Bug 13375

Joey Hess wants better doc for the existing backlight control method in Intel driver 2.2. He says:

It's hard to figure out from this documentation what value should be passed
to xrandr to enable the different backlight control methods. It turns out
to be the first word of each phrase ("kernel", "native", "legacy", or
"combination").

      LVDS   Laptop panel.  Properties:
                BACKLIGHT - set backlight level
                BACKLIGHT_CONTROL - set backlight level control method  (i.e.
              use  kernel interfaces, native LVDS power register, legacy reg‐
              ister, or combination)

The documentation in the source is much more helpful than this brief line
in the man page:

+ * Four different methods are available:
+ *   NATIVE:  only ever touch the native backlight control registers
+ *     This method may be susceptible to problem (2) above if the firmware
+ *     modifies the legacy registers.
+ *   LEGACY:  only ever touch the legacy backlight control registers
+ *     This method may be susceptible to problem (1) above if the firmware
+ *     also modifies the legacy registers.
+ *   COMBO:  try to use both sets
+ *     In this case, the driver will try to modify both sets of registers
+ *     if needed.  To avoid problem (2) above it may set the LBB register
+ *     to a non-zero value if the brightness is to be increased.  It's still
+ *     susceptible to problem (1), but to a lesser extent than the LEGACY only
+ *     method.
+ *   KERNEL:  use kernel methods for controlling the backlight
+ *     This is only available on some platforms, but where present this can
+ *     provide the best user experience.