Design notes on dedicated P2P interface API

Note: The design on this page is WIP: http://thread.gmane.org/gmane.linux.kernel.wireless.general/93044

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.

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 <code>struct wireless_dev</code> but **without** a netdev
* modify cfg80211 API (e.g. <code>scan</code>, <code>remain_on_channel</code>) to take <code>struct wireless_dev</code> 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? :-) )