// SPDX-License-Identifier: GPL-2.0-only
/*
 * TCP HYBLA
 *
 * TCP-HYBLA Congestion control algorithm, based on:
 *   C.Caini, R.Firrincieli, "TCP-Hybla: A TCP Enhancement
 *   for Heterogeneous Networks",
 *   International Journal on satellite Communications,
 *				       September 2004
 *    Daniele Lacamera
 *    root at danielinux.net
 */

#include <linux/module.h>
#include <net/tcp.h>

/* Tcp Hybla structure. */
struct hybla {
	bool  hybla_en;
	u32   snd_cwnd_cents; /* Keeps increment values when it is <1, <<7 */
	u32   rho;	      /* Rho parameter, integer part  */
	u32   rho2;	      /* Rho * Rho, integer part */
	u32   rho_3ls;	      /* Rho parameter, <<3 */
	u32   rho2_7ls;	      /* Rho^2, <<7	*/
	u32   minrtt_us;      /* Minimum smoothed round trip time value seen */
};

/* Hybla reference round trip time (default= 1/40 sec = 25 ms), in ms */
static int rtt0 = 25;
module_param(rtt0, int, 0644);
MODULE_PARM_DESC(rtt0, "reference rout trip time (ms)");

/* This is called to refresh values for hybla parameters */
static inline void hybla_recalc_param (struct sock *sk)
{
	struct hybla *ca = inet_csk_ca(sk);

	ca->rho_3ls = max_t(u32,
			    tcp_sk(sk)->srtt_us / (rtt0 * USEC_PER_MSEC),
			    8U);
	ca->rho = ca->rho_3ls >> 3;
	ca->rho2_7ls = (ca->rho_3ls * ca->rho_3ls) << 1;
	ca->rho2 = ca->rho2_7ls >> 7;
}

static void hybla_init(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct hybla *ca = inet_csk_ca(sk);

	ca->rho = 0;
	ca->rho2 = 0;
	ca->rho_3ls = 0;
	ca->rho2_7ls = 0;
	ca->snd_cwnd_cents = 0;
	ca->hybla_en = true;
	tcp_snd_cwnd_set(tp, 2);
	tp->snd_cwnd_clamp = 65535;

	/* 1st Rho measurement based on initial srtt */
	hybla_recalc_param(sk);

	/* set minimum rtt as this is the 1st ever seen */
	ca->minrtt_us = tp->srtt_us;
	tcp_snd_cwnd_set(tp, ca->rho);
}

static void hybla_state(struct sock *sk, u8 ca_state)
{
	struct hybla *ca = inet_csk_ca(sk);

	ca->hybla_en = (ca_state == TCP_CA_Open);
}

static inline u32 hybla_fraction(u32 odds)
{
	static const u32 fractions[] = {
		128, 139, 152, 165, 181, 197, 215, 234,
	};

	return (odds < ARRAY_SIZE(fractions)) ? fractions[odds] : 128;
}

/* TCP Hybla main routine.
 * This is the algorithm behavior:
 *     o Recalc Hybla parameters if min_rtt has changed
 *     o Give cwnd a new value based on the model proposed
 *     o remember increments <1
 */
static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct hybla *ca = inet_csk_ca(sk);
	u32 increment, odd, rho_fractions;
	int is_slowstart = 0;

	/*  Recalculate rho only if this srtt is the lowest */
	if (tp->srtt_us < ca->minrtt_us) {
		hybla_recalc_param(sk);
		ca->minrtt_us = tp->srtt_us;
	}

	if (!tcp_is_cwnd_limited(sk))
		return;

	if (!ca->hybla_en) {
		tcp_reno_cong_avoid(sk, ack, acked);
		return;
	}

	if (ca->rho == 0)
		hybla_recalc_param(sk);

	rho_fractions = ca->rho_3ls - (ca->rho << 3);

	if (tcp_in_slow_start(tp)) {
		/*
		 * slow start
		 *      INC = 2^RHO - 1
		 * This is done by splitting the rho parameter
		 * into 2 parts: an integer part and a fraction part.
		 * Inrement<<7 is estimated by doing:
		 *	       [2^(int+fract)]<<7
		 * that is equal to:
		 *	       (2^int)	*  [(2^fract) <<7]
		 * 2^int is straightly computed as 1<<int,
		 * while we will use hybla_slowstart_fraction_increment() to
		 * calculate 2^fract in a <<7 value.
		 */
		is_slowstart = 1;
		increment = ((1 << min(ca->rho, 16U)) *
			hybla_fraction(rho_fractions)) - 128;
	} else {
		/*
		 * congestion avoidance
		 * INC = RHO^2 / W
		 * as long as increment is estimated as (rho<<7)/window
		 * it already is <<7 and we can easily count its fractions.
		 */
		increment = ca->rho2_7ls / tcp_snd_cwnd(tp);
		if (increment < 128)
			tp->snd_cwnd_cnt++;
	}

	odd = increment % 128;
	tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + (increment >> 7));
	ca->snd_cwnd_cents += odd;

	/* check when fractions goes >=128 and increase cwnd by 1. */
	while (ca->snd_cwnd_cents >= 128) {
		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
		ca->snd_cwnd_cents -= 128;
		tp->snd_cwnd_cnt = 0;
	}
	/* check when cwnd has not been incremented for a while */
	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tcp_snd_cwnd(tp)) {
		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
		tp->snd_cwnd_cnt = 0;
	}
	/* clamp down slowstart cwnd to ssthresh value. */
	if (is_slowstart)
		tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), tp->snd_ssthresh));

	tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), tp->snd_cwnd_clamp));
}

static struct tcp_congestion_ops tcp_hybla __read_mostly = {
	.init		= hybla_init,
	.ssthresh	= tcp_reno_ssthresh,
	.undo_cwnd	= tcp_reno_undo_cwnd,
	.cong_avoid	= hybla_cong_avoid,
	.set_state	= hybla_state,

	.owner		= THIS_MODULE,
	.name		= "hybla"
};

static int __init hybla_register(void)
{
	BUILD_BUG_ON(sizeof(struct hybla) > ICSK_CA_PRIV_SIZE);
	return tcp_register_congestion_control(&tcp_hybla);
}

static void __exit hybla_unregister(void)
{
	tcp_unregister_congestion_control(&tcp_hybla);
}

module_init(hybla_register);
module_exit(hybla_unregister);

MODULE_AUTHOR("Daniele Lacamera");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP Hybla");