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Subject: Re: [PATCH 13/17] RISC-V: Add include subdirectory
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Palmer Dabbelt <palmer@dabbelt.com>, Arnd Bergmann <arnd@arndb.de>
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        Olof Johansson <olof@lixom.net>, albert@sifive.com,
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Date: Sat, 24 Jun 2017 10:42:05 -0500
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On Fri, 2017-06-23 at 19:01 -0700, Palmer Dabbelt wrote:
> > > +#define mmiowb()       __asm__ __volatile__ ("fence io,io" : : : "memory");

I forgot if we already mentioned that but mmiowb is primarily intended
to order MMIO stores vs. a subsequent spin_unlock.

I'm not sure an IO only fence is sufficient here.

Note that I've never trusted drivers to get that right, it's a rather
bad abstraction to begin with, so on powerpc, instead, I just set a
per-cpu flag on every non-relaxed MMIO write and test it in spin_unlock
in order to "beef up" the barrier in there if necessary.

Cheers,
Ben.