Fixing Kernel Panics on the Dell Latitude 7490 with NixOS

I had to buy a laptop to use for college. My desktop PC at home is ridiculously over powered, so I wanted the cheapest laptop with modern security features that I could use as a thin client.¹ I figured the 7490 would do. To my dismay, the 7490 is not as trouble-free as the Arch Wiki suggested.

Shortly after booting into any Linux system, after either a few minutes or an attempt to suspend the laptop, the entire system hangs indefinitely until you hard reboot.

The Solution

This issue is caused by the i915 graphics driver and the cpu power states. Disabling these power management features solves the problem. Of course with the major caveat that your laptop will not be as power efficient. On NixOS the fix is as simple as:

{ config, pkgs, ... }:

{
  boot.kernelParams = [
    "i915.enable_dc=0"
    "i915.enable_psr=0"
    "intel_idle.max_cstate=1"
  ];
}

If you find that you cannot even reach or apply the configuration in time before the system hangs, just add those same kernel parameters right before you boot. This step depends on your bootloader.

Extra Trivia

It seems that Google themselves have struggled getting the 7490 to behave as well! On the ChromeOS Flex certified models list, the 7490 is listed as one of the few uncertified Dell models. It is not outright decertified, but Google says that “minor issues [are to be] expected.” Of course, I did test ChromeOS Flex, and it exhibited the exact same issue. I also tried to have a peristant kernel boot parameter applied, which was rejected due to boot safety reasons.

On a positive note, the 7490 actually supports WPA3 on Linux! On Windows, the networking driver was artificially limited to WPA2 only for this model. This kind of discrepancy is common due to how vendor driver support is implemented on Windows.