ARM atomics overhaul for musl

[Arnd Bergmann]

On Monday 17 November 2014 11:48:33 Catalin Marinas wrote:
> On Sun, Nov 16, 2014 at 04:33:56PM +0000, Russell King - ARM Linux wrote:
> > On Sun, Nov 16, 2014 at 12:56:56AM -0500, Rich Felker wrote:
> > > Aside from that, the only case among the above that's "right" already
> > > is v7+. Hard-coding the mcr-based barrier on v6 is wrong because it's
> > 
> > I don't think it's wrong at all.  The instruction isn't going away from
> > ARMv7, because ARMv7 deprecates it, but it _still_ has to be implemented
> > by a CPU conforming to ARMv7.  As ARMv7 is going to be the last 32-bit
> > ARM architecture, we aren't going to see the MCR instruction disappearing
> > on 32-bit CPUs.
> 
> You are wrong here. ARMv8-A supports 32-bit at all levels. ARMv8-R is
> 32-bit only (and it even has an MMU at EL1). And there is a slight
> chance that we may even see 32-bit only ARMv8-A implementations (I'm not
> really giving a hint and I'm not aware of any but I don't see anything
> preventing this, it's all marketing driven).

FWIW, both Samsung EXYNOS and Qualcomm Snapdragon SoCs based on Cortex-A53
have been shipped in 32-bit only devices.

	Arnd