On my laptop, the screen's backlight brightness can be adjusted via "/sys/class/backlight/intel_backlight/". But, I also have a "/sys/class/backlight/acpi_video0/" which has no effect whatsoever.

Unfortunately, everything in the system tries to adjust acpi_video0 instead of intel_backlight. So the only way I can adjust the brightness is by doing it manually.
3.14-2-amd64
I realize I can make a simple script to modify intel_backlight and setup a global key combination to trigger the script. But I'd rather do a more "proper" fix.

So, how can I *correctly* tell my system to use intel_backlight? (Or fix acpi_video0 so that it does work, if that's a non-hackish thing to do. I really don't know anything about acpi_video0.)

If it matters, I'm on Debian Testing, Kernel 3.14-2-amd64

A symlink or three? There is 'lndir' which is used
lndir dir1 dir2
which makes a tree of symlinks of dir1 in the directory dir2

Maybe, I don't know, but when it comes to my drivers I'd rather not start throwing guesses at the wall without hearing from someone who (unlike me) actually has some understanding of them. Is there seriously nobody familiar with "/sys/class/backlight/" and how it's supposed to work? Somebody has to, it didn't just pop into existance!

The kernel guys replaced /proc/acpi with /sys in one of their major rewrites. Search the LKML archives.

/sys is poorly documented in searchable stuff. The fact is, the rest of /sys doesn't matter - it's the bit you want that matters. Look around in /sys - they are not afraid of symlinks.

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Seems to have been fixed now in the current Debian Testing which now uses kernel 3.16-3-amd64. (I'm assuming it was the kernel update that fixed it.) The "acpi_video0" doesn't even exist for me anymore, and even the keyboard's brightness buttons are now working automagically. Nice.