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net: Increase default NET_SKB_PAD to 32.

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Several devices need to insert some "pre headers" in front of the
main packet data when they transmit a packet.

Currently we allocate only 16 bytes of pad room and this ends up not
being enough for some types of hardware (NIU, usb-net, s390 qeth,
etc.)

So increase this to 32.

Note that drivers still need to check in their transmit routine
whether enough headroom exists, and if not use skb_realloc_headroom().
Tunneling, IPSEC, and other encapsulation methods can cause the
padding area to be used up.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2009-02-08 19:24:13 -08:00
commit d6301d3dd1
1 changed files with 3 additions and 3 deletions
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6
include/linux/skbuff.h
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@ -1287,7 +1287,7 @@ static inline int skb_network_offset(const struct sk_buff *skb)
* The networking layer reserves some headroom in skb data (via * The networking layer reserves some headroom in skb data (via
* dev_alloc_skb). This is used to avoid having to reallocate skb data when * dev_alloc_skb). This is used to avoid having to reallocate skb data when
* the header has to grow. In the default case, if the header has to grow * the header has to grow. In the default case, if the header has to grow
* 16 bytes or less we avoid the reallocation. * 32 bytes or less we avoid the reallocation.
* *
* Unfortunately this headroom changes the DMA alignment of the resulting * Unfortunately this headroom changes the DMA alignment of the resulting
* network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive * network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive
@ -1295,11 +1295,11 @@ static inline int skb_network_offset(const struct sk_buff *skb)
* perhaps setting it to a cacheline in size (since that will maintain * perhaps setting it to a cacheline in size (since that will maintain
* cacheline alignment of the DMA). It must be a power of 2. * cacheline alignment of the DMA). It must be a power of 2.
* *
* Various parts of the networking layer expect at least 16 bytes of * Various parts of the networking layer expect at least 32 bytes of
* headroom, you should not reduce this. * headroom, you should not reduce this.
*/ */
#ifndef NET_SKB_PAD #ifndef NET_SKB_PAD
#define NET_SKB_PAD 16 #define NET_SKB_PAD 32
#endif #endif
extern int ___pskb_trim(struct sk_buff *skb, unsigned int len); extern int ___pskb_trim(struct sk_buff *skb, unsigned int len);