[Daniel Vetter]
Sounds like. Please file a bug report against ACPI -> Video on bugzilla.kernel.org.
When you file that bug please add me and Jani Nikula to the cc list. We have a few other inverted brightness quirks all on similar machines. So this could all be due to the same strange interaction between drm/i915 and teh specific ACPI implementation on these machines.
Thanks for the pointers. I've now reported the bug as <URL: https://bugzilla.kernel.org/show_bug.cgi?id=60221 >.