Comment # 27 on bug 1180591 from LAURENT LE POITTEVIN

Ok thanks for the info. I was able to find out about it and start to understand
a little bit how it works.
I followed the test list on this page and I was able to make some changes.
Notably, a start at the brightness I set just before rebooting. But the
brightness slider remains inoperative.
I could visualize a change of brightness during the boot with the following
parameters in the boot line of the grub :
_video.use_bios_initial_backlight=0 (the parameter appears as unknow in dmesg
but the brightness is set to the requested value, the sliders is inoperative)
_ acpi_osi= (the brightness is set to the requested value, the sliders are
inoperative)
video.use_bios_initial_backlight=1 (the parameter appears as unknow in dmesg
but the brightness is set to the requested value, the sliders are inoperative)
_ acpi.brightness_switch_enabled=0 (brightness is set to the requested value,
sliders are inoperative).

I attach the different files made during the test in case.