Bug 80987

Backlight in console changes to minimum when switching from xorg to console. 

But when switching back from console to xorg, brightness goes back to max as it was before switching. 

Initial boot keeps maximum brightness in console through the boot process. 

Fn+brightness keys (ThinkPad T410) are functional to increase and decrease brightness, both in console and in xorg.

kernel commandline "acpi_backlight=vendor" keeps brightness level the same in console and in xorg, but disable brightness buttons (Fn+Home and Fn+End in my case).

zcat /proc/config.gz | grep -i think 
CONFIG_THINKPAD_ACPI=m 
CONFIG_THINKPAD_ACPI_ALSA_SUPPORT=y 
# CONFIG_THINKPAD_ACPI_DEBUGFACILITIES is not set 
# CONFIG_THINKPAD_ACPI_DEBUG is not set 
CONFIG_THINKPAD_ACPI_UNSAFE_LEDS=y 
CONFIG_THINKPAD_ACPI_VIDEO=y 
CONFIG_THINKPAD_ACPI_HOTKEY_POLL=y

$ zgrep -i fb_backlight /proc/config.gz 
# CONFIG_FB_BACKLIGHT is not set

# lsmod | grep think 
thinkpad_ec             3503  2 hdaps,tp_smapi 
thinkpad_acpi          53325  0

ls -1 /sys/class/backlight/
acpi_video0
intel_backlight

When in X mode, turn on backlight debug message by:
# cd /sys/kernel/debug/dynamic_debug
# echo 'module backlight +pft' > control
# dmesg
[  179.294484] [3209] brightness_store: backlight: set brightness to 0
[  179.606036] [3209] brightness_store: backlight: set brightness to 15
[  183.627956] [3209] brightness_store: backlight: set brightness to 15

ps aux | grep 3209
root      3098  0.0  0.0  12380  2260 pts/0    S+   15:51   0:00 grep --colour=auto 3209
root      3209  1.1  0.8 228012 33208 tty7     Ssl+ 15:30   0:15 /usr/bin/X -nolisten tcp -br -deferglyphs 16 vt07 -auth /var/run/slim.auth

initial workup done at https://bugzilla.kernel.org/show_bug.cgi?id=79541