In the default case (no special open-time flags), the 'read' handler in your userspace driver should block (make the userspace thread sleep until data is available). If the userspace application has specified the O_ASYNC open()/fcntl() flag, raise a signal. If the userspace application has specified the O_NONBLOCK open()/fnctl() flag, return EWOULDBLOCK instead.
For examples, see the Linux Kernel Module Programming Guide, Blocking Processes
section. There is even an example character driver
in the guide.
For detailed examples on character devices, blocking and signaling, see chapters 4
in Linux Device Drivers, 3rd edition