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Date: Wed, 24 Jun 2009 12:54:21 -0400 From: Neil Horman <nhorman@...driver.com> To: Nicholas Van Orton <turanammo@...il.com> Cc: netdev@...r.kernel.org Subject: Re: e100 skb allocation On Wed, Jun 24, 2009 at 09:58:38PM +0530, Nicholas Van Orton wrote: > On Wed, Jun 24, 2009 at 4:17 PM, Neil Horman<nhorman@...driver.com> wrote: > > On Wed, Jun 24, 2009 at 02:19:48PM +0530, Nicholas Van Orton wrote: > >> Hello, > >> > >> For the Intel 8256 Ethernet Controller (e100.c), I notice that at the > >> time of skb allocation using netdev_alloc_skb() and just before > >> calling skb_reserve(), rx->skb->head and rx->skb->tail do not point to > >> the same location. In fact they are already 16bytes apart. After > >> calling skb_reserve(), which would add a 2 byte padding, head and > >> tail would be 18 bytes apart. Is this behaviour correct? Shouldn't the > >> ethernet header be 14+2 bytes after padding? Why aren't head and tail > >> at the same location at the beginning? > >> > > Yes, its fine (although IIRC it should be 32, rather than 16 bytes). > > netdev_alloc_skb, in its implementation, allocates an skb that is NET_SKB_PAD > > bytes larger than the caller requested, then it calls skb_reserve internally to > > add NET_SKB_PAD bytes of data to the skb's buffer. > > > > Thanks a bunch for steering me in the right direction. Things are much > clearer now. > No problem :) Neil -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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