Documentation/staging/rpmsg.rst

Source file repositories/reference/linux-study-clean/Documentation/staging/rpmsg.rst

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Documentation/staging/rpmsg.rst
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============================================
Remote Processor Messaging (rpmsg) Framework
============================================

.. note::

  This document describes the rpmsg bus and how to write rpmsg drivers.
  To learn how to add rpmsg support for new platforms, check out remoteproc.txt
  (also a resident of Documentation/).

Introduction
============

Modern SoCs typically employ heterogeneous remote processor devices in
asymmetric multiprocessing (AMP) configurations, which may be running
different instances of operating system, whether it's Linux or any other
flavor of real-time OS.

OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP.
Typically, the dual cortex-A9 is running Linux in a SMP configuration,
and each of the other three cores (two M3 cores and a DSP) is running
its own instance of RTOS in an AMP configuration.

Typically AMP remote processors employ dedicated DSP codecs and multimedia
hardware accelerators, and therefore are often used to offload CPU-intensive
multimedia tasks from the main application processor.

These remote processors could also be used to control latency-sensitive
sensors, drive random hardware blocks, or just perform background tasks
while the main CPU is idling.

Users of those remote processors can either be userland apps (e.g. multimedia
frameworks talking with remote OMX components) or kernel drivers (controlling
hardware accessible only by the remote processor, reserving kernel-controlled
resources on behalf of the remote processor, etc..).

Rpmsg is a virtio-based messaging bus that allows kernel drivers to communicate
with remote processors available on the system. In turn, drivers could then
expose appropriate user space interfaces, if needed.

When writing a driver that exposes rpmsg communication to userland, please
keep in mind that remote processors might have direct access to the
system's physical memory and other sensitive hardware resources (e.g. on
OMAP4, remote cores and hardware accelerators may have direct access to the
physical memory, gpio banks, dma controllers, i2c bus, gptimers, mailbox
devices, hwspinlocks, etc..). Moreover, those remote processors might be
running RTOS where every task can access the entire memory/devices exposed
to the processor. To minimize the risks of rogue (or buggy) userland code
exploiting remote bugs, and by that taking over the system, it is often
desired to limit userland to specific rpmsg channels (see definition below)
it can send messages on, and if possible, minimize how much control
it has over the content of the messages.

Every rpmsg device is a communication channel with a remote processor (thus
rpmsg devices are called channels). Channels are identified by a textual name
and have a local ("source") rpmsg address, and remote ("destination") rpmsg
address.

When a driver starts listening on a channel, its rx callback is bound with
a unique rpmsg local address (a 32-bit integer). This way when inbound messages
arrive, the rpmsg core dispatches them to the appropriate driver according
to their destination address (this is done by invoking the driver's rx handler
with the payload of the inbound message).


User API
========

::

Annotation

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