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Although the ARM SWP instruction may exist for ARMv6 and above, it's not guaranteed to work, especially on SMP systems where it's use may lead to instability at runtime, etc: https://community.arm.com/processors/b/blog/posts/locks-swps-and-two-smoking-barriers Keeping the optimisation for architecture levels which pre-date SMP (ie <= ARMv5) may be safe, however other distros (Buildroot, Debian, Fedora, etc) are not doing so and mutex contention is likely to be less of an issue on uniprocessor systems anyway, so the benefits of this micro optimisations are not clear. Since OE uses ARMv5 qemu as a proxy for testing all 32bit ARM architecture levels, it's desirable to keep the ARMv5 builds aligned with later ARM architecture levels wherever possible. (From OE-Core rev: 7aa94abac09be6beb7ce14a2b9a409e934465706) Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
QEMU Emulation Targets ====================== To simplify development, the build system supports building images to work with the QEMU emulator in system emulation mode. Several architectures are currently supported in 32 and 64 bit variants: * ARM (qemuarm + qemuarm64) * x86 (qemux86 + qemux86-64) * PowerPC (qemuppc only) * MIPS (qemumips + qemumips64) Use of the QEMU images is covered in the Yocto Project Reference Manual. The appropriate MACHINE variable value corresponding to the target is given in brackets.