. https://bbs.archlinux.org/viewtopic.php?id=302499 ↩

  2. https://community.frame.work/t/responded-framework-13th-gen-diy-ubuntu-screen-super-laggy-1fps-on-occasion/31767/7 ↩

Graph View