From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Tue, 21 May 2013 19:02:01 +0100
Subject: [PATCH 14/14] ARM: elf: add new hwcap for identifying atomic
 ldrd/strd instructions
In-Reply-To: <20130520171349.GM3639@MacBook-Pro.local>
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 <20130520142459.GN31359@mudshark.cambridge.arm.com>
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 <20130520160407.GS31359@mudshark.cambridge.arm.com>
 <20130520171349.GM3639@MacBook-Pro.local>
Message-ID: <20130521180201.GB26251@mudshark.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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! :)

Will