[Buildroot] [RFC] toolchain: libatomic needs NTPL threads support

[Thomas Petazzoni]

Hello,

On Tue,  2 Aug 2016 19:25:55 +0200, Romain Naour wrote:

>  	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 && \
> -		BR2_TOOLCHAIN_HAS_THREADS
> +		BR2_TOOLCHAIN_HAS_THREADS_NPTL

So, I had a closer look. The crux of the matter is in
libatomic/configure.tgt. There you can see that if a platform uses the
"linux" OS part of the target tuple, libatomic will be generated.
However, if "uclinux" is used, libatomic will not be generated.

So it's not about having NPTL or not, it's about using BR2_BINFMT_FLAT
or not.

Example: the Microblaze architecture does not support NPTL, but
libatomic is generated:

br-microblaze-full-2016.05-1162-g94c7298-build.log:checking for libatomic support... yes

However, on ARM Cortex-M4, and m68k noMMU, we see:

br-arm-cortex-m4-full-2016.05-1162-g94c7298-build.log:checking for libatomic support... no
br-m68k-5208-full-2016.05-1162-g94c7298-build.log:checking for libatomic support... no

Note that it's also not about noMMU vs. MMU, because a noMMU platform
can use FDPIC, in which case the OS part of the tuple would be "linux".

I've therefore updated your patch with this, and applied.

However, it is worth mentioning that changing the gcc code would be
pretty simple: it's about adding *-uclinux-* in
libatomic/configure.tgt. Indeed, libatomic provides a generic posix
implementation, which simply relies on pthread mutex locks to emulate
atomicity. This works on all platforms, as long as thread support is
available, including BR2_BINFMT_FLAT platforms.

People interested in testing this are encouraged to submit patches :-)

Thanks!

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com