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Subject: Re: [patch 2.6.17-rc5 1/2] i386 memcpy: use as few moves as  possible for I/O
Date: Wed, 31 May 2006 07:39:02 -0400
Message-ID: <Pine.LNX.4.61.0605310732110.28667@chaos.analogic.com>
In-Reply-To: <200605302103_MC3-1-BF0E-59B@compuserve.com>
Thread-Topic: [patch 2.6.17-rc5 1/2] i386 memcpy: use as few moves as  possible for I/O
Thread-Index: AcaEptZBZ8+wCUiIRpugrCHiZoXkFA==
References: <200605302103_MC3-1-BF0E-59B@compuserve.com>
From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Chuck Ebbert" <76306.1226@compuserve.com>
Cc: "linux-kernel" <linux-kernel@vger.kernel.org>,
       "Chris Lesiak" <chris.lesiak@licor.com>,
       "H. Peter Anvin" <hpa@zytor.com>, "Andrew Morton" <akpm@osdl.org>,
       "Linus Torvalds" <torvalds@osdl.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 3853
Lines: 108


On Tue, 30 May 2006, Chuck Ebbert wrote:

> Chris Lesiak reported that changes to i386's __memcpy() broke his device
> because it can't handle byte moves and the new code uses them for
> all trailing bytes when the length is not divisible by four.  The old
> code tried to use a 16-bit move and/or a byte move as needed.
>
> H. Peter Anvin:
> "There are only a few semantics that make sense: fixed 8, 16, 32, or 64
> bits, plus "optimal"; the latter to be used for anything that doesn't
> require a specific transfer size.  Logically, an unqualified
> "memcpy_to/fromio" should be the optimal size (as few transfers as
> possible)"
>
> So add back the old code as __minimal_memcpy and have IO transfers
> use that.
>
> Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
>
> ---
>
> include/asm-i386/io.h     |    4 ++--
> include/asm-i386/string.h |   21 +++++++++++++++++++++
> 2 files changed, 23 insertions(+), 2 deletions(-)
>
> --- 2.6.17-rc5-32.orig/include/asm-i386/io.h
> +++ 2.6.17-rc5-32/include/asm-i386/io.h
> @@ -202,11 +202,11 @@ static inline void memset_io(volatile vo
> }
> static inline void memcpy_fromio(void *dst, const volatile void __iomem *src, int count)
> {
> -	__memcpy(dst, (void __force *) src, count);
> +	__minimal_memcpy(dst, (void __force *) src, count);
> }
> static inline void memcpy_toio(volatile void __iomem *dst, const void *src, int count)
> {
> -	__memcpy((void __force *) dst, src, count);
> +	__minimal_memcpy((void __force *) dst, src, count);
> }
>
> /*
> --- 2.6.17-rc5-32.orig/include/asm-i386/string.h
> +++ 2.6.17-rc5-32/include/asm-i386/string.h
> @@ -220,6 +220,28 @@ return (to);
> }
>
> /*
> + * Do memcpy with as few moves as possible (for transfers to/from IO space.)
> + */
> +static inline void * __minimal_memcpy(void * to, const void * from, size_t n)
> +{
> +int d0, d1, d2;
> +__asm__ __volatile__(
> +	"rep ; movsl\n\t"
> +	"testb $2,%b4\n\t"
> +	"jz 1f\n\t"
> +	"movsw\n"
> +	"1:\n\t"
> +	"testb $1,%b4\n\t"
> +	"jz 2f\n\t"
> +	"movsb\n"
> +	"2:"
> +	: "=&c" (d0), "=&D" (d1), "=&S" (d2)
> +	:"0" (n/4), "q" (n), "1" ((long) to), "2" ((long) from)
> +	: "memory");
> +return to;
> +}
> +
> +/*
>  * This looks ugly, but the compiler can optimize it totally,
>  * as the count is constant.
>  */

> +	"rep ; movsl\n\t"
^^^^^^^^^^^^^^^^^^^^^^^^
Huh, you copyied stuff into three variables, then you needed to make
memory accesses to those variables, and you claim "the compiler can
optimize it totally". Sure. I got a bridge I'd like to sell you.

Please, instead of guessing, why don't measure the exact number of
CPU cycles with rdtsc?

> --
> Chuck
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.73 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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