linux-kernel - Re: alloc skb based on a given data buffer

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Message-Id: <1253006111.7549.61.camel@debian>
Date:	Tue, 15 Sep 2009 17:15:11 +0800
From:	Zhu Yi <yi.zhu@...el.com>
To:	David Miller <davem@...emloft.net>
Cc:	"mel@....ul.ie" <mel@....ul.ie>,
	"Chatre, Reinette" <reinette.chatre@...el.com>,
	"elendil@...net.nl" <elendil@...net.nl>,
	"Larry.Finger@...inger.net" <Larry.Finger@...inger.net>,
	"linville@...driver.com" <linville@...driver.com>,
	"penberg@...helsinki.fi" <penberg@...helsinki.fi>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"ipw3945-devel@...ts.sourceforge.net" 
	<ipw3945-devel@...ts.sourceforge.net>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"cl@...ux-foundation.org" <cl@...ux-foundation.org>,
	"Krauss, Assaf" <assaf.krauss@...el.com>,
	"johannes@...solutions.net" <johannes@...solutions.net>,
	"Abbas, Mohamed" <mohamed.abbas@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: alloc skb based on a given data buffer

On Tue, 2009-09-15 at 17:09 +0800, David Miller wrote:
> 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.

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

Thanks,
-yi

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