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kernel. The ability to dynamically
control the number of cores used by
the kernel is a primary reason AMP
developers prefer the SMP kernel over
the UP kernel.
The Remote Processor Framework
The Remote Processor (remoteproc)
Framework is the Linux component
that is responsible for starting and
stopping individual cores (remote
processors), as well as for loading a
core's software in an AMP system.
For example, we can dynamically
reconfigure the SMP system shown in
Figure 1 into the AMP system shown in
Figure 2, and then back again to SMP,
using the capabilities of remoteproc.
We can fully control reconfiguration
via a user-space application or system
initialization script. This allows user
applications to stop, reload and run a
variety of RTOS applications based on
the dynamic needs of the system.
The core’s software (in our example,
the RTOS and user application) is
loaded from a standard Executable
and Linkable Format (ELF) file that
contains a special section known as
the resource table. The resource table
Figure 1. Symmetric multiprocessing. The SMP
kernel can run simultaneously on multiple cores.
Figure 2. AMP with the Linux SMP kernel
is analogous to the PCI configuration
space in that it describes the resources
that the RTOS requires. Among those
resources is the memory needed for
the RTOS code and data.
Trace buffers
Trace buffers are regions of memory
that automatically appear as files in
a Linux file system. As their name
suggests, trace buffers provide basic
tracing capabilities to the remote
processor. A remote processor writes
trace, debug and status messages
to the buffers, where the messages
are available for inspection via the
Linux command line or by custom
applications.
One or more trace buffers may be
requested via entries in the resource
table. Although they typically contain
plain text, trace buffers may also
contain binary data such as application
state information or alarm indications.
Virtual I/O devices
The resource table also supports the
definition virtual input/output devices
(VDEVs), which are basically pairs of
shared memory queues that support
message transfer between the Linux
kernel and the remote processor. The
VDEV definition includes fields that
negotiate the size of the queues as
well as the interrupts used to signal
between the processors.
The Linux kernel handles initialization
of the virtual I/O queues. The software
running on a remote processor need
only include a VDEV description in
its resource table and then use the
queues once it begins execution; the
kernel handles the rest.
Remote
Processor
Messaging
Framework
(rpmsg) Framework is a messaging
“bus” based on the Linux kernel’s
virtual I/O system. The messaging bus
is similar to a local area subnetwork in
which individual processors can create
addressable endpoints and exchange
messages, all via shared memory.
The kernel’s rpmsg framework acts
as a switch, routing messages to
the appropriate endpoint based on
the destination address contained in
the message. Because the message
header includes a source address, ad
hoc connections can be established
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