linux-kernel - Re: [PATCH] powerpc/32: fix csum_partial_copy

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Message-ID: <20160801143103.GB1742@gate.crashing.org>
Date:	Mon, 1 Aug 2016 09:31:04 -0500
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Scott Wood <oss@...error.net>, antoine.blangy@....fr,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] powerpc/32: fix csum_partial_copy_generic()

On Mon, Aug 01, 2016 at 03:45:59PM +0200, Christophe Leroy wrote:
> >>--- a/arch/powerpc/lib/checksum_32.S
> >>+++ b/arch/powerpc/lib/checksum_32.S
> >>@@ -127,7 +127,8 @@ _GLOBAL(csum_partial_copy_generic)
> >> 	stw	r7,12(r1)
> >> 	stw	r8,8(r1)
> >>
> >>-	andi.	r0,r4,1			/* is destination address even ? */
> >>+	rlwinm	r0,r4,3,0x8		/* is destination address even ? */
> >>+	rlwnm	r6,r6,r0,0,31		/* swap bytes for odd destination */
> >> 	cmplwi	cr7,r0,0
> >> 	addic	r12,r6,0
> >> 	addi	r6,r4,-4
> >
> >That does not "swap bytes"; it shifts the word up by 8 bits, instead.
> >That may or may not do what is intended.
> 
> Indeed it does what is intended, similar to what is done at the end of 
> the function:
> 
> 	...
> 	beqlr+	cr7
> 	rlwinm	r3,r3,8,0,31	/* swap bytes for odd destination */
>  	blr
> 
> Should I fix the (both) comment(s) ?

It's quite confusing comment, so yes please.  The code is hard enough to
understand as is!


Segher