Hans de Goede and Drew DeVault Kernel Hacking (UPDATED)
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“Old/weird laptops” sought to help test Linux kernel backlight drivers | Ars Technica
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?
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Kernel 6.1-rc# might break backlight control on old/weird laptops, please test: hansdegoede — LiveJournal
I have landed a large(ish) refactor of the ACPI/x86 backlight detection code in the kernel for 6.1. I have been very careful to try and not break things but there is a special group of laptops where the ability to control the backlight brightness may disappear because of this.
The most likely laptops to be hit by this are laptops which are either pretty old and or which are weird in some other way (e.g. flashed with coreboot, did not ship with Windows as factory os, ...). Note Chromebooks are affected by this too, but that special category has already been fixed.
You can check if your laptop is affected by this by running "ls /sys/class/backlight" if this shows only 1 entry and that entry is named "intel_backlight", "nouveau_bl", "amdgpu_bl0" or "radeon_bl0" then your laptop might be affected.
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Notes from kernel hacking in Hare, part 3: serial driver
Today I would like to show you the implementation of the first userspace driver for Helios: a simple serial driver. All of the code we’re going to look at today runs in userspace, not in the kernel, so strictly speaking this should be “notes from OS hacking in Hare”, but I won’t snitch if you don’t.
Note: In the previous entry to this series, I promised to cover the userspace threading API in this post. I felt like covering this instead. Sorry!
A serial port provides a simple protocol for transferring data between two systems. It generalizes a bit, but for our purposes we can just think of this as a terminal which you can use over a simple cable and a simple protocol. It’s a standard x86_64 feature (though one which has been out of style for a couple of decades now), and its simple design (and high utility) makes it a good choice for the first driver to write for Helios.
UPDATE
Slashdot should have linked directly to the original, not the Conde Nast piece: