Genuine question: who restricts the sourcecode language in which a kernel driver is written? I'm thinking machine opcodes in a .o file, to be linked wherever,

Genuine question: who restricts the sourcecode language in which a kernel driver is written? I'm thinking machine opcodes in a .o file, to be linked wherever, are only of a concern if they're doing function call linkages (and maybe kernel space vs user space transition bookkeeping) improperly?