Subject: Re: alloc skb based on a given data buffer

From: Zhu Yi <[email-masked]>
Date: Tue, 15 Sep 2009 17:15:11 +0800

On Tue, 2009-09-15 at 17:09 +0800, David Miller wrote:
> From: Zhu Yi <[email-masked]>
> 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.

I see. Thanks for this info. I'll try implementing the same for iwlagn.

Thanks,
-yi