David S. Miller wrote:
From: Feyd <[EMAIL PROTECTED]>
Date: Mon, 11 Jul 2005 07:43:10 +0200
can I assume that hard_start_xmit will always get skbs with hard_header_len
reserved? I need two more bytes at the start of the packet and I'm getting
spurious panics in skb_push.
Typically, no. ->hard_start_xmit() has a fully built packet, hardware
headers and all.
Protocols push the hardware header and copy it into the packet
long before you get called. For example, look at
net/ipv4/ip_output.c:ip_finish_output2(), it takes the cached
ARP response hardware header and copies it into the packet like
so:
read_lock_bh(&hh->hh_lock);
hh_alen = HH_DATA_ALIGN(hh->hh_len);
memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
read_unlock_bh(&hh->hh_lock);
skb_push(skb, hh->hh_len);
return hh->hh_output(skb);
Your driver's ->hard_start_xmit() gets the packet after this
has occured.
If you do find yourself needing to grow the skb, it can be done,
but it is not too efficient. Look at the 8021q vlan transmit
path code...it will insert it's 4 byte header in certain cases, and
has logic to grow the skb if needed...
Ben
--
Ben Greear <[EMAIL PROTECTED]>
Candela Technologies Inc http://www.candelatech.com
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