[RFC] wpa_supplicant: add fast reconnect support

[Sam Leffler]

On Wed, Mar 23, 2011 at 6:07 PM, Sam Leffler <sleffler at google.com> wrote:
> When dropped by an AP for inactivity or because the session has timed
> out, try to re-join directly before falling back to a full scan and the
> normal work. ?This reduces the time to re-join a network from 5+ seconds
> to ~300ms (not counting any time to renew DHCP).
>
> Note that for mac80211 we cannot directly re-authenticate because by the
> time we issue a request mac80211 has discarded the BSS state which is
> required to process an MLME request. ?Instead we scan the channel where
> we last saw the AP and then use the normal join mechanism. ?Since this
> is not true for all drivers it might be worthwhile to try a direct Auth
> first and only then fall back to scanning. ?Also, note that if the scan
> request collides with another scan then we just piggyback on that scan.

Beware I ported this change from an older version of HEAD and much has
changed in this area.  The main issue I'm looking for feedback on is
how to handle scan collisions and/or errors.  I modified the SME code
to return an error code so we can recognize collisions but left the
other code path (into the drv_scan op) unchanged.

-Sam