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Old 02-24-2009, 06:48 PM   #1
Registered: May 2007
Location: New Zealand
Distribution: Ubuntu
Posts: 88

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Question Ethernet device driver hardware rx- am I doing the right thing with my socket buffer?

Hi there,

I am working on an Ethernet device driver for uClinux, currently I am not sure about the hardware receive function, this is what I do:

define a socket buffer:
struct sk_buff *skb;
perform some checks on the device specific header (from which I get the rxLen)

if all okay, allocate memory for the skb:
if (goodPacket && ((skb = dev_alloc_skb(rxLen+x)) != NULL)) { ...
align IP header to 16 bit boundary - not too sure about this but other drivers seem to do it:
skb_reserve(skb, 2);
get a pointer to the start of the skb data section:
rdptr = (uint8_t *) skb_put(skb, rxLen+x);
I then loop, reading one word from hardware (into rxWord), and then writing it to the skb:
	memcpy(rdptr, &rxWord, 16); /* copy 2 bytes to socket buffer */
	rdptr += 2;	/* increment pointer by 2 bytes */
Once I have reached the end of the frame, fill skb structure:
skb->dev = dev;
skb->protocol = eth_type_trans(skb, dev);
skb->ip_summed = CHECKSUM_UNNECESSARY; /* don't check it */
priv->stats.rx_bytes += rxLen;
And pass it to the upper layer
ret = netif_rx(skb);

Does this look correct?

I do not know what to put for the offset to add to rxLen ("x"). If I put zero I get
skb_over_panic: text:fb00005e len:240 put:240 head:00bd6000 data:00bd6012 tail:0xbd6102 end:0xbd6100 dev:eth0
BUG: failure at /home/caleb/nios2-linux/linux-2.6/net/core/skbuff.c:128/skb_over_panic()!
Kernel panic - not syncing: BUG!
, but when I add an offset, when I print out the skb data at the end to check, it has the sequence "...bb00 efbe adde 0000..." at the end of the buffer after the packet data I have written. Is there a way I can clear the socket buffer memory? should I memcpy zeros in initially? Do I need to pad to some boundary?

netif_rx is returning 0 which indicates success, but I don't see the packet count increase in ifconfig.
Old 02-25-2009, 03:30 PM   #2
Registered: Feb 2002
Location: Grenoble
Distribution: Debian
Posts: 9,657

Rep: Reputation: 210Reputation: 210Reputation: 210
x=2. That's the same 2 you add to skb_reserve. Explanation: IP packet in the stack must be aligned. It doesn't match the 'raw' Ethernet packet you have. It means that you need to reserve 2 octets more and copy the data in such a way that the IP packet is formed as it should be.


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