Re: Disabling UDP checksums on HME

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

> > > So, it looks like we should disable UDP checksums for hme(4)
> > 
> > I guess disabling only udp4csum-tx is enough.
> 
> It's not clear from their bug report.  I thought that the same hardware
> routine would be used for both transmit and receive.

IIUC, a dumb check sum is okay on RX.

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).

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)
---
Izumi Tsutsui