Power management HOWTO claims that the default LCD brigtness path used by laptop_mode isn't supported by new kernels. That information is most probably pretty old already, so that path won't probably be supported by kernels run by Gentoo users already.
I can confirm the nonexistence of that path on my Lenovo x201, as well as existence of the replacement suggested by the document:
Could we please switch the default value to the path supported by current kernels?
I suggest you get an account on https://wiki.gentoo.org/ and simply change it. As for app-laptop/laptop-mode-tools still referring to the deprecated /proc/acpi, that's a matter for its upstream developers to fix.
I reported a bug in the [default] contents of a configuration file distributed as part of the app-laptop/laptop-mode-tools to help other Gentoo users avoid having to replace the obsolete path with the correct path.
It's a trivial replacement that can be handled by a single 'sed' line in the ebuild. I can supply a patch for the ebuild if requested.
(In reply to Jeroen Roovers from comment #1)
> I suggest you get an account on https://wiki.gentoo.org/
I do have one:
> and simply change it.
The Wiki correctly describes the very issue I'm reporting. We shouldn't change that until a fixed version of the package is available in the stable branch of Gentoo. The only sane thing I can do now is to refer to this closed bug report in the Wiki.
> As for app-laptop/laptop-mode-tools still referring to the deprecated
> /proc/acpi, that's a matter for its upstream developers to fix.
I admit a bug in default configuration is not a particularly blocking issue and it's documented in the HOWTO, after all, but why don't we apply a trivial fix when it's available?