rpmsg: add virtio-based remote processor messaging bus

Add a virtio-based inter-processor communication bus, which enables
kernel drivers to communicate with entities, running on remote
processors, over shared memory using a simple messaging protocol.

Every pair of AMP processors share two vrings, which are used to send
and receive the messages over shared memory.

The header of every message sent on the rpmsg bus contains src and dst
addresses, which make it possible to multiplex several rpmsg channels on
the same vring.

Every rpmsg channel is a device on this bus. When a channel is added,
and an appropriate rpmsg driver is found and probed, it is also assigned
a local rpmsg address, which is then bound to the driver's callback.

When inbound messages carry the local address of a bound driver,
its callback is invoked by the bus.

This patch provides a kernel interface only; user space interfaces
will be later exposed by kernel users of this rpmsg bus.

Designed with Brian Swetland <[email protected]>.

Signed-off-by: Ohad Ben-Cohen <[email protected]>
Acked-by: Rusty Russell <[email protected]> (virtio_ids.h)
Cc: Brian Swetland <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Grant Likely <[email protected]>
Cc: Tony Lindgren <[email protected]>
Cc: Russell King <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Stephen Boyd <[email protected]>

drivers/Kconfig

@@ -134,6 +134,8 @@ source "drivers/iommu/Kconfig"
 
 source "drivers/remoteproc/Kconfig"
 
+source "drivers/rpmsg/Kconfig"
+
 source "drivers/virt/Kconfig"
 
 source "drivers/devfreq/Kconfig"