I have since discovered that:
* On this machine the i915 driver is responsible for the backlight
control too.
* The i915 driver registers the backlight device in the modesetting
path (!)
* Although "nomodeset" was required to get the kernel from stretch
Alpha 5 to not produce a black screen shortly after booting into
the installed system, linux-image-4.4.0-1-amd64 4.4.6-1 works fine
without "nomodeset" (and seems to set the mode during boot as
expected). There seems little point in reporting any bug against
the stretch alpha 5 kernel as I'm sure it will be updated in due
course.
* Booting linux-image-4.4.0-1-amd64 4.4.6-1 without "nomodeset"
- produces /sys/class/backlight/intel_backlight
and working backlight control buttons
- seems to suspend and resume fine (4 trials so far)
I will close this bug. Sorry for the noise.
Ian.
