Subject: Re: [RFC 2/2] powerpc: copy_4K_page tweaked for Cell - add CPU feature

From: Michael Ellerman <[email-masked]>
Date: Thu, 14 Aug 2008 20:51:35 +1000

On Thu, 2008-08-14 at 16:18 +1000, Mark Nelson wrote:
> Add a new CPU feature, CPU_FTR_CP_USE_DCBTZ, to be added to the CPUs that benefit
> from having dcbt and dcbz instructions used in copy_4K_page(). So far Cell, PPC970
> and Power4 benefit.
> 
> This way all the other 64bit powerpc chips will have the whole prefetching loop
> nop'ed out.

> Index: upstream/arch/powerpc/lib/copypage_64.S
> ===================================================================
> --- upstream.orig/arch/powerpc/lib/copypage_64.S
> +++ upstream/arch/powerpc/lib/copypage_64.S
> @@ -18,6 +18,7 @@ PPC64_CACHES:
>  
>  _GLOBAL(copy_4K_page)
>  	li	r5,4096		/* 4K page size */
> +BEGIN_FTR_SECTION
>  	ld      r10,PPC64_CACHES@toc(r2)
>  	lwz	r11,DCACHEL1LOGLINESIZE(r10)	/* log2 of cache line size */
>  	lwz     r12,DCACHEL1LINESIZE(r10)	/* Get cache line size */
> @@ -30,7 +31,7 @@ setup:
>  	dcbz	r9,r3
>  	add	r9,r9,r12
>  	bdnz	setup
> -
> +END_FTR_SECTION_IFSET(CPU_FTR_CP_USE_DCBTZ)
>  	addi	r3,r3,-8
>  	srdi    r8,r5,7		/* page is copied in 128 byte strides */
>  	addi	r8,r8,-1	/* one stride copied outside loop */

Instead of nop'ing it out, we could use an alternative feature section
to either run it or jump over it. It would look something like:

_GLOBAL(copy_4K_page)
BEGIN_FTR_SECTION
        li      r5,4096         /* 4K page size */
        ld      r10,PPC64_CACHES@toc(r2)
        lwz     r11,DCACHEL1LOGLINESIZE(r10)    /* log2 of cache line size */
        lwz     r12,DCACHEL1LINESIZE(r10)       /* Get cache line size */
        li      r9,0
        srd     r8,r5,r11

        mtctr   r8
setup:
        dcbt    r9,r4
        dcbz    r9,r3
        add     r9,r9,r12
        bdnz    setup
FTR_SECTION_ELSE
	b	1f
ALT_FTR_SECTION_END_IFSET(CPU_FTR_CP_USE_DCBTZ)
1:
        addi    r3,r3,-8

So in the no-dcbtz case you'd get a branch instead of 11 nops.

Of course you'd need to benchmark it to see if skipping the nops is
better than executing them ;P

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person