source: rtems-libbsd/freebsd/sys/net80211/ieee80211_scan.c @ a241ea8

#include <machine/rtems-bsd-kernel-space.h>

/*-
 * 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("$FreeBSD$");

/*
 * IEEE 802.11 scanning support.
 */
#include <rtems/bsd/local/opt_wlan.h>

#include <rtems/bsd/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 */
#define ROAM_MCS_11AC_DEFAULT           (1 | IEEE80211_RATE_MCS) /* 11ac bss; XXX not used yet */

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 },
        [IEEE80211_MODE_VHT_2GHZ]       = { .rssi = ROAM_RSSI_11B_DEFAULT,
                                    .rate = ROAM_MCS_11AC_DEFAULT },
        [IEEE80211_MODE_VHT_5GHZ]       = { .rssi = ROAM_RSSI_11A_DEFAULT,
                                    .rate = ROAM_MCS_11AC_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.
 *
 * This is called during normal 802.11 data path to cancel
 * a scan so a newly arrived normal data packet can be sent.
 */
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);
}