osmo-bts/src/osmo-bts-trx/scheduler_trx.c

/* Scheduler worker functions for OsmoBTS-TRX */

/* (C) 2013 by Andreas Eversberg <jolly@eversberg.eu>
 * (C) 2015 by Alexander Chemeris <Alexander.Chemeris@fairwaves.co>
 * (C) 2015-2017 by Harald Welte <laforge@gnumonks.org>
 * (C) 2020-2021 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
 *
 * All Rights Reserved
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
#include <stdlib.h>
#include <unistd.h>
#include <limits.h>
#include <errno.h>
#include <stdint.h>
#include <ctype.h>
#include <inttypes.h>
#include <sys/timerfd.h>

#include <osmocom/core/msgb.h>
#include <osmocom/core/talloc.h>
#include <osmocom/core/timer_compat.h>
#include <osmocom/core/bits.h>
#include <osmocom/gsm/a5.h>
#include <osmocom/gsm/gsm0502.h>


#include <osmo-bts/gsm_data.h>
#include <osmo-bts/logging.h>
#include <osmo-bts/rsl.h>
#include <osmo-bts/bts.h>
#include <osmo-bts/l1sap.h>
#include <osmo-bts/scheduler.h>
#include <osmo-bts/scheduler_backend.h>
#include <osmo-bts/pcu_if.h>

#include "l1_if.h"
#include "trx_if.h"

#include "btsconfig.h"

#ifdef HAVE_SYSTEMTAP
/* include the generated probes header and put markers in code */
#include "probes.h"
#define TRACE(probe) probe
#define TRACE_ENABLED(probe) probe ## _ENABLED()
#else
/* Wrap the probe to allow it to be removed when no systemtap available */
#define TRACE(probe)
#define TRACE_ENABLED(probe) (0)
#endif /* HAVE_SYSTEMTAP */

#define SCHED_FH_PARAMS_FMT "hsn=%u, maio=%u, ma_len=%u"
#define SCHED_FH_PARAMS_VALS(ts) \
	(ts)->hopping.hsn, (ts)->hopping.maio, (ts)->hopping.arfcn_num

static void lchan_report_interf_meas(const struct gsm_lchan *lchan)
{
	const struct gsm_bts_trx_ts *ts = lchan->ts;
	const struct l1sched_ts *l1ts = ts->priv;
	enum trx_chan_type dcch, acch;
	int interf_avg;

	/* We're not interested in active CS channels */
	if (lchan->state == LCHAN_S_ACTIVE) {
		if (lchan->type != GSM_LCHAN_PDTCH)
			return;
	}

	switch (lchan->type) {
	case GSM_LCHAN_SDCCH:
		if (ts->pchan == GSM_PCHAN_CCCH_SDCCH4 ||
		    ts->pchan == GSM_PCHAN_CCCH_SDCCH4_CBCH) {
			dcch = TRXC_SDCCH4_0 + lchan->nr;
			acch = TRXC_SACCH4_0 + lchan->nr;
		} else { /* SDCCH/8 otherwise */
			dcch = TRXC_SDCCH8_0 + lchan->nr;
			acch = TRXC_SACCH8_0 + lchan->nr;
		}
		break;
	case GSM_LCHAN_TCH_F:
		dcch = TRXC_TCHF;
		acch = TRXC_SACCHTF;
		break;
	case GSM_LCHAN_TCH_H:
		dcch = TRXC_TCHH_0 + lchan->nr;
		acch = TRXC_SACCHTH_0 + lchan->nr;
		break;
	case GSM_LCHAN_PDTCH:
		/* We use idle TDMA frames on PDCH */
		dcch = TRXC_IDLE;
		acch = TRXC_IDLE;
		break;
	default:
		/* Skip other lchan types */
		return;
	}

	OSMO_ASSERT(dcch < ARRAY_SIZE(l1ts->chan_state));
	OSMO_ASSERT(acch < ARRAY_SIZE(l1ts->chan_state));

	interf_avg = (l1ts->chan_state[dcch].meas.interf_avg +
		      l1ts->chan_state[acch].meas.interf_avg) / 2;

	gsm_lchan_interf_meas_push((struct gsm_lchan *) lchan, interf_avg);
}

static void bts_report_interf_meas(const struct gsm_bts *bts,
				   const uint32_t fn)
{
	const struct gsm_bts_trx *trx;
	unsigned int tn, ln;

	llist_for_each_entry(trx, &bts->trx_list, list) {
		for (tn = 0; tn < ARRAY_SIZE(trx->ts); tn++) {
			const struct gsm_bts_trx_ts *ts = &trx->ts[tn];
			for (ln = 0; ln < ARRAY_SIZE(ts->lchan); ln++)
				lchan_report_interf_meas(&ts->lchan[ln]);
		}
	}
}

/* Find a route (PHY instance) for a given Downlink burst request */
static struct phy_instance *dlfh_route_br(const struct trx_dl_burst_req *br,
					  struct gsm_bts_trx_ts *ts)
{
	const struct gsm_bts_trx *trx;
	struct gsm_time time;
	uint16_t idx;

	gsm_fn2gsmtime(&time, br->fn);

	/* Check the "cache" first, so we eliminate frequent lookups */
	idx = gsm0502_hop_seq_gen(&time, SCHED_FH_PARAMS_VALS(ts), NULL);
	if (ts->fh_trx_list[idx] != NULL)
		return ts->fh_trx_list[idx]->pinst;

	/* The "cache" may not be filled yet, lookup the transceiver */
	llist_for_each_entry(trx, &ts->trx->bts->trx_list, list) {
		if (trx->arfcn == ts->hopping.arfcn_list[idx]) {
			ts->fh_trx_list[idx] = trx;
			return trx->pinst;
		}
	}

	LOGPTRX(ts->trx, DL1C, LOGL_FATAL, "Failed to find the transceiver (RF carrier) "
		"for a Downlink burst (fn=%u, tn=%u, " SCHED_FH_PARAMS_FMT ")\n",
		br->fn, br->tn, SCHED_FH_PARAMS_VALS(ts));

	struct bts_trx_priv *priv = (struct bts_trx_priv *) ts->trx->bts->model_priv;
	rate_ctr_inc(rate_ctr_group_get_ctr(priv->ctrs, BTSTRX_CTR_SCHED_DL_FH_NO_CARRIER));

	return NULL;
}

static void bts_sched_init_buffers(struct gsm_bts *bts, const uint32_t fn)
{
	struct gsm_bts_trx *trx;
	uint8_t tn;

	llist_for_each_entry(trx, &bts->trx_list, list) {
		struct phy_instance *pinst = trx->pinst;
		const struct phy_link *plink = pinst->phy_link;

		/* Advance frame number, so the PHY has more time to process bursts */
		const uint32_t sched_fn = GSM_TDMA_FN_SUM(fn, plink->u.osmotrx.clock_advance);

		for (tn = 0; tn < ARRAY_SIZE(trx->ts); tn++) {
			struct trx_dl_burst_req *br = &pinst->u.osmotrx.br[tn];

			*br = (struct trx_dl_burst_req) {
				.trx_num = trx->nr,
				.fn = sched_fn,
				.tn = tn,
			};
		}
	}

	/* Initialize all timeslots on C0/TRX0 with dummy burst */
	for (tn = 0; tn < ARRAY_SIZE(trx->ts); tn++) {
		struct phy_instance *pinst = bts->c0->pinst;
		struct trx_dl_burst_req *br = &pinst->u.osmotrx.br[tn];
		const struct gsm_bts_trx_ts *ts = &bts->c0->ts[tn];

		memcpy(br->burst, _sched_dummy_burst, GSM_BURST_LEN);
		br->burst_len = GSM_BURST_LEN;

		/* BCCH carrier power reduction for this timeslot */
		br->att = ts->c0_power_red_db;
	}
}

static void bts_sched_flush_buffers(struct gsm_bts *bts)
{
	const struct gsm_bts_trx *trx;
	unsigned int tn;

	llist_for_each_entry(trx, &bts->trx_list, list) {
		const struct phy_instance *pinst = trx->pinst;
		struct trx_l1h *l1h = pinst->u.osmotrx.hdl;

		for (tn = 0; tn < TRX_NR_TS; tn++) {
			const struct trx_dl_burst_req *br;

			br = &pinst->u.osmotrx.br[tn];
			if (!br->burst_len)
				continue;
			trx_if_send_burst(l1h, br);
		}

		/* Batch all timeslots into a single TRXD PDU */
		trx_if_send_burst(l1h, NULL);
	}
}

/* schedule one frame for a shadow timeslot, merge bursts */
static void _sched_dl_shadow_burst(const struct gsm_bts_trx_ts *ts,
				   struct trx_dl_burst_req *br)
{
	struct l1sched_ts *l1ts = ts->priv;

	/* For the shadow timeslots, physical channel type can be either
	 * GSM_PCHAN_TCH_{F,H} or GSM_PCHAN_NONE.  Even if the primary
	 * timeslot is a dynamic timeslot, it's always a concrete value. */
	if (ts->pchan == GSM_PCHAN_NONE)
		return;

	struct trx_dl_burst_req sbr = {
		.trx_num = br->trx_num,
		.fn = br->fn,
		.tn = br->tn,
	};

	_sched_dl_burst(l1ts, &sbr);

	if (br->burst_len != 0 && sbr.burst_len != 0) { /* Both present */
		memcpy(br->burst + GSM_BURST_LEN, sbr.burst, GSM_BURST_LEN);
		br->burst_len = 2 * GSM_BURST_LEN;
		br->mod = TRX_MOD_T_AQPSK;
		/* FIXME: SCPIR is hard-coded to 0 */
	} else if (br->burst_len == 0) {
		/* No primary burst, send shadow burst alone */
		memcpy(br, &sbr, sizeof(sbr));
	} else if (sbr.burst_len == 0) {
		/* No shadow burst, send primary burst alone */
		return;
	}
}

/* schedule all frames of all TRX for given FN */
static void bts_sched_fn(struct gsm_bts *bts, const uint32_t fn)
{
	struct gsm_bts_trx *trx;
	unsigned int tn;

	/* Report interference measurements */
	if (fn % 104 == 0) /* SACCH period */
		bts_report_interf_meas(bts, fn);

	/* send time indication */
	l1if_mph_time_ind(bts, fn);

	/* Initialize Downlink burst buffers */
	bts_sched_init_buffers(bts, fn);

	/* Populate Downlink burst buffers for each TRX/TS */
	llist_for_each_entry(trx, &bts->trx_list, list) {
		const struct phy_link *plink = trx->pinst->phy_link;
		struct trx_l1h *l1h = trx->pinst->u.osmotrx.hdl;

		/* we don't schedule, if power is off */
		if (!trx_if_powered(l1h))
			continue;

		/* process every TS of TRX */
		for (tn = 0; tn < ARRAY_SIZE(trx->ts); tn++) {
			struct phy_instance *pinst = trx->pinst;
			struct gsm_bts_trx_ts *ts = &trx->ts[tn];
			struct l1sched_ts *l1ts = ts->priv;
			struct trx_dl_burst_req *br;

			/* ready-to-send */
			TRACE(OSMO_BTS_TRX_DL_RTS_START(trx->nr, tn, fn));
			_sched_rts(l1ts, GSM_TDMA_FN_SUM(fn, plink->u.osmotrx.clock_advance
							   + plink->u.osmotrx.rts_advance));
			TRACE(OSMO_BTS_TRX_DL_RTS_DONE(trx->nr, tn, fn));

			/* pre-initialized buffer for the Downlink burst */
			br = &pinst->u.osmotrx.br[tn];

			/* resolve PHY instance if freq. hopping is enabled */
			if (ts->hopping.enabled) {
				pinst = dlfh_route_br(br, ts);
				if (pinst == NULL)
					continue;
				/* simply use a different buffer */
				br = &pinst->u.osmotrx.br[tn];
			}

			/* get burst for the primary timeslot */
			_sched_dl_burst(l1ts, br);

			/* get burst for the shadow timeslot */
			_sched_dl_shadow_burst(ts->vamos.peer, br);
		}
	}

	/* Send everything to the PHY */
	bts_sched_flush_buffers(bts);
}

/* Find a route (TRX instance) for a given Uplink burst indication */
static struct gsm_bts_trx *ulfh_route_bi(const struct trx_ul_burst_ind *bi,
					 const struct gsm_bts_trx *src_trx)
{
	struct gsm_bts_trx *trx;
	struct gsm_time time;
	uint16_t arfcn;

	gsm_fn2gsmtime(&time, bi->fn);

	llist_for_each_entry(trx, &src_trx->bts->trx_list, list) {
		const struct gsm_bts_trx_ts *ts = &trx->ts[bi->tn];
		if (!ts->hopping.enabled)
			continue;

		arfcn = gsm0502_hop_seq_gen(&time, SCHED_FH_PARAMS_VALS(ts), ts->hopping.arfcn_list);
		if (src_trx->arfcn == arfcn)
			return trx;
	}

	LOGPTRX(src_trx, DL1C, LOGL_DEBUG, "Failed to find the transceiver (RF carrier) "
		"for an Uplink burst (fn=%u, tn=%u, " SCHED_FH_PARAMS_FMT ")\n",
		bi->fn, bi->tn, SCHED_FH_PARAMS_VALS(&src_trx->ts[bi->tn]));

	struct bts_trx_priv *priv = (struct bts_trx_priv *) src_trx->bts->model_priv;
	rate_ctr_inc(rate_ctr_group_get_ctr(priv->ctrs, BTSTRX_CTR_SCHED_UL_FH_NO_CARRIER));

	return NULL;
}

/* Route a given Uplink burst indication to the scheduler depending on freq. hopping state */
int trx_sched_route_burst_ind(const struct gsm_bts_trx *trx, struct trx_ul_burst_ind *bi)
{
	/* no frequency hopping => nothing to do */
	if (!trx->ts[bi->tn].hopping.enabled)
		return trx_sched_ul_burst(trx->ts[bi->tn].priv, bi);

	trx = ulfh_route_bi(bi, trx);
	if (trx == NULL)
		return -ENODEV;

	return trx_sched_ul_burst(trx->ts[bi->tn].priv, bi);
}

/*! maximum number of 'missed' frame periods we can tolerate of OS doesn't schedule us*/
#define MAX_FN_SKEW		50
/*! maximum number of frame periods we can tolerate without TRX Clock Indication*/
#define TRX_LOSS_FRAMES		400

/*! compute the number of micro-seconds difference elapsed between \a last and \a now */
static inline int64_t compute_elapsed_us(const struct timespec *last, const struct timespec *now)
{
	struct timespec elapsed;

	timespecsub(now, last, &elapsed);
	return (int64_t)(elapsed.tv_sec * 1000000) + (elapsed.tv_nsec / 1000);
}

/*! compute the number of frame number intervals elapsed between \a last and \a now */
static inline int compute_elapsed_fn(const uint32_t last, const uint32_t now)
{
	int elapsed_fn = GSM_TDMA_FN_SUB(now, last);
	if (elapsed_fn >= 135774)
		elapsed_fn -= GSM_TDMA_HYPERFRAME;
	return elapsed_fn;
}

/*! normalise given 'struct timespec', i.e. carry nanoseconds into seconds */
static inline void normalize_timespec(struct timespec *ts)
{
	ts->tv_sec += ts->tv_nsec / 1000000000;
	ts->tv_nsec = ts->tv_nsec % 1000000000;
}

/*! this is the timerfd-callback firing for every FN to be processed */
static int trx_fn_timer_cb(struct osmo_fd *ofd, unsigned int what)
{
	struct gsm_bts *bts = ofd->data;
	struct bts_trx_priv *bts_trx = (struct bts_trx_priv *)bts->model_priv;
	struct osmo_trx_clock_state *tcs = &bts_trx->clk_s;
	struct timespec tv_now;
	uint64_t expire_count;
	int64_t elapsed_us, error_us;
	int rc, i;

	if (!(what & OSMO_FD_READ))
		return 0;

	/* read from timerfd: number of expirations of periodic timer */
	rc = read(ofd->fd, (void *) &expire_count, sizeof(expire_count));
	if (rc < 0 && errno == EAGAIN)
		return 0;
	OSMO_ASSERT(rc == sizeof(expire_count));

	if (expire_count > 1) {
		LOGP(DL1C, LOGL_NOTICE, "FN timer expire_count=%"PRIu64": We missed %"PRIu64" timers\n",
		     expire_count, expire_count - 1);
		rate_ctr_add(rate_ctr_group_get_ctr(bts_trx->ctrs, BTSTRX_CTR_SCHED_DL_MISS_FN), expire_count - 1);
	}

	/* check if transceiver is still alive */
	if (tcs->fn_without_clock_ind++ == TRX_LOSS_FRAMES) {
		LOGP(DL1C, LOGL_NOTICE, "No more clock from transceiver\n");
		goto no_clock;
	}

	/* compute actual elapsed time and resulting OS scheduling error */
	clock_gettime(CLOCK_MONOTONIC, &tv_now);
	elapsed_us = compute_elapsed_us(&tcs->last_fn_timer.tv, &tv_now);
	error_us = elapsed_us - GSM_TDMA_FN_DURATION_uS;
#ifdef DEBUG_CLOCK
	printf("%s(): %09ld, elapsed_us=%05" PRId64 ", error_us=%-d: fn=%d\n", __func__,
		tv_now.tv_nsec, elapsed_us, error_us, tcs->last_fn_timer.fn+1);
#endif
	tcs->last_fn_timer.tv = tv_now;

	/* if someone played with clock, or if the process stalled */
	if (elapsed_us > GSM_TDMA_FN_DURATION_uS * MAX_FN_SKEW || elapsed_us < 0) {
		LOGP(DL1C, LOGL_ERROR, "PC clock skew: elapsed_us=%" PRId64 ", error_us=%" PRId64 "\n",
			elapsed_us, error_us);
		goto no_clock;
	}

	/* call bts_sched_fn() for all expired FN */
	for (i = 0; i < expire_count; i++)
		bts_sched_fn(bts, GSM_TDMA_FN_INC(tcs->last_fn_timer.fn));

	return 0;

no_clock:
	osmo_timerfd_disable(&tcs->fn_timer_ofd);
	bts_shutdown(bts, "No clock from osmo-trx");
	return -1;
}

/*! \brief This is the cb of the initial timer set upon start. On timeout, it
 *  means it wasn't replaced and hence no CLOCK IND was received. */
static int trx_start_noclockind_to_cb(struct osmo_fd *ofd, unsigned int what)
{
	struct gsm_bts *bts = ofd->data;
	struct bts_trx_priv *bts_trx = (struct bts_trx_priv *)bts->model_priv;
	struct osmo_trx_clock_state *tcs = &bts_trx->clk_s;

	osmo_fd_close(&tcs->fn_timer_ofd); /* Avoid being called again */
	bts_shutdown(bts, "No clock since TRX was started");
	return -1;
}

/*! \brief PHY informs us clock indications should start to be received */
int trx_sched_clock_started(struct gsm_bts *bts)
{
	struct bts_trx_priv *bts_trx = (struct bts_trx_priv *)bts->model_priv;
	struct osmo_trx_clock_state *tcs = &bts_trx->clk_s;
	const struct timespec it_val = {3, 0};
	const struct timespec it_intval = {0, 0};

	LOGP(DL1C, LOGL_NOTICE, "GSM clock started, waiting for clock indications\n");
	osmo_fd_close(&tcs->fn_timer_ofd);
	memset(tcs, 0, sizeof(*tcs));
	tcs->fn_timer_ofd.fd = -1;
	/* Set up timeout to shutdown BTS if no clock ind is received in a few
	 * seconds. Upon clock ind receival, fn_timer_ofd will be reused and
	 * timeout won't trigger.
	 */
	osmo_timerfd_setup(&tcs->fn_timer_ofd, trx_start_noclockind_to_cb, bts);
	osmo_timerfd_schedule(&tcs->fn_timer_ofd, &it_val, &it_intval);
	return 0;
}

/*! \brief PHY informs us no more clock indications should be received anymore */
int trx_sched_clock_stopped(struct gsm_bts *bts)
{
	struct bts_trx_priv *bts_trx = (struct bts_trx_priv *)bts->model_priv;
	struct osmo_trx_clock_state *tcs = &bts_trx->clk_s;

	LOGP(DL1C, LOGL_NOTICE, "GSM clock stopped\n");
	osmo_fd_close(&tcs->fn_timer_ofd);

	return 0;
}

/*! reset clock with current fn and schedule it. Called when trx becomes
 *  available or when max clock skew is reached */
static int trx_setup_clock(struct gsm_bts *bts, struct osmo_trx_clock_state *tcs,
	struct timespec *tv_now, const struct timespec *interval, uint32_t fn)
{
	/* schedule first FN clock timer */
	osmo_timerfd_setup(&tcs->fn_timer_ofd, trx_fn_timer_cb, bts);
	osmo_timerfd_schedule(&tcs->fn_timer_ofd, NULL, interval);

	tcs->last_fn_timer.fn = fn;
	tcs->last_fn_timer.tv = *tv_now;
	/* call trx scheduler function for new 'last' FN */
	bts_sched_fn(bts, tcs->last_fn_timer.fn);

	return 0;
}

/*! called every time we receive a clock indication from TRX */
int trx_sched_clock(struct gsm_bts *bts, uint32_t fn)
{
	struct bts_trx_priv *bts_trx = (struct bts_trx_priv *)bts->model_priv;
	struct osmo_trx_clock_state *tcs = &bts_trx->clk_s;
	struct timespec tv_now;
	int elapsed_fn;
	int64_t elapsed_us, elapsed_us_since_clk, elapsed_fn_since_clk, error_us_since_clk;
	unsigned int fn_caught_up = 0;
	const struct timespec interval = { .tv_sec = 0, .tv_nsec = GSM_TDMA_FN_DURATION_nS };

	/* reset lost counter */
	tcs->fn_without_clock_ind = 0;

	clock_gettime(CLOCK_MONOTONIC, &tv_now);

	/* calculate elapsed time +fn since last timer */
	elapsed_us = compute_elapsed_us(&tcs->last_fn_timer.tv, &tv_now);
	elapsed_fn = compute_elapsed_fn(tcs->last_fn_timer.fn, fn);
#ifdef DEBUG_CLOCK
	printf("%s(): LAST_TIMER %9ld, elapsed_us=%7d, elapsed_fn=%+3d\n", __func__,
		tv_now.tv_nsec, elapsed_us, elapsed_fn);
#endif
	/* negative elapsed_fn values mean that we've already processed
	 * more FN based on the local interval timer than what the TRX
	 * now reports in the clock indication.   Positive elapsed_fn
	 * values mean we still have a backlog to process */

	/* calculate elapsed time +fn since last clk ind */
	elapsed_us_since_clk = compute_elapsed_us(&tcs->last_clk_ind.tv, &tv_now);
	elapsed_fn_since_clk = compute_elapsed_fn(tcs->last_clk_ind.fn, fn);
	/* error (delta) between local clock since last CLK and CLK based on FN clock at TRX */
	error_us_since_clk = elapsed_us_since_clk - (GSM_TDMA_FN_DURATION_uS * elapsed_fn_since_clk);
	LOGP(DL1C, LOGL_INFO, "TRX Clock Ind: elapsed_us=%7"PRId64", "
		"elapsed_fn=%3"PRId64", error_us=%+5"PRId64"\n",
		elapsed_us_since_clk, elapsed_fn_since_clk, error_us_since_clk);

	/* TODO: put this computed error_us_since_clk into some filter
	 * function and use that to adjust our regular timer interval to
	 * compensate for clock drift between the PC clock and the
	 * TRX/SDR clock */

	tcs->last_clk_ind.tv = tv_now;
	tcs->last_clk_ind.fn = fn;

	/* check for max clock skew */
	if (elapsed_fn > MAX_FN_SKEW || elapsed_fn < -MAX_FN_SKEW) {
		LOGP(DL1C, LOGL_NOTICE, "GSM clock skew: old fn=%u, "
			"new fn=%u\n", tcs->last_fn_timer.fn, fn);
		return trx_setup_clock(bts, tcs, &tv_now, &interval, fn);
	}

	LOGP(DL1C, LOGL_INFO, "GSM clock jitter: %" PRId64 "us (elapsed_fn=%d)\n",
		elapsed_fn * GSM_TDMA_FN_DURATION_uS - elapsed_us, elapsed_fn);

	/* too many frames have been processed already */
	if (elapsed_fn < 0) {
		struct timespec first = interval;
		/* set clock to the time or last FN should have been
		 * transmitted. */
		first.tv_nsec += (0 - elapsed_fn) * GSM_TDMA_FN_DURATION_nS;
		normalize_timespec(&first);
		LOGP(DL1C, LOGL_NOTICE, "We were %d FN faster than TRX, compensating\n", -elapsed_fn);
		/* set time to the time our next FN has to be transmitted */
		osmo_timerfd_schedule(&tcs->fn_timer_ofd, &first, &interval);
		return 0;
	}

	/* transmit what we still need to transmit */
	while (fn != tcs->last_fn_timer.fn) {
		bts_sched_fn(bts, GSM_TDMA_FN_INC(tcs->last_fn_timer.fn));
		fn_caught_up++;
	}

	if (fn_caught_up) {
		LOGP(DL1C, LOGL_NOTICE, "We were %d FN slower than TRX, compensated\n", elapsed_fn);
		tcs->last_fn_timer.tv = tv_now;
	}

	return 0;
}

void _sched_act_rach_det(struct gsm_bts_trx *trx, uint8_t tn, uint8_t ss, int activate)
{
	struct phy_instance *pinst = trx_phy_instance(trx);
	struct trx_l1h *l1h = pinst->u.osmotrx.hdl;

	if (activate)
		trx_if_cmd_handover(l1h, tn, ss);
	else
		trx_if_cmd_nohandover(l1h, tn, ss);
}

/* Add a set of UL burst measurements to the history */
void trx_sched_meas_push(struct l1sched_chan_state *chan_state,
			 const struct trx_ul_burst_ind *bi)
{
	unsigned int hist_size = ARRAY_SIZE(chan_state->meas.buf);
	unsigned int current = chan_state->meas.current;

	chan_state->meas.buf[current] = (struct l1sched_meas_set) {
		.ci_cb = (bi->flags & TRX_BI_F_CI_CB) ? bi->ci_cb : 0,
		.toa256 = bi->toa256,
		.rssi = bi->rssi,
	};

	chan_state->meas.current = (current + 1) % hist_size;
}

/* Calculate the AVG of n measurements from the history */
void trx_sched_meas_avg(const struct l1sched_chan_state *chan_state,
			struct l1sched_meas_set *avg,
			enum sched_meas_avg_mode mode)
{
	unsigned int hist_size = ARRAY_SIZE(chan_state->meas.buf);
	unsigned int current = chan_state->meas.current;
	const struct l1sched_meas_set *set;
	unsigned int shift, pos, i, n;

	float rssi_sum = 0;
	int toa256_sum = 0;
	int ci_cb_sum = 0;

	switch (mode) {
	/* last 4 bursts (default for xCCH, TCH/H, PTCCH and PDTCH) */
	case SCHED_MEAS_AVG_M_QUAD:
		n = 4; shift = n;
		break;
	/* last 8 bursts (default for TCH/F and FACCH/F) */
	case SCHED_MEAS_AVG_M_OCTO:
		n = 8; shift = n;
		break;
	/* last 6 bursts (default for FACCH/H) */
	case SCHED_MEAS_AVG_M_SIX:
		n = 6; shift = n;
		break;
	/* first 4 of last 8 bursts */
	case SCHED_MEAS_AVG_M8_FIRST_QUAD:
		n = 4; shift = 8;
		break;
	/* first 2 of last 6 bursts */
	case SCHED_MEAS_AVG_M6_FIRST_TWO:
		n = 2; shift = 6;
		break;
	/* middle 2 of last 6 bursts */
	case SCHED_MEAS_AVG_M6_MIDDLE_TWO:
		n = 2; shift = 4;
		break;
	default:
		/* Shall not happen */
		OSMO_ASSERT(false);
	}

	/* Calculate the sum of n entries starting from pos */
	for (i = 0; i < n; i++) {
		pos = (current + hist_size - shift + i) % hist_size;
		set = &chan_state->meas.buf[pos];

		rssi_sum   += set->rssi;
		toa256_sum += set->toa256;
		ci_cb_sum  += set->ci_cb;
	}

	/* Calculate the average for each value */
	*avg = (struct l1sched_meas_set) {
		.rssi   = (rssi_sum   / n),
		.toa256 = (toa256_sum / n),
		.ci_cb  = (ci_cb_sum  / n),
	};

	LOGP(DMEAS, LOGL_DEBUG, "Measurement AVG (num=%u, shift=%u): "
	     "RSSI %f, ToA256 %d, C/I %d cB\n", n, shift,
	     avg->rssi, avg->toa256, avg->ci_cb);
}