Taking a quick look at RPmsg, it seems it's from 2016. Apple have been doing coprocessors longer than that; Apple had their Secure Enclave in 2013 and that was

Taking a quick look at RPmsg, it seems it's from 2016. Apple have been doing coprocessors longer than that; Apple had their Secure Enclave in 2013 and that was already using their mailbox system, which is fundamentally different from the IRQs-only RPmsg that keeps all message passing entirely in shared memory. In general, this kind of message passing thing is easy to reinvent and everyone does it differently. You'll find tons of mailbox drivers in Linux already, all different.