netdev - Re: [RESEND][PATCH 1/3] PPPoE: improved hashing routine

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Date:	Tue, 31 Jul 2007 11:01:59 +0200
From:	Florian Zumbiehl <florz@...rz.de>
To:	David Miller <davem@...emloft.net>
Cc:	mostrows@...thlink.net, netdev@...r.kernel.org
Subject: Re: [RESEND][PATCH 1/3] PPPoE: improved hashing routine

Hi,

> > Erm, I'd say this not only produces different results than the old
> > version, but it also produces "wrong" results, in that it ignores quite
> > a bit of the data that's supposed to be hashed. If I didn't overlook
> > something, it only considers addr&0x0f0f0f0f0f00 and sid&0x0f0f, given
> > the right endianness of addr and that PPPOE_HASH_SIZE stays 16.
> 
> You're right, I need to fix the shifts up.  How does this version
> look?
> 
> 	hash ^= (hash >> 4) ^ (hash >> 12) ^ (hash >> 20);
> 	return (head ^ (hash >> 8) ^ (hash >> 24)) &
> 		(PPPOE_HASH_SIZE - 1);

Assuming that it was supposed to read s/head/hash/: Same disclaimers
apply, but I'd say this considers only addr&0xff0fff0f000f and
sid&0x0fff, so, well, yes, it's better, but still not quite what I
think it should be ;-)

> Actually it might be simpler and more efficient to just make
> PPPOE_HASH_SHIFT be 8.

SHIFT? SIZE? BITS?

Florian
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