Dynamically register IOCTLs
When using IOCTL, can we use dynamic registration for device drivers?
You do not need major numbers to define ioctl, they are not relevant.
What you need is a (somewhat) unique (or at least not built-in) number or letter to identify your IOCTL range. These can be optionally registered (registered in
Documentation/ioctl/ioctl-number.txt , in which there are also instructions for use) to prevent (or at least reduce) the possibility that others will also use it.
I highly recommend reading the documentation in any case, as it explains common conventions and usages.