FreeBSD/Linux Kernel Cross Reference
sys/net80211/ieee80211_scan.c

     /*-
      * Copyright (c) 2002-2008 Sam Leffler, Errno Consulting
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
      *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
     * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
     * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
     * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
     * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/11.1/sys/net80211/ieee80211_scan.c 298392 2016-04-21 06:19:33Z avos $");
    
    /*
     * IEEE 802.11 scanning support.
     */
    #include "opt_wlan.h"
    
    #include <sys/param.h>
    #include <sys/systm.h> 
    #include <sys/proc.h>
    #include <sys/kernel.h>
    #include <sys/malloc.h>
    #include <sys/condvar.h>
     
    #include <sys/socket.h>
    
    #include <net/if.h>
    #include <net/if_var.h>
    #include <net/if_media.h>
    #include <net/ethernet.h>
    
    #include <net80211/ieee80211_var.h>
    
    /* XXX until it's implemented as attach ops */
    #include <net80211/ieee80211_scan_sw.h>
    
    #include <net/bpf.h>
    
    /*
     * Roaming-related defaults.  RSSI thresholds are as returned by the
     * driver (.5dBm).  Transmit rate thresholds are IEEE rate codes (i.e
     * .5M units) or MCS.
     */
    /* rssi thresholds */
    #define ROAM_RSSI_11A_DEFAULT           14      /* 11a bss */
    #define ROAM_RSSI_11B_DEFAULT           14      /* 11b bss */
    #define ROAM_RSSI_11BONLY_DEFAULT       14      /* 11b-only bss */
    /* transmit rate thresholds */
    #define ROAM_RATE_11A_DEFAULT           2*12    /* 11a bss */
    #define ROAM_RATE_11B_DEFAULT           2*5     /* 11b bss */
    #define ROAM_RATE_11BONLY_DEFAULT       2*1     /* 11b-only bss */
    #define ROAM_RATE_HALF_DEFAULT          2*6     /* half-width 11a/g bss */
    #define ROAM_RATE_QUARTER_DEFAULT       2*3     /* quarter-width 11a/g bss */
    #define ROAM_MCS_11N_DEFAULT            (1 | IEEE80211_RATE_MCS) /* 11n bss */
    
    void
    ieee80211_scan_attach(struct ieee80211com *ic)
    {
            /*
             * If there's no scan method pointer, attach the
             * swscan set as a default.
             */
            if (ic->ic_scan_methods == NULL)
                    ieee80211_swscan_attach(ic);
            else
                    ic->ic_scan_methods->sc_attach(ic);
    }
    
    void
    ieee80211_scan_detach(struct ieee80211com *ic)
    {
    
            /*
             * Ideally we'd do the ss_ops detach call here;
             * but then sc_detach() would need to be split in two.
             *
             * I'll do that later.
             */
            ic->ic_scan_methods->sc_detach(ic);
    }
    
    static const struct ieee80211_roamparam defroam[IEEE80211_MODE_MAX] = {
            [IEEE80211_MODE_11A]    = { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_11A_DEFAULT },
           [IEEE80211_MODE_11G]    = { .rssi = ROAM_RSSI_11B_DEFAULT,
                                       .rate = ROAM_RATE_11B_DEFAULT },
           [IEEE80211_MODE_11B]    = { .rssi = ROAM_RSSI_11BONLY_DEFAULT,
                                       .rate = ROAM_RATE_11BONLY_DEFAULT },
           [IEEE80211_MODE_TURBO_A]= { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_11A_DEFAULT },
           [IEEE80211_MODE_TURBO_G]= { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_11A_DEFAULT },
           [IEEE80211_MODE_STURBO_A]={ .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_11A_DEFAULT },
           [IEEE80211_MODE_HALF]   = { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_HALF_DEFAULT },
           [IEEE80211_MODE_QUARTER]= { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_RATE_QUARTER_DEFAULT },
           [IEEE80211_MODE_11NA]   = { .rssi = ROAM_RSSI_11A_DEFAULT,
                                       .rate = ROAM_MCS_11N_DEFAULT },
           [IEEE80211_MODE_11NG]   = { .rssi = ROAM_RSSI_11B_DEFAULT,
                                       .rate = ROAM_MCS_11N_DEFAULT },
   };
   
   void
   ieee80211_scan_vattach(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           vap->iv_bgscanidle = (IEEE80211_BGSCAN_IDLE_DEFAULT*1000)/hz;
           vap->iv_bgscanintvl = IEEE80211_BGSCAN_INTVAL_DEFAULT*hz;
           vap->iv_scanvalid = IEEE80211_SCAN_VALID_DEFAULT*hz;
   
           vap->iv_roaming = IEEE80211_ROAMING_AUTO;
           memcpy(vap->iv_roamparms, defroam, sizeof(defroam));
   
           ic->ic_scan_methods->sc_vattach(vap);
   }
   
   void
   ieee80211_scan_vdetach(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
           struct ieee80211_scan_state *ss;
   
           IEEE80211_LOCK(ic);
           ss = ic->ic_scan;
   
           ic->ic_scan_methods->sc_vdetach(vap);
   
           if (ss != NULL && ss->ss_vap == vap) {
                   if (ss->ss_ops != NULL) {
                           ss->ss_ops->scan_detach(ss);
                           ss->ss_ops = NULL;
                   }
                   ss->ss_vap = NULL;
           }
           IEEE80211_UNLOCK(ic);
   }
   
   /*
    * Simple-minded scanner module support.
    */
   static const char *scan_modnames[IEEE80211_OPMODE_MAX] = {
           "wlan_scan_sta",        /* IEEE80211_M_IBSS */
           "wlan_scan_sta",        /* IEEE80211_M_STA */
           "wlan_scan_wds",        /* IEEE80211_M_WDS */
           "wlan_scan_sta",        /* IEEE80211_M_AHDEMO */
           "wlan_scan_ap",         /* IEEE80211_M_HOSTAP */
           "wlan_scan_monitor",    /* IEEE80211_M_MONITOR */
           "wlan_scan_sta",        /* IEEE80211_M_MBSS */
   };
   static const struct ieee80211_scanner *scanners[IEEE80211_OPMODE_MAX];
   
   const struct ieee80211_scanner *
   ieee80211_scanner_get(enum ieee80211_opmode mode)
   {
           if (mode >= IEEE80211_OPMODE_MAX)
                   return NULL;
           if (scanners[mode] == NULL)
                   ieee80211_load_module(scan_modnames[mode]);
           return scanners[mode];
   }
   
   void
   ieee80211_scanner_register(enum ieee80211_opmode mode,
           const struct ieee80211_scanner *scan)
   {
           if (mode >= IEEE80211_OPMODE_MAX)
                   return;
           scanners[mode] = scan;
   }
   
   void
   ieee80211_scanner_unregister(enum ieee80211_opmode mode,
           const struct ieee80211_scanner *scan)
   {
           if (mode >= IEEE80211_OPMODE_MAX)
                   return;
           if (scanners[mode] == scan)
                   scanners[mode] = NULL;
   }
   
   void
   ieee80211_scanner_unregister_all(const struct ieee80211_scanner *scan)
   {
           int m;
   
           for (m = 0; m < IEEE80211_OPMODE_MAX; m++)
                   if (scanners[m] == scan)
                           scanners[m] = NULL;
   }
   
   /*
    * Update common scanner state to reflect the current
    * operating mode.  This is called when the state machine
    * is transitioned to RUN state w/o scanning--e.g. when
    * operating in monitor mode.  The purpose of this is to
    * ensure later callbacks find ss_ops set to properly
    * reflect current operating mode.
    */
   void
   ieee80211_scan_update_locked(struct ieee80211vap *vap,
           const struct ieee80211_scanner *scan)
   {
           struct ieee80211com *ic = vap->iv_ic;
           struct ieee80211_scan_state *ss = ic->ic_scan;
   
           IEEE80211_LOCK_ASSERT(ic);
   
   #ifdef IEEE80211_DEBUG
           if (ss->ss_vap != vap || ss->ss_ops != scan) {
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
                       "%s: current scanner is <%s:%s>, switch to <%s:%s>\n",
                       __func__,
                       ss->ss_vap != NULL ?
                           ss->ss_vap->iv_ifp->if_xname : "none",
                       ss->ss_vap != NULL ?
                           ieee80211_opmode_name[ss->ss_vap->iv_opmode] : "none",
                       vap->iv_ifp->if_xname,
                       ieee80211_opmode_name[vap->iv_opmode]);
           }
   #endif
           ss->ss_vap = vap;
           if (ss->ss_ops != scan) {
                   /*
                    * Switch scanners; detach old, attach new.  Special
                    * case where a single scan module implements multiple
                    * policies by using different scan ops but a common
                    * core.  We assume if the old and new attach methods
                    * are identical then it's ok to just change ss_ops
                    * and not flush the internal state of the module.
                    */
                   if (scan == NULL || ss->ss_ops == NULL ||
                       ss->ss_ops->scan_attach != scan->scan_attach) {
                           if (ss->ss_ops != NULL)
                                   ss->ss_ops->scan_detach(ss);
                           if (scan != NULL && !scan->scan_attach(ss)) {
                                   /* XXX attach failure */
                                   /* XXX stat+msg */
                                   scan = NULL;
                           }
                   }
                   ss->ss_ops = scan;
           }
   }
   
   void
   ieee80211_scan_dump_channels(const struct ieee80211_scan_state *ss)
   {
           struct ieee80211com *ic = ss->ss_ic;
           const char *sep;
           int i;
   
           sep = "";
           for (i = ss->ss_next; i < ss->ss_last; i++) {
                   const struct ieee80211_channel *c = ss->ss_chans[i];
   
                   printf("%s%u%c", sep, ieee80211_chan2ieee(ic, c),
                       ieee80211_channel_type_char(c));
                   sep = ", ";
           }
   }
   
   #ifdef IEEE80211_DEBUG
   void
   ieee80211_scan_dump(struct ieee80211_scan_state *ss)
   {
           struct ieee80211vap *vap = ss->ss_vap;
   
           if_printf(vap->iv_ifp, "scan set ");
           ieee80211_scan_dump_channels(ss);
           printf(" dwell min %lums max %lums\n",
               ticks_to_msecs(ss->ss_mindwell), ticks_to_msecs(ss->ss_maxdwell));
   }
   #endif /* IEEE80211_DEBUG */
   
   void
   ieee80211_scan_copy_ssid(struct ieee80211vap *vap, struct ieee80211_scan_state *ss,
           int nssid, const struct ieee80211_scan_ssid ssids[])
   {
           if (nssid > IEEE80211_SCAN_MAX_SSID) {
                   /* XXX printf */
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
                       "%s: too many ssid %d, ignoring all of them\n",
                       __func__, nssid);
                   return;
           }
           memcpy(ss->ss_ssid, ssids, nssid * sizeof(ssids[0]));
           ss->ss_nssid = nssid;
   }
   
   /*
    * Start a scan unless one is already going.
    */
   int
   ieee80211_start_scan(struct ieee80211vap *vap, int flags,
           u_int duration, u_int mindwell, u_int maxdwell,
           u_int nssid, const struct ieee80211_scan_ssid ssids[])
   {
           const struct ieee80211_scanner *scan;
           struct ieee80211com *ic = vap->iv_ic;
   
           scan = ieee80211_scanner_get(vap->iv_opmode);
           if (scan == NULL) {
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
                       "%s: no scanner support for %s mode\n",
                       __func__, ieee80211_opmode_name[vap->iv_opmode]);
                   /* XXX stat */
                   return 0;
           }
   
           return ic->ic_scan_methods->sc_start_scan(scan, vap, flags, duration,
               mindwell, maxdwell, nssid, ssids);
   }
   
   /*
    * Check the scan cache for an ap/channel to use; if that
    * fails then kick off a new scan.
    */
   int
   ieee80211_check_scan(struct ieee80211vap *vap, int flags,
           u_int duration, u_int mindwell, u_int maxdwell,
           u_int nssid, const struct ieee80211_scan_ssid ssids[])
   {
           struct ieee80211com *ic = vap->iv_ic;
           struct ieee80211_scan_state *ss = ic->ic_scan;
           const struct ieee80211_scanner *scan;
           int result;
   
           scan = ieee80211_scanner_get(vap->iv_opmode);
           if (scan == NULL) {
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
                       "%s: no scanner support for %s mode\n",
                       __func__, vap->iv_opmode);
                   /* XXX stat */
                   return 0;
           }
   
           /*
            * Check if there's a list of scan candidates already.
            * XXX want more than the ap we're currently associated with
            */
   
           IEEE80211_LOCK(ic);
           IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
               "%s: %s scan, %s%s%s%s%s\n"
               , __func__
               , flags & IEEE80211_SCAN_ACTIVE ? "active" : "passive"
               , flags & IEEE80211_SCAN_FLUSH ? "flush" : "append"
               , flags & IEEE80211_SCAN_NOPICK ? ", nopick" : ""
               , flags & IEEE80211_SCAN_NOJOIN ? ", nojoin" : ""
               , flags & IEEE80211_SCAN_PICK1ST ? ", pick1st" : ""
               , flags & IEEE80211_SCAN_ONCE ? ", once" : ""
           );
   
           if (ss->ss_ops != scan) {
                   /* XXX re-use cache contents? e.g. adhoc<->sta */
                   flags |= IEEE80211_SCAN_FLUSH;
           }
   
           /*
            * XXX TODO: separate things out a bit better.
            */
           ieee80211_scan_update_locked(vap, scan);
   
           result = ic->ic_scan_methods->sc_check_scan(scan, vap, flags, duration,
               mindwell, maxdwell, nssid, ssids);
   
           IEEE80211_UNLOCK(ic);
   
           return (result);
   }
   
   /*
    * Check the scan cache for an ap/channel to use; if that fails
    * then kick off a scan using the current settings.
    */
   int
   ieee80211_check_scan_current(struct ieee80211vap *vap)
   {
           return ieee80211_check_scan(vap,
               IEEE80211_SCAN_ACTIVE,
               IEEE80211_SCAN_FOREVER, 0, 0,
               vap->iv_des_nssid, vap->iv_des_ssid);
   }
   
   /*
    * Restart a previous scan.  If the previous scan completed
    * then we start again using the existing channel list.
    */
   int
   ieee80211_bg_scan(struct ieee80211vap *vap, int flags)
   {
           struct ieee80211com *ic = vap->iv_ic;
           const struct ieee80211_scanner *scan;
   
           // IEEE80211_UNLOCK_ASSERT(sc);
   
           scan = ieee80211_scanner_get(vap->iv_opmode);
           if (scan == NULL) {
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
                       "%s: no scanner support for %s mode\n",
                       __func__, vap->iv_opmode);
                   /* XXX stat */
                   return 0;
           }
   
           /*
            * XXX TODO: pull apart the bgscan logic into whatever
            * belongs here and whatever belongs in the software
            * scanner.
            */
           return (ic->ic_scan_methods->sc_bg_scan(scan, vap, flags));
   }
   
   /*
    * Cancel any scan currently going on for the specified vap.
    */
   void
   ieee80211_cancel_scan(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           ic->ic_scan_methods->sc_cancel_scan(vap);
   }
   
   /*
    * Cancel any scan currently going on.
    */
   void
   ieee80211_cancel_anyscan(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           ic->ic_scan_methods->sc_cancel_anyscan(vap);
   }
   
   /*
    * Manually switch to the next channel in the channel list.
    * Provided for drivers that manage scanning themselves
    * (e.g. for firmware-based devices).
    */
   void
   ieee80211_scan_next(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           ic->ic_scan_methods->sc_scan_next(vap);
   }
   
   /*
    * Manually stop a scan that is currently running.
    * Provided for drivers that are not able to scan single channels
    * (e.g. for firmware-based devices).
    */
   void
   ieee80211_scan_done(struct ieee80211vap *vap)
   {
           struct ieee80211com *ic = vap->iv_ic;
           struct ieee80211_scan_state *ss;
   
           IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN, "%s: called\n", __func__);
   
           IEEE80211_LOCK(ic);
           ss = ic->ic_scan;
           ss->ss_next = ss->ss_last; /* all channels are complete */
   
           ic->ic_scan_methods->sc_scan_done(vap);
   
           IEEE80211_UNLOCK(ic);
   }
   
   /*
    * Probe the current channel, if allowed, while scanning.
    * If the channel is not marked passive-only then send
    * a probe request immediately.  Otherwise mark state and
    * listen for beacons on the channel; if we receive something
    * then we'll transmit a probe request.
    */
   void
   ieee80211_probe_curchan(struct ieee80211vap *vap, int force)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           if ((ic->ic_curchan->ic_flags & IEEE80211_CHAN_PASSIVE) && !force) {
                   ic->ic_flags_ext |= IEEE80211_FEXT_PROBECHAN;
                   return;
           }
   
           ic->ic_scan_methods->sc_scan_probe_curchan(vap, force);
   }
   
   #ifdef IEEE80211_DEBUG
   static void
   dump_country(const uint8_t *ie)
   {
           const struct ieee80211_country_ie *cie =
              (const struct ieee80211_country_ie *) ie;
           int i, nbands, schan, nchan;
   
           if (cie->len < 3) {
                   printf(" <bogus country ie, len %d>", cie->len);
                   return;
           }
           printf(" country [%c%c%c", cie->cc[0], cie->cc[1], cie->cc[2]);
           nbands = (cie->len - 3) / sizeof(cie->band[0]);
           for (i = 0; i < nbands; i++) {
                   schan = cie->band[i].schan;
                   nchan = cie->band[i].nchan;
                   if (nchan != 1)
                           printf(" %u-%u,%u", schan, schan + nchan-1,
                               cie->band[i].maxtxpwr);
                   else
                           printf(" %u,%u", schan, cie->band[i].maxtxpwr);
           }
           printf("]");
   }
   
   void
   ieee80211_scan_dump_probe_beacon(uint8_t subtype, int isnew,
           const uint8_t mac[IEEE80211_ADDR_LEN],
           const struct ieee80211_scanparams *sp, int rssi)
   {
   
           printf("[%s] %s%s on chan %u (bss chan %u) ",
               ether_sprintf(mac), isnew ? "new " : "",
               ieee80211_mgt_subtype_name(subtype), sp->chan, sp->bchan);
           ieee80211_print_essid(sp->ssid + 2, sp->ssid[1]);
           printf(" rssi %d\n", rssi);
   
           if (isnew) {
                   printf("[%s] caps 0x%x bintval %u erp 0x%x", 
                           ether_sprintf(mac), sp->capinfo, sp->bintval, sp->erp);
                   if (sp->country != NULL)
                           dump_country(sp->country);
                   printf("\n");
           }
   }
   #endif /* IEEE80211_DEBUG */
   
   /*
    * Process a beacon or probe response frame.
    */
   void
   ieee80211_add_scan(struct ieee80211vap *vap,
           struct ieee80211_channel *curchan,
           const struct ieee80211_scanparams *sp,
           const struct ieee80211_frame *wh,
           int subtype, int rssi, int noise)
   {
           struct ieee80211com *ic = vap->iv_ic;
   
           return (ic->ic_scan_methods->sc_add_scan(vap, curchan, sp, wh, subtype,
               rssi, noise));
   }
   
   /*
    * Timeout/age scan cache entries; called from sta timeout
    * timer (XXX should be self-contained).
    */
   void
   ieee80211_scan_timeout(struct ieee80211com *ic)
   {
           struct ieee80211_scan_state *ss = ic->ic_scan;
   
           if (ss->ss_ops != NULL)
                   ss->ss_ops->scan_age(ss);
   }
   
   /*
    * Mark a scan cache entry after a successful associate.
    */
   void
   ieee80211_scan_assoc_success(struct ieee80211vap *vap, const uint8_t mac[])
   {
           struct ieee80211_scan_state *ss = vap->iv_ic->ic_scan;
   
           if (ss->ss_ops != NULL) {
                   IEEE80211_NOTE_MAC(vap, IEEE80211_MSG_SCAN,
                           mac, "%s",  __func__);
                   ss->ss_ops->scan_assoc_success(ss, mac);
           }
   }
   
   /*
    * Demerit a scan cache entry after failing to associate.
    */
   void
   ieee80211_scan_assoc_fail(struct ieee80211vap *vap,
           const uint8_t mac[], int reason)
   {
           struct ieee80211_scan_state *ss = vap->iv_ic->ic_scan;
   
           if (ss->ss_ops != NULL) {
                   IEEE80211_NOTE_MAC(vap, IEEE80211_MSG_SCAN, mac,
                           "%s: reason %u", __func__, reason);
                   ss->ss_ops->scan_assoc_fail(ss, mac, reason);
           }
   }
   
   /*
    * Iterate over the contents of the scan cache.
    */
   void
   ieee80211_scan_iterate(struct ieee80211vap *vap,
           ieee80211_scan_iter_func *f, void *arg)
   {
           struct ieee80211_scan_state *ss = vap->iv_ic->ic_scan;
   
           if (ss->ss_ops != NULL)
                   ss->ss_ops->scan_iterate(ss, f, arg);
   }
   
   /*
    * Flush the contents of the scan cache.
    */
   void
   ieee80211_scan_flush(struct ieee80211vap *vap)
   {
           struct ieee80211_scan_state *ss = vap->iv_ic->ic_scan;
   
           if (ss->ss_ops != NULL && ss->ss_vap == vap) {
                   IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN, "%s\n",  __func__);
                   ss->ss_ops->scan_flush(ss);
           }
   }
   
   /*
    * Check the scan cache for an ap/channel to use; if that
    * fails then kick off a new scan.
    */
   struct ieee80211_channel *
   ieee80211_scan_pickchannel(struct ieee80211com *ic, int flags)
   {
           struct ieee80211_scan_state *ss = ic->ic_scan;
   
           IEEE80211_LOCK_ASSERT(ic);
   
           if (ss == NULL || ss->ss_ops == NULL || ss->ss_vap == NULL) {
                   /* XXX printf? */
                   return NULL;
           }
           if (ss->ss_ops->scan_pickchan == NULL) {
                   IEEE80211_DPRINTF(ss->ss_vap, IEEE80211_MSG_SCAN,
                       "%s: scan module does not support picking a channel, "
                       "opmode %s\n", __func__, ss->ss_vap->iv_opmode);
                   return NULL;
           }
           return ss->ss_ops->scan_pickchan(ss, flags);
   }

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