[pawadu]

> Everything worked great in Linux for me until kernel 4.2 or something when backlight brightness stopped working.

Interestingly, this also happened to a lot of Vaios when people upgraded to Windows 10. Now, your Linux was eventually fixed while my Vaio is still running at 0% backlight with zero response from Sony.

[bisby]

I dont know what changed in linux at kernel 4.2 (or maybe it was 4.1 or 4.3? I dunno. it was early 4.x), but I was able to fix it with a kernel boot parameter (acpi_osi=linux acpi_backlight=video). This tells the GPU to control the backlight, and everything works. Theres a lot of different laptop configurations of who powers and who runs the backlight. linux has all these options set up, but it has to pick one by default. So my laptop didnt "just work" on a fresh installation, but I was able to fix it.

I think I know what happened in windows: windows itself has no backlight driver, and the standard GPU drivers have no backlight handling (or do, but its disabled by default). So previously, these laptops would ship with a special Windows 7 driver that would handle the backlight. But its "so much work" to port that driver to windows 10... so they don't. So in this scenario, my laptop originally worked. A fresh installation would require proprietary Samsung drivers, and samsung has stopped providing those for windows 10... literally no fix available (it's possible the windows 7 drivers still work? to be clear, Ive never checked if there are work arounds or other drivers available. I switched to linux before windows 10 came out, and my dad is happy with linux resolving his backlight issues, so Ive never had to look it up)