The Linux-3.2 merge window may be the first time that two new CPU
architectures get merged at the same time: the
from Texas Instruments and the Hexagon architecture
Following the recently merged OpenRisc
platform, the two submissions look very solid and should see no major
obstacles getting merged into Linux after the usual review comments have
been resolved, but there is still some
over how to best add glibc
support for the architectures.
Interestingly, there is a lot that these two architectures have in common,
far beyond coincidentally
implementing the same bugs.
Both are not regular CPU cores designed to run an operating systems but
instead are essentially digital signal
similar to the Blackfin architecture that was merged into linux-2.6.22
a few years ago. Further, both Hexagon and c6x are already widely available
in systems running Linux on the ARM core of a TI OMAP or Qualcomm MSM
system-on-a-chip, where they are used for offloading CPU intensive work
such as that required for video codecs.
It will be interesting to see how Linux can coexist in the long run when
the same SoC can run Linux on either of the two CPU architectures. The
ARM architecture is currently transitioning to probing based on the
dts device tree format and all new
architectures merged into Linux will have to use that format as well
when they have devices that cannot be automatically detected. If the device
tree vision comes true, a single board will actually be able to use
the same device tree binary on either one, independent of which CPU
actually runs the kernel.
Another intriguing scenario is running Linux on both architectures
(ARM and DSP) simultaneously, using shared memory to communicate between
them. Ohad Ben-Cohen has recently posted a
framework based on virtio
to allow just that on the TI platform.
While virtio was intended to be used for communication between a virtual
machine and the host operating system, it turns out to be flexible enough
to allow the same drivers for communication between operating system
instances on the same hardware.
Looking closer at the actual DSP architectures, there are some major
differences between Hexagon and C6x. The former is quite capable,
with support for symmetric multiprocessing, a memory management
unit and even a hypervisor. It can be seen as a competitor to
established CPU architectures like ARM, x86 or powerpc, at least in
the embedded space. In contrast, C6x is a rather minimalistic architecture
dating back to the TMS320 introduced in 1983. So far, its kernel supports
neither SMP nor an MMU, which means it is restricted to running μClibc
instead of glibc, and it has a very limited set of
applications that can be supported as long as it is missing the MMU.
Beyond Linux 3.2, there are still more architectures that have been
around for a long time and could get merged if the respective maintainers
were interested. FPGA-based Nios2 is
apparently close to getting submitted, while the similar lm32 architecture
saw a lot
of activity in 2010 but does not seem to be actively worked on now.
and Imagination META are
to have Linux and Android support, but there is no indication that the
authors are actively working on upstream submission or even on making
the patches for current kernels easily available.
Finally, Donald Knuth's MMIX
architecture has seen some occasional work in
the past but
now appears to be stalled, the latest kernel source version being 2.6.18.
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