netdev - RE: [net-next PATCH] csum: Update csum_block_add to use rotate instead of byteswap

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Date:	Wed, 9 Mar 2016 10:54:49 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Joe Perches' <joe@...ches.com>,
	Alexander Duyck <aduyck@...antis.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [net-next PATCH] csum: Update csum_block_add to use rotate
 instead of byteswap

From: Joe Perches
> Sent: 08 March 2016 23:26
...
> > +
> > +	if (offset & 1)
> > +		sum = (sum << 24) + (sum >> 8);
> 
> Maybe use ror32(sum, 8);
> 
> or maybe something like:
> 
> {
> 	u32 sum;
> 
> 	/* rotated csum2 of odd offset will be the right checksum */
> 	if (offset & 1)
> 		sum = ror32((__force u32)csum2, 8);
> 	else
> 		sum = (__force u32)csum2;

Or even:
	sum = ror32((__force u32)csum2, (offset & 1) * 8);
to remove the conditional.
Assuming 'rotate by 0 bits' is valid.
If not add 16 to rotate by 16 or 24.

	David