Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

From: Trent Piepho
Date: Wed May 21 2008 - 15:50:15 EST

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) {[0] = 44;
out_be32(baz+100, 200);[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
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