linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Message-ID: <Pine.LNX.4.64.0805201511300.7453@t2.domain.actdsltmp>
Date:	Tue, 20 May 2008 15:21:13 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Scott Wood <scottwood@...escale.com>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

On Tue, 20 May 2008, Benjamin Herrenschmidt wrote:
> On Tue, 2008-05-20 at 16:38 -0500, Scott Wood wrote:
>> It looks like we rely on -fno-strict-aliasing to prevent reordering
>> ordinary memory accesses (such as to DMA descriptors) past the I/O
>> access.  It won't prevent reordering of memory reads around an I/O
>> read,
>> though, which could be a problem if the I/O read result determines
>> the
>> validity of the DMA buffer.  IMHO, a memory clobber would be better.
>
> We probably want a full "memory" clobber then...

As far as I could tell, no other arch has a full memory clobber.  I can see
the argument for changing the Linux model to be stricter and less efficient,
but easier to program for.  Not that I entirely agree with it.

But I don't see a good reason for why powerpc should be different than
everything else.
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