[PATCH 14/14] ARM: elf: add new hwcap for identifying atomic ldrd/strd instructions

Will Deacon will.deacon at arm.com
Tue May 21 14:02:01 EDT 2013

On Mon, May 20, 2013 at 06:13:52PM +0100, Catalin Marinas wrote:
> On Mon, May 20, 2013 at 05:04:07PM +0100, Will Deacon wrote:
> > On Mon, May 20, 2013 at 04:11:58PM +0100, Catalin Marinas wrote:
> > > Given that the ARM ARM isn't clear (though this is the case on the
> > > actual implementations), user space may not necessarily assume that
> > > LPAE==atomic doubles. That's why I think reporting the actual atomic
> > > feature is better.
> > 
> > The ARM ARM isn't too bad: it's just avoiding mandating 64-bit-wide paths
> > around the entire SoC (and I've checked this with the architects). The only
> > way we can probe this feature is using the MMFR0 and checking if LPAE is
> > supported, and that's exactly what userspace will need to rely on.
> Well, LPAE implies atomic doubles but I wouldn't say that's the "only"
> way, it can always be a feature of the CPU. Now, would the user
> developers fully understand the implications of LPAE?

I don't think it *can* be a feature of the CPU, because it depends on
system-wide support. It could be a feature of an SoC, but per-SoC hwcaps
isn't something we currently support. As I said, the only reason we can even
probe this is because the architecture helps us out.

> > We can
> > change the name, but the probe code will remain the same so I'm not sure it
> > makes anything clearer. We had "atomicd" originally, but that sounds like a
> > techno band.
> We can make it longer, 'atomicdbl', if that's the issue ;).

Argh! :)


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