Re: alloc skb based on a given data buffer

David Miller
2009-09-15T09:08:56Z

From: Zhu Yi <yi.zhu@intel.com>
Date: Tue, 15 Sep 2009 16:57:29 +0800

> Thanks. So we can put the 8K buffer into 2 skb_shinfo()->frags[] slots
> and set nr_frags to 2, right? Is this supported allover the network code
> already? At a first glance, I didn't find any frags handling in mac80211
> stack.

You have to pre-pull the link level protocol headers into the
linear area, but that's it.

Again, see niu.c for details, it does:

static void niu_rx_skb_append(struct sk_buff *skb, struct page *page,
u32 offset, u32 size)
{
int i = skb_shinfo(skb)->nr_frags;
skb_frag_t *frag = &skb_shinfo(skb)->frags[i];

frag->page = page;
frag->page_offset = offset;
frag->size = size;

skb->len += size;
skb->data_len += size;
skb->truesize += size;

skb_shinfo(skb)->nr_frags = i + 1;
}

to add pages to SKBs and then at the end it goes:

skb_reserve(skb, NET_IP_ALIGN);
__pskb_pull_tail(skb, min(len, NIU_RXPULL_MAX));

Right before giving the SKB to the networking stack. NIU_RXPULL_MAX
should be a value that will be large enough to cover the largest
possible link level header.