On Mon, 2014-12-15 at 19:14 +0100, Ralf Baechle wrote:
> On Mon, Dec 01, 2014 at 04:09:36AM +0000, Ben Hutchings wrote:
>
> > > /* Same as tx - compute csum of pseudo header */
> > > csum = hwsum +
> > > - (ih->tot_len - (ih->ihl << 2)) +
> > > - htons((uint16_t)ih->protocol) +
> > > + (ih->tot_len - (ih->ihl << 2)) + ih->protocol +
> > > (ih->saddr >> 16) + (ih->saddr & 0xffff) +
> > > (ih->daddr >> 16) + (ih->daddr & 0xffff);
> > >
> >
> > The pseudo-header is specified as:
> >
> > +--------+--------+--------+--------+
> > | Source Address |
> > +--------+--------+--------+--------+
> > | Destination Address |
> > +--------+--------+--------+--------+
> > | zero | PTCL | TCP Length |
> > +--------+--------+--------+--------+
> >
> > The current code zero-extends the protocol number to produce the 5th
> > 16-bit word of the pseudo-header, then uses htons() to put it in
> > big-endian order, consistent with the other fields. (Yes, it's doing
> > addition on big-endian words; this works even on little-endian machines
> > due to the way the checksum is specified.)
> >
> > The driver should not be doing this at all, though. It should set
> > skb->csum = hwsum; skb->ip_summed = CHECKSUM_COMPLETE; and let the
> > network stack adjust the hardware checksum.
>
> Really? The IOC3 isn't the exactly the smartest NIC around; it does add up
> everything and the kitchen sink, that is ethernet headers, IP headers and
> on RX the frame's trailing CRC.
That is almost exactly what CHECKSUM_COMPLETE means on receive; only the
CRC would need to be subtracted. Then the driver can validate
{TCP,UDP}/IPv{4,6} checksums without any header parsing.
> All that needs to be subtracted in software
> which is what this does. I think others NICs are all smarted and don't
> need this particular piece of magic.
It may not be smart, but that allows it to cover more cases than most
smart network controllers!
On transmit, the driver should:
- Calculate the partial checksum of data up to offset csum_start
- Subtract this from the checksum at offset (csum_start + csum_offset)
It should set the NETIF_F_GEN_CSUM feature flag rather than
NETIF_F_IP_CSUM. Then it will be able to generate {TCP,UDP}/IPv{4,6}
checksums.
Ben.
> I agree with your other comment wrt. to htons().
--
Ben Hutchings
The two most common things in the universe are hydrogen and stupidity.
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