We tried to write a simple class driver framework which will create a directory in /sys/class, we were able to compile the code but got the following error while trying to install the driver as,
$ sudo insmod hello.ko insmod: ERROR: could not insert module hello.ko: Unknown symbol in module
$ dmesg [ 7514.340407] hello: Unknown symbol __class_register (err 0) [ 7514.340436] hello: Unknown symbol class_unregister (err 0)
The function __class_create is exported only for GPL modules (exported with EXPORT_SYMBOL_GPL). So, you need to use a GPL license with MODULE_LICENSE macro to make use of that function.
Hence if you append your driver code with
[c] MODULE_LICENSE("GPL"); [/c]
and recompile driver and try insmod again, it will definately work and you will not get such errors.