Re: H/W requirements for NETIF_F_HW_CSUM

[Stephen Hemminger <shemminger@osdl.org>]

On Wed, 26 Jul 2006 10:28:00 -0700
"Ravinandan Arakali" <ravinandan.arakali@neterion.com> wrote:

> Hello,
> Our current NIC does not provide the actual checksum value on receive path.
> Hence we only claim NETIF_F_IP_CSUM instead of the more general
> NETIF_F_HW_CSUM.
> 
> To support this in a future adapter, we would like to know what exactly are
> the requirements (on both Rx and Tx )to claim NETIF_F_HW_CSUM ?

If you set NETIF_F_HW_CSUM, on transmit the adapter if ip_summed is set
will be handed an unchecksummed frame with the offset to stuff the checksum at.
Only difference between NETIF_F_HW_CSUM and NETIF_F_IP_CSUM is that IP_CSUM
means the device can handle IPV4 only.

NETIF_F_HW_CSUM has no impact on receive. The form of receive checksumming format
is up to the device. It can either put one's complement in skb->csum and
set ip_summed to CHECKSUM_HW or if device only reports checksum good then
set CHECKSUM_UNNECESSARY.

Several are a couple of subtleties to the receive processing:
* Meaning of ip_summed changes from tx to rx path and that has to be handled
  in code that does forwarding like bridges.
* If device only reports checksum okay vs bad. The packets marked bad might
  be another protocol, so should be passed up with CHECKSUM_NONE and let any
  checksum errors get detected in software.
* Checksum HW on receive should work better since then IPV6 and nested protocols like
  VLAN's can be handled.

> Following are some specific questions:
> 1. On Tx, our adapter supports checksumming of TCP/UDP over IPv4 and IPv6.
> This computation is TCP/UDP specific. Does the checksum calculation need to
> be more generic ? Also, skbuff.h says that the checksum needs to be placed
> at a specific location(skb->h.raw+skb->csum). I guess this means the adapter
> needs to pass back the checksum to host driver after transmission. What
> happens in case of TSO ?
> 2. On Rx, is it suffficient if we place the L4 checksum in skb->csum ? What
> about L3 checksum ?
> 

The L3 checksum (IP) is always computed. Since the header is in CPU cache anyway
it is faster that way.