netdev - Re: alloc skb based on a given data buffer

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Message-Id: <20090915.020903.93643290.davem@davemloft.net>
Date:	Tue, 15 Sep 2009 02:09:03 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	yi.zhu@...el.com
Cc:	mel@....ul.ie, reinette.chatre@...el.com, elendil@...net.nl,
	Larry.Finger@...inger.net, linville@...driver.com,
	penberg@...helsinki.fi, linux-kernel@...r.kernel.org,
	linux-wireless@...r.kernel.org,
	ipw3945-devel@...ts.sourceforge.net, akpm@...ux-foundation.org,
	cl@...ux-foundation.org, assaf.krauss@...el.com,
	johannes@...solutions.net, mohamed.abbas@...el.com,
	netdev@...r.kernel.org
Subject: Re: alloc skb based on a given data buffer

From: Zhu Yi <yi.zhu@...el.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.
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