Re: Disabling UDP checksums on HME

[Julian Coleman <jdc%coris.org.uk@localhost>]

Hi,

> On TX, the UDP protocols requires a tweak, i.e. if the result
> becomes 0x0000 (+0), it should be replaced with 0xffff (-0)
> because 0x0000 in cksum field has some special meanings
> (no checksum on UDPv4). On the other hand, TCP doesn't
> require it (0x0000 is a valid sum).

I have done some tests to try and replicate the bad checksum with short
packets.  I am also unable to replicate it.  However, that still leaves
the case where the checksum result is 0x0000.
 
> On RX, we have to check whole a sum of packets including
> a value in the cksum field, so no tweaks on it even on UDP.
> (hardware doesn't modify the cksum field in a RX packet
>  but just provides a dumb sum value, I think)

There is also a test in hme_get() where we don't set the hardware checksum
flag if the UDP checksum is 0x0000.  This will catch the case where the
hardware fails to generate the correct UDP sum.

So, I think that we should just disallow IFCAP_CSUM_UDPv4_Tx for HME.  And,
I should revisit the GEM driver, and look to see if it makes sense to turn
on the hardware receive checksums there.

Thanks,

J

-- 
  My other computer also runs NetBSD    /        Sailing at Newbiggin
        http://www.netbsd.org/        /   http://www.newbigginsailingclub.org/