Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()

From: Scott Wood <[email-masked]>
Date: Fri, 22 May 2015 14:32:42 -0500

On Fri, 2015-05-22 at 15:57 +0000, David Laight wrote:
> From: Linuxppc-dev Christophe Leroy
> > Sent: 19 May 2015 16:19
> ...
> > diff --git a/arch/powerpc/include/asm/checksum.h b/arch/powerpc/include/asm/checksum.h
> > index 5e43d2d..e8d9ef4 100644
> > --- a/arch/powerpc/include/asm/checksum.h
> > +++ b/arch/powerpc/include/asm/checksum.h
> > @@ -130,6 +130,22 @@ static inline __sum16 csum_tcpudp_magic(__be32 saddr, __be32 daddr,
> >  	return csum_fold(csum_tcpudp_nofold(saddr, daddr, len, proto, sum));
> >  }
> > 
> > +#define HAVE_ARCH_CSUM_ADD
> > +static inline __wsum csum_add(__wsum csum, __wsum addend)
> > +{
> > +#ifdef __powerpc64__
> > +	u64 res = (__force u64)csum;
> > +
> > +	res += (__force u64)addend;
> > +	return (__force __wsum)((u32)res + (res >> 32));
> > +#else
> > +	asm("addc %0,%0,%1;"
> > +	    "addze %0,%0;"
> > +	    : "+r" (csum) : "r" (addend));
> > +	return csum;
> > +#endif
> 
> I'd have thought it better to test for the cpu type where you want the
> 'asm' variant, and then fall back on the C version for all others.
> I know (well suspect) there are only two cases here.

Usually it's more readable to see "if (x) ... else ..." than "if (!
x) ... else ..." and 64-bit is what has a symbol defined.

> I'd also have thought that the 64bit C version above would be generally 'good'.

It doesn't generate the addc/addze sequence.  At least with GCC 4.8.2,
it does something like:

	mr	tmp0, csum
	li	tmp1, 0
	li	tmp2, 0
	addc	tmp3, addend, tmp0
	adde	csum, tmp2, tmp1
	add	csum, csum, tmp3

-Scott