[Daniel Vetter]
The buttons might do something fancy behind the scenes (kernel or userspace), so can you please also check whether directly changing the backlight values in /sys/class/backlight works correctly?
There is full brightness when I set the value of max_brightness into the brightness file (as in 'echo 4882 > brightness'), and complete darkness when it is set to 0. This was using acpi_backlight=vendor. I suspect that mean the intel_backlight driver is working correctly, but am not sure if 0 is supposed to be completely dark or not.
acpi can't invert, and the different backlight drivers can affect each another in funny ways. Which is way we have a clearly defined priority order that userspace should use, and then _only_ touch the selected backlight. ACPI wins over i915, so if the ACPI backlight is broken (but the raw i915 backlight driver works) we need to blacklist it.
Right. I guess my findings above mean the hardware should be blacklisted (from acpi?).
Is there no way to discover using ACPI if the backlight brightness setting should be inverted or not?