Comment # 1 on bug 1209296 from Takashi Iwai

Could you give the dmesg output from the last working kernel, too?

Through a quick glance, I couldn't spot out the suspicious changes.  There have
been a few changes in drivers/acpi/video_detect.c, but acpi_backlight=native
behavior should be kept, and there is no significant change in nouveau's
backlight code, either.

Maybe it's about the probing of the nouveau itself; please boot with
"drm.debug=0x1e log_buf_len=1M" options and get the dmesg output for both
working and non-working cases.

Last but not least, please give hwinfo output, too.