RPMsg
The Linux kernel's message bus between the main processor and the auxiliary cores running their own firmware on the same chip.
Remote Processor Messaging landed in Linux in 2012 alongside remoteproc, the framework that loads and boots firmware on another core. Ohad Ben-Cohen wrote both for Texas Instruments' OMAP parts, which paired the main ARM core with a DSP and a Cortex-M. The transport was borrowed from virtualisation: virtio queues (vrings) in shared memory, plus an interrupt to knock on the other side's door when something new has arrived.
On top sit addressed endpoints and named channels, so firmware can announce "there is a service called rpmsg-tty here" and Linux creates a matching device. Buffers are small — 512 bytes by default — which suits commands and status lines, not streaming samples. People who try pushing audio through it usually end up with their own ring buffer in shared memory, using RPMsg only to say where it lives.