ddcci-driver-linux builds two out-of-tree kernel modules, "ddcci" and
"ddcci-backlight".

dkms is an optional dependency. If it is installed on the build
machine, this SlackBuild packages the driver as a DKMS module instead
of a prebuilt one: only source is shipped, doinst.sh runs "dkms
install" for the kernel being installed onto, and DKMS then rebuilds
it again automatically for every kernel installed afterwards. Set
USE_DKMS=no to force the plain build below even when DKMS is present.

Without DKMS, the modules are built once against a specific kernel's
build tree (/lib/modules/$KERNEL/build, defaulting to the kernel
running on the build machine; set KERNEL=<version> to target a
different installed kernel/headers) and only load on that kernel. The
package name/version then includes the kernel version, and you must
rebuild and reinstall this package after every kernel upgrade.

Either way, the "ddcci" module is an I2C bus driver and is not
autoloaded. Load it yourself, e.g. by adding to
/etc/rc.d/rc.modules.local:

  modprobe ddcci-backlight

("ddcci" comes along automatically, since "ddcci-backlight" depends on
it; omit it if you don't need backlight control.)

You also need your motherboard/GPU's I2C bus driver (e.g. i2c-i801)
loaded so the monitor's DDC/CI I2C bus is exposed.