On the first login (before restarting gdm), where brightness cannot be changed using the Fn+up/down shortcut (but can using setpci), I did the follwing:
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# rmmod samsung_backlight
# modprobe samsung_backlight debug=1
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Set brightness level 0 (hw 10)
Brightness power 0
Restarting gdm and logging back in (brightness can now be changed), I can now set the brightness levels using the shortcut keys, but the dmesg output (with samsung_backlight loaded with the debug=1 option) is different:
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Set brightness level 3 (hw 102)
Brightness power 0
In both cases the driver responds to the shortcut, but only once gdm has been restarted do the level and hw numbers vary and the brightness actually change.
My coding skills aren't up to taking a look at the driver source myself, but does anyone have any ideas regarding this?