Re: mac80211 hardware encryption

[Ivo van Doorn <ivdoorn@gmail.com>]

On Monday 07 April 2008, Johannes Berg wrote:
> 
> > I'm not sure if I understand completely what you mean but rt61pci/rt73usb hardware does the following:
> > 
> > 1) Receive frame
> > 2) Determine key from register
> > 3) Put IV/EIV into descriptor
> > 4) Decrypt
> > 5) Notifies driver about the frame +  decryption status
> 
> Ok, so you get those out-of-band.
> 
> > Well as an alternative to adding it to tx_control, perhaps a callback function for drivers
> > could be provided? At the moment mac80211 calls:
> > 
> > ieee80211_tkip_add_iv(pos, key,
> >                       (u8) (key->u.tkip.iv16 >> 8),
> >                       (u8) (((key->u.tkip.iv16 >> 8) | 0x20) &
> >                             0x7f),
> >                       (u8) key->u.tkip.iv16);
> > 
> > to insert the IV into the skb, if the driver could get a calback function that calls the above
> > function and writes the result into a char* buffer you get the same effect except that the
> > driver can put the iv wherever it wants.
> > That way rt2x00 doesn't have to set the IEEE80211_KEY_FLAG_GENERATE_IV flag, and
> > can request the IV manually from mac80211.
> 
> No, you can't do that because the IV is already incremented at that
> point, so you wouldn't be able to perfectly match things up because
> ops->tx() need not be serialised with this due to deferring to the
> master interface.

Ok, so just to summarize (so I have everything right :) )

TX path)
rt2x00 sets the IEEE80211_KEY_FLAG_GENERATE_IV flag
grabs the IV/EIV from behind the ieee80211 header
memmove ieee80211 header to remove the IV/EIV

TX done path)
memmove ieee80211 header to make room for IV/EIV
reinsert IV/EIV behind ieee80211 header
call ieee80211_tx_status() for mac80211

RX path)
memmove ieee80211 header to make room for IV/EIV
insert IV/EIV behind ieee80211 header
call ieee80211_rx() for mac80211

Thanks,

Ivo