Do you have a laptop that’s either “pretty old” or “weird in some other way”? Did it ship without Windows from the factory, or did you flash its firmware with coreboot? You could help the Linux kernel move its backlight code forward without abandoning quirky gear like yours.
Hans de Goede, a longtime Linux developer and principal engineer at Red Hat, writes on his Livejournal about the need to test “a special group of laptops” to prevent their backlight controls from disappearing in Linux kernel 6.1.
Old laptop tests are needed because de Goede is initiating some major changes to user-space backlight controls, something he has been working on since 2014. As detailed at Linux blog Phoronix, there are multiple issues with how Linux tries to address the wide variety of backlight schemes in displays, which de Goede laid out at the recent Linux Plumbers Conference. There can be multiple backlight devices operating a single display, leaving high-level controls to “guess which one will work.” Brightness control requires root permissions at the moment. And “0” passed along as a backlight value remains a conundrum, as the engineer pointed out in 2014: Is that entirely off, or as low as the display can be lit?
The changes that de Goede has proposed for kernel 6.1 would allow for setting maximum brightness amounts, communicating clearly when brightness controls are not supported, and responding to hotplug events, like plugging in a monitor that supports a different brightness control scheme.
If your (old, weird) laptop is running Linux already, or you can boot it into a live USB session, you can check to see if your laptop might be affected by running ls /sys/class/backlight. If there’s only one entry, and it’s named intel_backlight, nouveau_bl, amdgpu_bl0, or radeon_bl0, there’s a chance your laptop could be affected by the big upcoming backlight change. To test further, follow the instructions in de Goede’s post.
As seen in a recent emergency update to the Linux kernel, providing the right code to properly power a laptop display is a tricky affair. There are a lot of laptop displays out there, and the kernel authors and maintainers can’t test them all. If you’ve got an old one you can boot into Linux, you might be able to help.
