bridge_native_dahdi.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2013 Digium, Inc.
 *
 * Richard Mudgett <[email protected]>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*!
 * \file
 * \brief Native DAHDI bridging support.
 *
 * \author Richard Mudgett <[email protected]>
 *
 * See Also:
 * \arg \ref AstCREDITS
 */

/*** MODULEINFO
   <support_level>core</support_level>
 ***/

#include "asterisk.h"

#include "../sig_analog.h"
#if defined(HAVE_PRI)
#include "../sig_pri.h"
#endif   /* defined(HAVE_PRI) */
#include "../chan_dahdi.h"

#include "bridge_native_dahdi.h"
#include "asterisk/bridge.h"
#include "asterisk/bridge_technology.h"
#include "asterisk/frame.h"
#include "asterisk/format_cache.h"

/* ------------------------------------------------------------------- */

static const struct ast_channel_tech *dahdi_tech;

struct native_pvt_chan {
   /*! Original private. */
   struct dahdi_pvt *pvt;
   /*! Original private owner. */
   struct ast_channel *owner;
   /*! Original owner index. */
   int index;
   /*! Original file descriptor 0. */
   int fd0;
   /*! Original channel state. */
   int state;
   /*! Original inthreeway. */
   unsigned int inthreeway:1;
};

struct native_pvt_bridge {
   /*! Master channel in the native bridge. */
   struct dahdi_pvt *master;
   /*! Slave channel in the native bridge. */
   struct dahdi_pvt *slave;
   /*! TRUE if the bridge can start when ready. */
   unsigned int saw_start:1;
   /*! TRUE if the channels are connected in a conference. */
   unsigned int connected:1;
#if defined(HAVE_PRI) && defined(PRI_2BCT)
   /*!
    * \brief TRUE if tried to eliminate possible PRI tromboned call.
    *
    * \note A tromboned call uses two B channels of the same ISDN
    * span.  One leg comes into Asterisk, the other leg goes out of
    * Asterisk, and Asterisk is natively bridging the two legs.
    */
   unsigned int tried_trombone_removal:1;
#endif   /* defined(HAVE_PRI) && defined(PRI_2BCT) */
};

/*!
 * \internal
 * \brief Create a bridge technology instance for a bridge.
 * \since 12.0.0
 *
 * \retval 0 on success
 * \retval -1 on failure
 *
 * \note On entry, bridge may or may not already be locked.
 * However, it can be accessed as if it were locked.
 */
static int native_bridge_create(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;

   ast_assert(!bridge->tech_pvt);

   tech_pvt = ast_calloc(1, sizeof(*tech_pvt));
   if (!tech_pvt) {
      return -1;
   }

   bridge->tech_pvt = tech_pvt;
   return 0;
}

/*!
 * \internal
 * \brief Destroy a bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \note On entry, bridge must NOT be locked.
 */
static void native_bridge_destroy(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;

   tech_pvt = bridge->tech_pvt;
   bridge->tech_pvt = NULL;
   ast_free(tech_pvt);
}

/*!
 * \internal
 * \brief Stop native bridging activity.
 * \since 12.0.0
 *
 * \param bridge What to operate upon.
 *
 * \return Nothing
 *
 * \note On entry, bridge is already locked.
 */
static void native_stop(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *bridge_tech_pvt;
   struct ast_bridge_channel *cur;

   ast_assert(bridge->tech_pvt != NULL);

   AST_LIST_TRAVERSE(&bridge->channels, cur, entry) {
      struct native_pvt_chan *chan_tech_pvt;

      chan_tech_pvt = cur->tech_pvt;
      if (!chan_tech_pvt) {
         continue;
      }

      ast_mutex_lock(&chan_tech_pvt->pvt->lock);
      if (chan_tech_pvt->pvt == ast_channel_tech_pvt(cur->chan)) {
         dahdi_ec_enable(chan_tech_pvt->pvt);
      }
      if (chan_tech_pvt->index == SUB_REAL) {
         dahdi_dtmf_detect_enable(chan_tech_pvt->pvt);
      }
      ast_mutex_unlock(&chan_tech_pvt->pvt->lock);
   }

   bridge_tech_pvt = bridge->tech_pvt;
   dahdi_master_slave_unlink(bridge_tech_pvt->slave, bridge_tech_pvt->master, 1);

   ast_debug(2, "Stop native bridging %s and %s\n",
      ast_channel_name(AST_LIST_FIRST(&bridge->channels)->chan),
      ast_channel_name(AST_LIST_LAST(&bridge->channels)->chan));
}

/*!
 * \internal
 * \brief Request to stop native bridging activity.
 * \since 12.0.0
 *
 * \param bridge What to operate upon.
 *
 * \return Nothing
 *
 * \note On entry, bridge is already locked.
 */
static void native_request_stop(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;

   ast_assert(bridge->tech_pvt != NULL);

   tech_pvt = bridge->tech_pvt;
   if (!tech_pvt->connected) {
      return;
   }
   tech_pvt->connected = 0;

   /* Now to actually stop the bridge. */
   native_stop(bridge);
}

/*!
 * \internal
 * \brief Start native bridging activity.
 * \since 12.0.0
 *
 * \param bridge What to operate upon.
 *
 * \retval 0 on success.
 * \retval -1 on error.  Could not start the bridge.
 *
 * \note On entry, bridge may or may not already be locked.
 * However, it can be accessed as if it were locked.
 */
static int native_start(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;
   struct ast_bridge_channel *bc0;
   struct ast_bridge_channel *bc1;
   struct native_pvt_chan *npc0;
   struct native_pvt_chan *npc1;
   struct ast_channel *c0;
   struct ast_channel *c1;
   struct dahdi_pvt *p0;
   struct dahdi_pvt *p1;
   struct dahdi_pvt *master;
   struct dahdi_pvt *slave;
   int inconf;
   int nothing_ok;

   ast_assert(bridge->tech_pvt != NULL);

   bc0 = AST_LIST_FIRST(&bridge->channels);
   bc1 = AST_LIST_LAST(&bridge->channels);
   c0 = bc0->chan;
   c1 = bc1->chan;

   /* Lock channels and privates */
   for (;;) {
      ast_channel_lock(c0);
      if (!ast_channel_trylock(c1)) {
         p0 = ast_channel_tech_pvt(c0);
         if (!ast_mutex_trylock(&p0->lock)) {
            p1 = ast_channel_tech_pvt(c1);
            if (!ast_mutex_trylock(&p1->lock)) {
               /* Got all locks */
               break;
            }
            ast_mutex_unlock(&p0->lock);
         }
         ast_channel_unlock(c1);
      }
      ast_channel_unlock(c0);
      sched_yield();
   }

   npc0 = bc0->tech_pvt;
   ast_assert(npc0 != NULL);
   npc0->pvt = p0;
   npc0->owner = p0->owner;
   npc0->index = dahdi_get_index(c0, p0, 0);
   npc0->fd0 = ast_channel_fd(c0, 0);
   npc0->state = -1;
   npc0->inthreeway = p0->subs[SUB_REAL].inthreeway;

   npc1 = bc1->tech_pvt;
   ast_assert(npc1 != NULL);
   npc1->pvt = p1;
   npc1->owner = p1->owner;
   npc1->index = dahdi_get_index(c1, p1, 0);
   npc1->fd0 = ast_channel_fd(c1, 0);
   npc1->state = -1;
   npc1->inthreeway = p1->subs[SUB_REAL].inthreeway;

   /*
    * Check things that can change on the privates while in native
    * bridging and cause native to not activate.
    */
   if (npc0->index < 0 || npc1->index < 0
#if defined(HAVE_PRI)
      /*
       * PRI nobch channels (hold and call waiting) are equivalent to
       * pseudo channels and cannot be nativly bridged.
       */
      || (dahdi_sig_pri_lib_handles(p0->sig)
         && ((struct sig_pri_chan *) p0->sig_pvt)->no_b_channel)
      || (dahdi_sig_pri_lib_handles(p1->sig)
         && ((struct sig_pri_chan *) p1->sig_pvt)->no_b_channel)
#endif   /* defined(HAVE_PRI) */
      ) {
      ast_mutex_unlock(&p0->lock);
      ast_mutex_unlock(&p1->lock);
      ast_channel_unlock(c0);
      ast_channel_unlock(c1);
      return -1;
   }

   inconf = 0;
   nothing_ok = 1;
   master = NULL;
   slave = NULL;
   if (npc0->index == SUB_REAL && npc1->index == SUB_REAL) {
      if (p0->owner && p1->owner) {
         /*
          * If we don't have a call-wait in a 3-way, and we aren't in a
          * 3-way, we can be master.
          */
         if (!p0->subs[SUB_CALLWAIT].inthreeway && !p1->subs[SUB_REAL].inthreeway) {
            master = p0;
            slave = p1;
            inconf = 1;
         } else if (!p1->subs[SUB_CALLWAIT].inthreeway && !p0->subs[SUB_REAL].inthreeway) {
            master = p1;
            slave = p0;
            inconf = 1;
         } else {
            ast_log(LOG_WARNING, "Huh?  Both calls are callwaits or 3-ways?  That's clever...?\n");
            ast_log(LOG_WARNING, "p0: chan %d/%d/CW%d/3W%d, p1: chan %d/%d/CW%d/3W%d\n",
               p0->channel,
               npc0->index, (p0->subs[SUB_CALLWAIT].dfd > -1) ? 1 : 0,
               p0->subs[SUB_REAL].inthreeway,
               p0->channel,
               npc0->index, (p1->subs[SUB_CALLWAIT].dfd > -1) ? 1 : 0,
               p1->subs[SUB_REAL].inthreeway);
         }
         nothing_ok = 0;
      }
   } else if (npc0->index == SUB_REAL && npc1->index == SUB_THREEWAY) {
      if (p1->subs[SUB_THREEWAY].inthreeway) {
         master = p1;
         slave = p0;
         nothing_ok = 0;
      }
   } else if (npc0->index == SUB_THREEWAY && npc1->index == SUB_REAL) {
      if (p0->subs[SUB_THREEWAY].inthreeway) {
         master = p0;
         slave = p1;
         nothing_ok = 0;
      }
   } else if (npc0->index == SUB_REAL && npc1->index == SUB_CALLWAIT) {
      /*
       * We have a real and a call wait.  If we're in a three way
       * call, put us in it, otherwise, don't put us in anything.
       */
      if (p1->subs[SUB_CALLWAIT].inthreeway) {
         master = p1;
         slave = p0;
         nothing_ok = 0;
      }
   } else if (npc0->index == SUB_CALLWAIT && npc1->index == SUB_REAL) {
      /* Same as previous */
      if (p0->subs[SUB_CALLWAIT].inthreeway) {
         master = p0;
         slave = p1;
         nothing_ok = 0;
      }
   }
   ast_debug(3, "master: %d, slave: %d, nothing_ok: %d\n",
      master ? master->channel : 0,
      slave ? slave->channel : 0,
      nothing_ok);
   if (master && slave) {
      /*
       * Stop any tones, or play ringtone as appropriate.  If they are
       * bridged in an active threeway call with a channel that is
       * ringing, we should indicate ringing.
       */
      if (npc1->index == SUB_THREEWAY
         && p1->subs[SUB_THREEWAY].inthreeway
         && p1->subs[SUB_REAL].owner
         && p1->subs[SUB_REAL].inthreeway
         && ast_channel_state(p1->subs[SUB_REAL].owner) == AST_STATE_RINGING) {
         ast_debug(2,
            "Playing ringback on %d/%d(%s) since %d/%d(%s) is in a ringing three-way\n",
            p0->channel, npc0->index, ast_channel_name(c0),
            p1->channel, npc1->index, ast_channel_name(c1));
         tone_zone_play_tone(p0->subs[npc0->index].dfd, DAHDI_TONE_RINGTONE);
         npc1->state = ast_channel_state(p1->subs[SUB_REAL].owner);
      } else {
         ast_debug(2, "Stopping tones on %d/%d(%s) talking to %d/%d(%s)\n",
            p0->channel, npc0->index, ast_channel_name(c0),
            p1->channel, npc1->index, ast_channel_name(c1));
         tone_zone_play_tone(p0->subs[npc0->index].dfd, -1);
      }

      if (npc0->index == SUB_THREEWAY
         && p0->subs[SUB_THREEWAY].inthreeway
         && p0->subs[SUB_REAL].owner
         && p0->subs[SUB_REAL].inthreeway
         && ast_channel_state(p0->subs[SUB_REAL].owner) == AST_STATE_RINGING) {
         ast_debug(2,
            "Playing ringback on %d/%d(%s) since %d/%d(%s) is in a ringing three-way\n",
            p1->channel, npc1->index, ast_channel_name(c1),
            p0->channel, npc0->index, ast_channel_name(c0));
         tone_zone_play_tone(p1->subs[npc1->index].dfd, DAHDI_TONE_RINGTONE);
         npc0->state = ast_channel_state(p0->subs[SUB_REAL].owner);
      } else {
         ast_debug(2, "Stopping tones on %d/%d(%s) talking to %d/%d(%s)\n",
            p1->channel, npc1->index, ast_channel_name(c1),
            p0->channel, npc0->index, ast_channel_name(c0));
         tone_zone_play_tone(p1->subs[npc1->index].dfd, -1);
      }

      if (npc0->index == SUB_REAL && npc1->index == SUB_REAL) {
         if (!p0->echocanbridged || !p1->echocanbridged) {
            /* Disable echo cancellation if appropriate */
            dahdi_ec_disable(p0);
            dahdi_ec_disable(p1);
         }
      }
      dahdi_master_slave_link(slave, master);
      master->inconference = inconf;
   } else if (!nothing_ok) {
      ast_log(LOG_WARNING, "Can't link %d/%s with %d/%s\n",
         p0->channel, subnames[npc0->index],
         p1->channel, subnames[npc1->index]);
   }
   dahdi_conf_update(p0);
   dahdi_conf_update(p1);

   ast_channel_unlock(c0);
   ast_channel_unlock(c1);

   /* Native bridge failed */
   if ((!master || !slave) && !nothing_ok) {
      ast_mutex_unlock(&p0->lock);
      ast_mutex_unlock(&p1->lock);
      return -1;
   }

   if (npc0->index == SUB_REAL) {
      dahdi_dtmf_detect_disable(p0);
   }
   if (npc1->index == SUB_REAL) {
      dahdi_dtmf_detect_disable(p1);
   }

   ast_mutex_unlock(&p0->lock);
   ast_mutex_unlock(&p1->lock);

   tech_pvt = bridge->tech_pvt;
   tech_pvt->master = master;
   tech_pvt->slave = slave;

   ast_debug(2, "Start native bridging %s and %s\n",
      ast_channel_name(c0), ast_channel_name(c1));

#if defined(HAVE_PRI) && defined(PRI_2BCT)
   if (!tech_pvt->tried_trombone_removal) {
      tech_pvt->tried_trombone_removal = 1;

      if (p0->pri && p0->pri == p1->pri && p0->pri->transfer) {
         q931_call *q931_c0;
         q931_call *q931_c1;

         /* Try to eliminate the tromboned call. */
         ast_mutex_lock(&p0->pri->lock);
         ast_assert(dahdi_sig_pri_lib_handles(p0->sig));
         ast_assert(dahdi_sig_pri_lib_handles(p1->sig));
         q931_c0 = ((struct sig_pri_chan *) (p0->sig_pvt))->call;
         q931_c1 = ((struct sig_pri_chan *) (p1->sig_pvt))->call;
         if (q931_c0 && q931_c1) {
            pri_channel_bridge(q931_c0, q931_c1);
            ast_debug(2, "Attempt to eliminate tromboned call with %s and %s\n",
               ast_channel_name(c0), ast_channel_name(c1));
         }
         ast_mutex_unlock(&p0->pri->lock);
      }
   }
#endif   /* defined(HAVE_PRI) && defined(PRI_2BCT) */
   return 0;
}

/*!
 * \internal
 * \brief Request to start native bridging activity.
 * \since 12.0.0
 *
 * \param bridge What to operate upon.
 *
 * \return Nothing
 *
 * \note On entry, bridge may or may not already be locked.
 * However, it can be accessed as if it were locked.
 */
static void native_request_start(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;
   struct ast_bridge_channel *cur;

   ast_assert(bridge->tech_pvt != NULL);

   tech_pvt = bridge->tech_pvt;

   if (bridge->num_channels != 2 || !tech_pvt->saw_start || tech_pvt->connected) {
      return;
   }
   AST_LIST_TRAVERSE(&bridge->channels, cur, entry) {
      if (cur->suspended || !cur->tech_pvt) {
         return;
      }
   }

   /* Actually try starting the native bridge. */
   if (native_start(bridge)) {
      return;
   }
   tech_pvt->connected = 1;
}

/*!
 * \internal
 * \brief Request a bridge technology instance start operations.
 * \since 12.0.0
 *
 * \retval 0 on success
 * \retval -1 on failure
 *
 * \note On entry, bridge may or may not already be locked.
 * However, it can be accessed as if it were locked.
 */
static int native_bridge_start(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;

   ast_assert(bridge->tech_pvt != NULL);

   tech_pvt = bridge->tech_pvt;
   tech_pvt->saw_start = 1;

   native_request_start(bridge);
   return 0;
}

/*!
 * \internal
 * \brief Request a bridge technology instance stop in preparation for being destroyed.
 * \since 12.0.0
 *
 * \note On entry, bridge is already locked.
 */
static void native_bridge_stop(struct ast_bridge *bridge)
{
   struct native_pvt_bridge *tech_pvt;

   tech_pvt = bridge->tech_pvt;
   if (!tech_pvt) {
      return;
   }

   tech_pvt->saw_start = 0;
   native_request_stop(bridge);
}

/*!
 * \internal
 * \brief Add a channel to a bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \retval 0 on success
 * \retval -1 on failure
 *
 * \note On entry, bridge is already locked.
 */
static int native_bridge_join(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel)
{
   struct native_pvt_chan *tech_pvt;
   struct ast_channel *c0;
   struct ast_channel *c1;

   ast_assert(!bridge_channel->tech_pvt);

   tech_pvt = ast_calloc(1, sizeof(*tech_pvt));
   if (!tech_pvt) {
      return -1;
   }

   bridge_channel->tech_pvt = tech_pvt;
   native_request_start(bridge);

   /*
    * Make the channels compatible in case the native bridge did
    * not start for some reason and we need to fallback to 1-1
    * bridging.
    */
   c0 = AST_LIST_FIRST(&bridge->channels)->chan;
   c1 = AST_LIST_LAST(&bridge->channels)->chan;
   if (c0 == c1) {
      return 0;
   }
   return ast_channel_make_compatible(c0, c1);
}

/*!
 * \internal
 * \brief Remove a channel from a bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \note On entry, bridge is already locked.
 */
static void native_bridge_leave(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel)
{
   struct native_pvt_chan *tech_pvt;

   native_request_stop(bridge);

   tech_pvt = bridge_channel->tech_pvt;
   bridge_channel->tech_pvt = NULL;
   ast_free(tech_pvt);
}

/*!
 * \internal
 * \brief Suspend a channel on a bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \note On entry, bridge is already locked.
 */
static void native_bridge_suspend(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel)
{
   native_request_stop(bridge);
}

/*!
 * \internal
 * \brief Unsuspend a channel on a bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \note On entry, bridge is already locked.
 */
static void native_bridge_unsuspend(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel)
{
   native_request_start(bridge);
}

/*!
 * \internal
 * \brief Check if channel is compatible.
 * \since 12.0.0
 *
 * \param bridge_channel Is this channel compatible.
 *
 * \retval TRUE if channel is compatible with native DAHDI bridge.
 */
static int native_bridge_is_capable(struct ast_bridge_channel *bridge_channel)
{
   struct ast_channel *chan = bridge_channel->chan;
   struct dahdi_pvt *pvt;
   int is_capable;

   if (ao2_container_count(bridge_channel->features->dtmf_hooks)) {
      ast_debug(2, "Channel '%s' has DTMF hooks.\n", ast_channel_name(chan));
      return 0;
   }

   ast_channel_lock(chan);

   if (dahdi_tech != ast_channel_tech(chan)) {
      ast_debug(2, "Channel '%s' is not %s.\n",
         ast_channel_name(chan), dahdi_tech->type);
      ast_channel_unlock(chan);
      return 0;
   }
   if (ast_channel_has_audio_frame_or_monitor(chan)) {
      ast_debug(2, "Channel '%s' has an active monitor, audiohook, or framehook.\n",
         ast_channel_name(chan));
      ast_channel_unlock(chan);
      return 0;
   }
   pvt = ast_channel_tech_pvt(chan);
   if (!pvt || !pvt->sig) {
      /* No private; or signaling is for a pseudo channel. */
      ast_channel_unlock(chan);
      return 0;
   }

   is_capable = 1;
   ast_mutex_lock(&pvt->lock);

   if (pvt->callwaiting && pvt->callwaitingcallerid) {
      /*
       * Call Waiting Caller ID requires DTMF detection to know if it
       * can send the CID spill.
       */
      ast_debug(2, "Channel '%s' has call waiting caller ID enabled.\n",
         ast_channel_name(chan));
      is_capable = 0;
   }

   ast_mutex_unlock(&pvt->lock);
   ast_channel_unlock(chan);

   return is_capable;
}

/*!
 * \internal
 * \brief Check if a bridge is compatible with the bridging technology.
 * \since 12.0.0
 *
 * \retval 0 if not compatible
 * \retval non-zero if compatible
 *
 * \note On entry, bridge may or may not already be locked.
 * However, it can be accessed as if it were locked.
 */
static int native_bridge_compatible(struct ast_bridge *bridge)
{
   struct ast_bridge_channel *cur;

   /* We require two channels before even considering native bridging. */
   if (bridge->num_channels != 2) {
      ast_debug(1, "Bridge %s: Cannot use native DAHDI.  Must have two channels.\n",
         bridge->uniqueid);
      return 0;
   }

   AST_LIST_TRAVERSE(&bridge->channels, cur, entry) {
      if (!native_bridge_is_capable(cur)) {
         ast_debug(1, "Bridge %s: Cannot use native DAHDI.  Channel '%s' not compatible.\n",
            bridge->uniqueid, ast_channel_name(cur->chan));
         return 0;
      }
   }

   return -1;
}

/*!
 * \internal
 * \brief Check if something changed on the channel.
 * \since 12.0.0
 *
 * \param bridge_channel What to operate upon.
 *
 * \retval 0 Nothing changed.
 * \retval -1 Something changed.
 *
 * \note On entry, bridge_channel->bridge is already locked.
 */
static int native_chan_changed(struct ast_bridge_channel *bridge_channel)
{
   struct native_pvt_chan *tech_pvt;
   struct ast_channel *chan;
   struct dahdi_pvt *pvt;
   int idx = -1;

   ast_assert(bridge_channel->tech_pvt != NULL);

   tech_pvt = bridge_channel->tech_pvt;

   chan = bridge_channel->chan;
   ast_channel_lock(chan);
   pvt = ast_channel_tech_pvt(chan);
   if (tech_pvt->pvt == pvt) {
      idx = dahdi_get_index(chan, pvt, 1);
   }
   ast_channel_unlock(chan);

   if (/* Did chan get masqueraded or PRI change associated B channel? */
      tech_pvt->pvt != pvt
      /* Did the pvt active owner change? */
      || tech_pvt->owner != pvt->owner
      /* Did the pvt three way call status change? */
      || tech_pvt->inthreeway != pvt->subs[SUB_REAL].inthreeway
      /* Did the owner index change? */
      || tech_pvt->index != idx
      /*
       * Did chan file descriptor change? (This seems redundant with
       * masquerade and active owner change checks.)
       */
      || tech_pvt->fd0 != ast_channel_fd(chan, 0)
      /* Did chan state change? i.e. Did it stop ringing? */
      || (pvt->subs[SUB_REAL].owner
         && tech_pvt->state > -1
         && tech_pvt->state != ast_channel_state(pvt->subs[SUB_REAL].owner))) {
      return -1;
   }

   return 0;
}

/*!
 * \internal
 * \brief Check if something changed on the bridge channels.
 * \since 12.0.0
 *
 * \param bridge What to operate upon.
 *
 * \retval 0 Nothing changed.
 * \retval -1 Something changed.
 *
 * \note On entry, bridge is already locked.
 */
static int native_bridge_changed(struct ast_bridge *bridge)
{
   struct ast_bridge_channel *cur;

   AST_LIST_TRAVERSE(&bridge->channels, cur, entry) {
      if (native_chan_changed(cur)) {
         ast_debug(1, "Bridge %s: Something changed on channel '%s'.\n",
            bridge->uniqueid, ast_channel_name(cur->chan));
         return -1;
      }
   }
   return 0;
}

/*!
 * \internal
 * \brief Write a frame into the bridging technology instance for a bridge.
 * \since 12.0.0
 *
 * \note The bridge must be tolerant of bridge_channel being NULL.
 *
 * \retval 0 Frame accepted into the bridge.
 * \retval -1 Frame needs to be deferred.
 *
 * \note On entry, bridge is already locked.
 */
static int native_bridge_write(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel, struct ast_frame *frame)
{
   struct native_pvt_bridge *tech_pvt;

   /*
    * When we are not native bridged by DAHDI, we are like a normal
    * 1-1 bridge.
    */

   ast_assert(bridge->tech_pvt != NULL);

   /* Recheck native bridging validity. */
   tech_pvt = bridge->tech_pvt;
   switch (frame->frametype) {
   case AST_FRAME_VOICE:
   case AST_FRAME_VIDEO:
      if (!tech_pvt->connected) {
         /* Don't try to start native mode on media frames. */
         break;
      }
      if (native_bridge_changed(bridge)) {
         native_request_stop(bridge);
         native_request_start(bridge);
         if (!tech_pvt->connected) {
            break;
         }
      }

      /*
       * Native bridge handles voice frames in hardware.  However, it
       * also passes the frames up to Asterisk anyway.  Discard the
       * media frames.
       */
      return 0;
   default:
      if (!tech_pvt->connected) {
         native_request_start(bridge);
         break;
      }
      if (native_bridge_changed(bridge)) {
         native_request_stop(bridge);
         native_request_start(bridge);
      }
      break;
   }

   return ast_bridge_queue_everyone_else(bridge, bridge_channel, frame);
}

static struct ast_bridge_technology native_bridge = {
   .name = "native_dahdi",
   .capabilities = AST_BRIDGE_CAPABILITY_NATIVE,
   .preference = AST_BRIDGE_PREFERENCE_BASE_NATIVE,
   .create = native_bridge_create,
   .start = native_bridge_start,
   .stop = native_bridge_stop,
   .destroy = native_bridge_destroy,
   .join = native_bridge_join,
   .leave = native_bridge_leave,
   .suspend = native_bridge_suspend,
   .unsuspend = native_bridge_unsuspend,
   .compatible = native_bridge_compatible,
   .write = native_bridge_write,
};

/*!
 * \internal
 * \brief Destroy the DAHDI native bridge support.
 * \since 12.0.0
 *
 * \return Nothing
 */
void dahdi_native_unload(void)
{
   ast_bridge_technology_unregister(&native_bridge);
}

/*!
 * \internal
 * \brief Initialize the DAHDI native bridge support.
 * \since 12.0.0
 *
 * \retval 0 on success.
 * \retval -1 on error.
 */
int dahdi_native_load(const struct ast_channel_tech *tech)
{
   dahdi_tech = tech;

   if (ast_bridge_technology_register(&native_bridge)) {
      dahdi_native_unload();
      return -1;
   }

   return 0;
}