Messages in this thread | | | Subject | Re: [PATCH net-next 19/20] net: plip: slight optimization of addr compare | From | Joe Perches <> | Date | Fri, 27 Dec 2013 12:46:57 -0800 |
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On Fri, 2013-12-27 at 21:12 +0100, Julia Lawall wrote: > On Fri, 27 Dec 2013, Joe Perches wrote: > > > On Fri, 2013-12-27 at 07:48 -0800, Eric Dumazet wrote: > > > On Fri, 2013-12-27 at 14:49 +0800, Ding Tianhong wrote: > > > > Use possibly more efficient ether_addr_equal > > > > to instead of memcmp. > > [] > > > > diff --git a/drivers/net/plip/plip.c b/drivers/net/plip/plip.c > > [] > > > > @@ -549,7 +549,7 @@ static __be16 plip_type_trans(struct sk_buff *skb, struct net_device *dev) > > > > > > > > if(*eth->h_dest&1) > > > > { > > > > - if(memcmp(eth->h_dest,dev->broadcast, ETH_ALEN)==0) > > > > + if(ether_addr_equal(eth->h_dest, dev->broadcast)) > > > > skb->pkt_type=PACKET_BROADCAST; > > > > else > > > > skb->pkt_type=PACKET_MULTICAST; > > > > > > What about : > > > > > > if (is_multicast_ether_addr(eth->h_dest)) { > > > if (ether_addr_equal_64bits(eth->h_dest, dev->broadcast)) > > > skb->pkt_type = PACKET_BROADCAST; > > > else > > > skb->pkt_type = PACKET_MULTICAST; > > > } > > > > That is better though I wonder how many systems are > > still using laplink via parallel null-printer cables. > > > > No matter, better is better. > > > > The same optimization using ether_addr_equal_64bits > > may be possible to do in other places given other > > structs too. > > > > Perhaps it's a possible spatch/coccinelle conversion, > > > > I don't know spatch well enough to know if a > > mechanism to check if structure members have other > > fields that follow them in the structure or if the > > structure member is an array of a minimum size. > > > > Maybe Julia does. (cc'd) > > I'm not sure to competely understand the issues. Could you explain more?
Hi Julia.
Maybe this explanation is helpful?
ethernet addresses are u8[6] (48 bits)
ether_addr_equal_64bits gets passed a pointer to u8[8] and is more efficient on 64 bit architectures than ether_addr_equal because the test can be done with a single compare and shift.
The idea is not to access past the end of the ethernet address as appropriate (think pointer to eeprom or other such end-of-addressable memory conditions)
If a struct containing an ethernet address has additional members after the ethernet address, or the u8[6] address passed to ether_addr_equal is not going to access past the end of memory or the structure, then ether_addr_equal_64bits should be used in lieu of ether_addr_equal.
Joe
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