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

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Date:	Wed, 21 May 2008 12:44:07 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	Scott Wood <scottwood@...escale.com>
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

On Wed, 21 May 2008, Benjamin Herrenschmidt wrote:
>> Depends on what you define as "necessary".  It's seem clear that I/O accessors
>> _no not_ need to be strictly ordered with respect to normal memory accesses,
>> by what's defined in memory-barriers.txt.  So if by "necessary" you mean what
>> the Linux standard for I/O accessors requires (and what other archs provide),
>> then yes, they have the necessary ordering guarantees.
>>
>> But, if you want them to be strictly ordered w.r.t to normal memory, that's
>> not the case.
>
> They should be.

Someone should update memory-barriers.txt, because it doesn't say that, and
all I/O accessors for all the arches, because none of them are.

>> Here's a quick hack I stuck in a driver to test.  compile with -save-temps and
>> check the resulting asm.  gcc will do the optimization I described above.
>>
>> static void __iomem *baz = (void*)0x1234;
>> static struct bar {
>>      u32 bar[256];
>> } bar;
>>
>> void foo(void) {
>>      bar.bar[0] = 44;
>>      out_be32(baz+100, 200);
>>      bar.bar[0] = 45;
>>      out_be32(baz+101, 201);
>> }
>
> Have you removed -fno-strict-aliasing ? Just don't do that.

No, it's compiled with a normal kernel build, which includes
-fno-strict-aliasing.
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