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

["H. Peter Anvin" <hpa@zytor.com>]

[-- Attachment #1: Type: text/plain, Size: 978 bytes --]

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

[-- Attachment #2: memcpy_io.S --]
[-- Type: text/plain, Size: 1443 bytes --]

/*
 * 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