Re: [patch 2.6.17-rc5 1/2] i386 memcpy: use as few moves as possible for I/O

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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.


I was thinking some more about that, and I suspect the "right" way to do this looks something like the attached code. Note that it assymetric, and that it's probably too long to inline.

I haven't tested this yet, and I probably won't have time to do so this evening.

	-hpa
/*
 * arch/i386/lib/memcpy_io.S
 *
 * The most general form of memory copy to/from I/O space, used for
 * devices which can handle arbitrary transactions with appropriate
 * handling of byte enables.  The goal is to produce the minimum
 * number of naturally aligned transactions on the bus.
 */
	
#include <linux/config.h>
	
	.globl	memcpy_toio
	.type	memcpy_toio, @function
	
memcpy_toio:
	pushl	%edi
	pushl	%esi
	
#ifdef CONFIG_REGPARM
	movl	%eax, %edi
	movl	%edx, %esi
#else	
	movl	12(%esp), %eax
	movl	16(%esp), %edx
	movl	20(%esp), %ecx
#endif

	jecxz	1f
	
	testl	$1, %edi
	jz	2f
	movsb
	decl	%ecx
2:
	cmpl	$2, %ecx
	jb	3f
	testl	$2, %edi
	jz	4f
	movsw
	decl	%ecx
	decl	%ecx
4:
	movl	%ecx, %edx
	shrl	$2, %ecx
	jz	5f
	rep ; movsl
5:
	movl	%edx, %ecx
	testb	$2, %cl
	jz	3f
	movsw
3:
	testb	$1, %cl
	jz	1f
	movsb
1:
	pop	%esi
	pop	%edi
	ret

	.size	memcpy_toio, .-memcpy_toio

	.globl	memcpy_toio
	.type	memcpy_fromio, @function
	
memcpy_fromio:
	pushl	%edi
	pushl	%esi
	
#ifdef CONFIG_REGPARM
	movl	%eax, %edi
	movl	%edx, %esi
#else	
	movl	12(%esp), %eax
	movl	16(%esp), %edx
	movl	20(%esp), %ecx
#endif

	jecxz	1f
	
	testl	$1, %esi
	jz	2f
	movsb
	decl	%ecx
2:
	cmpl	$2, %ecx
	jb	3f
	testl	$2, %esi
	jz	4f
	movsw
	decl	%ecx
	decl	%ecx
4:
	movl	%ecx, %edx
	shrl	$2, %ecx
	jz	5f
	rep ; movsl
5:
	movl	%edx, %ecx
	testb	$2, %cl
	jz	3f
	movsw
3:
	testb	$1, %cl
	jz	1f
	movsb
1:
	pop	%esi
	pop	%edi
	ret

	.size	memcpy_fromio, .-memcpy_fromio

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