Design notes on dedicated P2P interface API

Rationale

Some drivers/devices would like to

use a separate MAC address
use a separate control path

for P2P usage. This could even help mac80211-based drivers like iwlagn since currently, iwlagn needs to enable P2P in the device when a remain-on-channel is done, and disable it after a timeout or when a P2P interface is used.

wpa_supplicant calls this the WPA_IF_P2P_GROUP but I think the term "P2P device interface" would be more appropriate here.

API notes

A separate netdev would be the most obvious choice, but can be confusing:

to the user – new interface is there, what does it do?
to the developer – no data traffic on this interface

Better: use dedicated API in nl80211:

start-P2P -> returns cookie
stop-P2P -> uses cookie

(or maybe don't have "stop-P2P" but simply stop when socket is closed like mgmt frame subscriptions)

The only issue with this is that things like scan, mgmt-tx etc. need a netdev index now. However, this can be changed, idea:

use cookie to identify the P2P device interface
internally, create a struct wireless_dev but without a netdev
modify cfg80211 API (e.g. scan, remain_on_channel) to take struct wireless_dev instead of netdev, driver can check what the type is etc.
this needs separate P2P-device iftype that can't really be used as an iftype, which is fine

Questions:

lifetime: does the P2P-device interface become the P2P-group/client interface like in wpa_supplicant, which means that it is removed before/when the real netdev is added? (personally I prefer it would stay around I think since I think discovery/public action things would still be done with it, not the real interface – Johannes)
pure software implementation of this in mac80211 for drivers that don't care, to unify API? but wpa_s needs old code anyway for backward compatibility

additional thoughts

This could also be a good framework for additional features that we'll need to add:

device-based P2P listen/search timing (soon)
maybe some more P2P offloads (WoP2P anyone? )