struct net_device结构中添加了needed_headroom/needed_tailroom成员

Linux Kernel 2.6.28，struct net_device结构中添加了needed_headroom/needed_tailroom成员，为此更新了许多地方。

struct net_device
{
    /*
     * 硬件可能需要额外的headroom与tailroom, 但并不是所有情况下都使用这两个成员，尤其是tailroom.
     * 在一些情况下不是通过分配skb，而是使用LL_MAX_HEADER.
     */
    unsigned short        needed_headroom;
    unsigned short        needed_tailroom;
}

以下是一些相关内容：
struct hh_cache
{
    /* cached hardware header; allow for machine alignment needs.        */
#define HH_DATA_MOD    16
#define HH_DATA_OFF(__len) \
    (HH_DATA_MOD - (((__len - 1) & (HH_DATA_MOD - 1)) + 1))
#define HH_DATA_ALIGN(__len) \
    (((__len)+(HH_DATA_MOD-1))&~(HH_DATA_MOD - 1))
    unsigned long    hh_data[HH_DATA_ALIGN(LL_MAX_HEADER) / sizeof(long)];
};

/* 必须保证以下关系成立：HH alignment <= LL alignment.
 *
 * LL_ALLOCATED_SPACE宏中考虑了device可能需要的tailroom.
 */
#define LL_RESERVED_SPACE(dev) \
    ((((dev)->hard_header_len+(dev)->needed_headroom)&~(HH_DATA_MOD - 1)) + HH_DATA_MOD)
#define LL_RESERVED_SPACE_EXTRA(dev,extra) \
    ((((dev)->hard_header_len+(dev)->needed_headroom+(extra))&~(HH_DATA_MOD - 1)) + HH_DATA_MOD)
#define LL_ALLOCATED_SPACE(dev) \
    ((((dev)->hard_header_len+(dev)->needed_headroom+(dev)->needed_tailroom)&~(HH_DATA_MOD - 1)) + HH_DATA_MOD)

为此，修改了以下文件：
af_econet.c (net\econet): skb = sock_alloc_send_skb(sk, len+LL_ALLOCATED_SPACE(dev),
af_packet.c (net\packet): skb = sock_alloc_send_skb(sk, len + LL_ALLOCATED_SPACE(dev),
arp.c (net\ipv4):      skb = alloc_skb(arp_hdr_len(dev) + LL_ALLOCATED_SPACE(dev), GFP_ATOMIC);
igmp.c (net\ipv4):      skb = alloc_skb(size + LL_ALLOCATED_SPACE(dev), GFP_ATOMIC);
igmp.c (net\ipv4):      skb=alloc_skb(IGMP_SIZE+LL_ALLOCATED_SPACE(dev), GFP_ATOMIC);
ip6_output.c (net\ipv6):  if ((frag = alloc_skb(len+hlen+sizeof(struct frag_hdr)+LL_ALLOCATED_SPACE(rt->u.dst.dev), GFP_ATOMIC)) == NULL) {
ipconfig.c (net\ipv4):      skb = alloc_skb(sizeof(struct bootp_pkt) + LL_ALLOCATED_SPACE(dev) + 15,
mcast.c (net\ipv6):      skb = sock_alloc_send_skb(sk, size + LL_ALLOCATED_SPACE(dev), 1, &err);
mcast.c (net\ipv6):      skb = sock_alloc_send_skb(sk, LL_ALLOCATED_SPACE(dev) + full_len, 1, &err);
ndisc.c (net\ipv6):           len + LL_ALLOCATED_SPACE(dev)),
ndisc.c (net\ipv6):            len + LL_ALLOCATED_SPACE(dev)),
netdevice.h (include\linux): * LL_ALLOCATED_SPACE also takes into account the tailroom the device
netdevice.h (include\linux):#define LL_ALLOCATED_SPACE(dev) \
netpoll.c (net\core):      send_skb = find_skb(np, size + LL_ALLOCATED_SPACE(np->dev),
raw.c (net\ipv4):          length + LL_ALLOCATED_SPACE(rt->u.dst.dev) + 15,
raw.c (net\ipv6):          length + LL_ALLOCATED_SPACE(rt->u.dst.dev) + 15,

参考资料：
[1] Johannes Berg, allow netdevices to specify needed head/tailroom, 05 May 2008, [url]http://lwn.net/Articles/280996/[/url]