netdev - Re: TX pre-headers...

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Message-ID: <498C26C5.7010003@linux.vnet.ibm.com>
Date:	Fri, 06 Feb 2009 13:02:13 +0100
From:	Frank Blaschka <blaschka@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org
Subject: Re: TX pre-headers...

David Miller schrieb:
> Some NIC hardware wants a pre-header pushed in front of the packet
> data on transmit.
> 
> When routing or bridging this will cause a reallocation of skb->data
> on every packet forwarded because there will only be NET_IP_ALIGN
> space reserved at the head by the device receive path.
> 
> NIU is one such NIC and I only noticed this because of some things I
> saw in some of Robert Olsson's routing stress test oprofile dumps.
> 
> Putting a hack into NIU is the wrong way to do this and would only fix
> cases where NIU is the receiver and transmitting device. e1000 to
> NIU would still be broken, for example.
> 
> I think the way to solve this is to have each device indicate how
> much TX slack space it neads for it's preheaders.  On device
> registration we have some global "netdev_max_tx_hdr_space" that
> records the maximum value seen.
> 
> We could decrease it on unregister (by walking the device list)
> but I don't think that is worth it.
> 
> We also round netdev_max_tx_hdr_space up to be a multiple of 4
> or something reasonable like that.
> 
> Then we get drivers to use a new interface:
> 
> 	struct sk_buff *netdev_alloc_rx_skb(struct net_device *dev, int size);
> 
> which is nearly identical to netdev_alloc_skb() except that it does:
> 
> 	size += NET_IP_ALIGN + netdev_max_tx_hdr_space;
> 	skb = netdev_alloc_skb(dev, size);
> 	if (skb)
> 		skb_reserve(skb, NET_IP_ALIGN + netdev_max_tx_hdr_space);
> 	return skb;
> 
> Seems reasonable?
Absolutely yes, this would help the s/390 qeth drivers too
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