/*
 * Copyright (c) 2016 Intel Corporation.
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * All references to the spec refer to IEEE 802.15.4-2020.
 */

#include <zephyr/logging/log.h>
LOG_MODULE_REGISTER(net_ieee802154_csma, CONFIG_NET_L2_IEEE802154_LOG_LEVEL);

#include <zephyr/net/ieee802154.h>
#include <zephyr/net/ieee802154_radio.h>
#include <zephyr/net/net_if.h>
#include <zephyr/random/random.h>
#include <zephyr/toolchain.h>

#include <errno.h>
#include <stdlib.h>

#include "ieee802154_priv.h"
#include "ieee802154_utils.h"

BUILD_ASSERT(CONFIG_NET_L2_IEEE802154_RADIO_CSMA_CA_MIN_BE <=
		     CONFIG_NET_L2_IEEE802154_RADIO_CSMA_CA_MAX_BE,
	     "The CSMA/CA min backoff exponent must be less or equal max backoff exponent.");

/* See section 6.2.5.1. */
static inline int unslotted_csma_ca_channel_access(struct net_if *iface)
{
	struct ieee802154_context *ctx = net_if_l2_data(iface);
	uint8_t be = CONFIG_NET_L2_IEEE802154_RADIO_CSMA_CA_MIN_BE;
	uint32_t turnaround_time, unit_backoff_period_us;

	turnaround_time = ieee802154_radio_get_a_turnaround_time(iface);
	unit_backoff_period_us = ieee802154_radio_get_multiple_of_symbol_period(
					 iface, ctx->channel,
					 IEEE802154_MAC_A_UNIT_BACKOFF_PERIOD(turnaround_time)) /
				 NSEC_PER_USEC;

	for (uint8_t nb = 0U; nb <= CONFIG_NET_L2_IEEE802154_RADIO_CSMA_CA_MAX_BO; nb++) {
		int ret;

		if (be) {
			uint8_t bo_n = sys_rand32_get() & ((1 << be) - 1);

			/* TODO: k_busy_wait() is too inaccurate on many platforms, the
			 * radio API should expose a precise radio clock instead (which may
			 * fall back to k_busy_wait() if the radio does not have a clock).
			 */
			k_busy_wait(bo_n * unit_backoff_period_us);
		}

		ret = ieee802154_radio_cca(iface);
		if (ret == 0) {
			/* Channel is idle -> CSMA Success */
			return 0;
		} else if (ret != -EBUSY) {
			/* CCA exited with failure code -> CSMA Abort */
			return -EIO;
		}

		/* Channel is busy -> CSMA Backoff */
		be = MIN(be + 1, CONFIG_NET_L2_IEEE802154_RADIO_CSMA_CA_MAX_BE);
	}

	/* Channel is still busy after max backoffs -> CSMA Failure */
	return -EBUSY;
}

/* Declare the public channel access algorithm function used by L2. */
FUNC_ALIAS(unslotted_csma_ca_channel_access, ieee802154_wait_for_clear_channel, int);