/*
 * Plugable TCP congestion control support and newReno
 * congestion control.
 * Based on ideas from I/O scheduler suport and Web100.
 *
 * Copyright (C) 2005 Stephen Hemminger <shemminger@osdl.org>
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <linux/module.h>
#include <linux/mm.h>
#include <linux/types.h>
#include <linux/list.h>
#include <linux/gfp.h>
#include <net/tcp.h>

int sysctl_tcp_max_ssthresh = 0;

static DEFINE_SPINLOCK(tcp_cong_list_lock);
static LIST_HEAD(tcp_cong_list);

/* Simple linear search, don't expect many entries! */
static struct tcp_congestion_ops *tcp_ca_find(const char *name)
{
        struct tcp_congestion_ops *e;

        list_for_each_entry_rcu(e, &tcp_cong_list, list) {
                if (strcmp(e->name, name) == 0)
                        return e;
        }

        return NULL;
}

/*
 * Attach new congestion control algorithm to the list
 * of available options.
 */
int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
{
        int ret = 0;

        /* all algorithms must implement ssthresh and cong_avoid ops */
        if (!ca->ssthresh || !ca->cong_avoid) {
                pr_err("%s does not implement required ops\n", ca->name);
                return -EINVAL;
        }

        spin_lock(&tcp_cong_list_lock);
        if (tcp_ca_find(ca->name)) {
                pr_notice("%s already registered\n", ca->name);
                ret = -EEXIST;
        } else {
                list_add_tail_rcu(&ca->list, &tcp_cong_list);
                pr_info("%s registered\n", ca->name);
        }
        spin_unlock(&tcp_cong_list_lock);

        return ret;
}
EXPORT_SYMBOL_GPL(tcp_register_congestion_control);

/*
 * Remove congestion control algorithm, called from
 * the module's remove function.  Module ref counts are used
 * to ensure that this can't be done till all sockets using
 * that method are closed.
 */
void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
{
        spin_lock(&tcp_cong_list_lock);
        list_del_rcu(&ca->list);
        spin_unlock(&tcp_cong_list_lock);
}
EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);

/* Assign choice of congestion control. */
void tcp_init_congestion_control(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_congestion_ops *ca;

        /* if no choice made yet assign the current value set as default */
        if (icsk->icsk_ca_ops == &tcp_init_congestion_ops) {
                rcu_read_lock();
                list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
                        if (try_module_get(ca->owner)) {
                                icsk->icsk_ca_ops = ca;
                                break;
                        }

                        /* fallback to next available */
                }
                rcu_read_unlock();
        }

        if (icsk->icsk_ca_ops->init)
                icsk->icsk_ca_ops->init(sk);
}

/* Manage refcounts on socket close. */
void tcp_cleanup_congestion_control(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        if (icsk->icsk_ca_ops->release)
                icsk->icsk_ca_ops->release(sk);
        module_put(icsk->icsk_ca_ops->owner);
}

/* Used by sysctl to change default congestion control */
int tcp_set_default_congestion_control(const char *name)
{
        struct tcp_congestion_ops *ca;
        int ret = -ENOENT;

        spin_lock(&tcp_cong_list_lock);
        ca = tcp_ca_find(name);
#ifdef CONFIG_MODULES
        if (!ca && capable(CAP_NET_ADMIN)) {
                spin_unlock(&tcp_cong_list_lock);

                request_module("tcp_%s", name);
                spin_lock(&tcp_cong_list_lock);
                ca = tcp_ca_find(name);
        }
#endif

        if (ca) {
                ca->flags |= TCP_CONG_NON_RESTRICTED;   /* default is always allowed */
                list_move(&ca->list, &tcp_cong_list);
                ret = 0;
        }
        spin_unlock(&tcp_cong_list_lock);

        return ret;
}

/* Set default value from kernel configuration at bootup */
static int __init tcp_congestion_default(void)
{
        return tcp_set_default_congestion_control(CONFIG_DEFAULT_TCP_CONG);
}
late_initcall(tcp_congestion_default);


/* Build string with list of available congestion control values */
void tcp_get_available_congestion_control(char *buf, size_t maxlen)
{
        struct tcp_congestion_ops *ca;
        size_t offs = 0;

        rcu_read_lock();
        list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
                offs += snprintf(buf + offs, maxlen - offs,
                                 "%s%s",
                                 offs == 0 ? "" : " ", ca->name);

        }
        rcu_read_unlock();
}

/* Get current default congestion control */
void tcp_get_default_congestion_control(char *name)
{
        struct tcp_congestion_ops *ca;
        /* We will always have reno... */
        BUG_ON(list_empty(&tcp_cong_list));

        rcu_read_lock();
        ca = list_entry(tcp_cong_list.next, struct tcp_congestion_ops, list);
        strncpy(name, ca->name, TCP_CA_NAME_MAX);
        rcu_read_unlock();
}

/* Built list of non-restricted congestion control values */
void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
{
        struct tcp_congestion_ops *ca;
        size_t offs = 0;

        *buf = '\0';
        rcu_read_lock();
        list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
                if (!(ca->flags & TCP_CONG_NON_RESTRICTED))
                        continue;
                offs += snprintf(buf + offs, maxlen - offs,
                                 "%s%s",
                                 offs == 0 ? "" : " ", ca->name);

        }
        rcu_read_unlock();
}

/* Change list of non-restricted congestion control */
int tcp_set_allowed_congestion_control(char *val)
{
        struct tcp_congestion_ops *ca;
        char *saved_clone, *clone, *name;
        int ret = 0;

        saved_clone = clone = kstrdup(val, GFP_USER);
        if (!clone)
                return -ENOMEM;

        spin_lock(&tcp_cong_list_lock);
        /* pass 1 check for bad entries */
        while ((name = strsep(&clone, " ")) && *name) {
                ca = tcp_ca_find(name);
                if (!ca) {
                        ret = -ENOENT;
                        goto out;
                }
        }

        /* pass 2 clear old values */
        list_for_each_entry_rcu(ca, &tcp_cong_list, list)
                ca->flags &= ~TCP_CONG_NON_RESTRICTED;

        /* pass 3 mark as allowed */
        while ((name = strsep(&val, " ")) && *name) {
                ca = tcp_ca_find(name);
                WARN_ON(!ca);
                if (ca)
                        ca->flags |= TCP_CONG_NON_RESTRICTED;
        }
out:
        spin_unlock(&tcp_cong_list_lock);
        kfree(saved_clone);

        return ret;
}


/* Change congestion control for socket */
int tcp_set_congestion_control(struct sock *sk, const char *name)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_congestion_ops *ca;
        int err = 0;

        rcu_read_lock();
        ca = tcp_ca_find(name);

        /* no change asking for existing value */
        if (ca == icsk->icsk_ca_ops)
                goto out;

#ifdef CONFIG_MODULES
        /* not found attempt to autoload module */
        if (!ca && capable(CAP_NET_ADMIN)) {
                rcu_read_unlock();
                request_module("tcp_%s", name);
                rcu_read_lock();
                ca = tcp_ca_find(name);
        }
#endif
        if (!ca)
                err = -ENOENT;

        else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) || capable(CAP_NET_ADMIN)))
                err = -EPERM;

        else if (!try_module_get(ca->owner))
                err = -EBUSY;

        else {
                tcp_cleanup_congestion_control(sk);
                icsk->icsk_ca_ops = ca;

                if (sk->sk_state != TCP_CLOSE && icsk->icsk_ca_ops->init)
                        icsk->icsk_ca_ops->init(sk);
        }
 out:
        rcu_read_unlock();
        return err;
}

/* RFC2861 Check whether we are limited by application or congestion window
 * This is the inverse of cwnd check in tcp_tso_should_defer
 */
int tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        u32 left;

        if (in_flight >= tp->snd_cwnd)
                return 1;

        left = tp->snd_cwnd - in_flight;
        if (sk_can_gso(sk) &&
            left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
            left * tp->mss_cache < sk->sk_gso_max_size)
                return 1;
        return left <= tcp_max_tso_deferred_mss(tp);
}
EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);

/*
 * Slow start is used when congestion window is less than slow start
 * threshold. This version implements the basic RFC2581 version
 * and optionally supports:
 *      RFC3742 Limited Slow Start        - growth limited to max_ssthresh
 *      RFC3465 Appropriate Byte Counting - growth limited by bytes acknowledged
 */
void tcp_slow_start(struct tcp_sock *tp)
{
        int cnt; /* increase in packets */

        /* RFC3465: ABC Slow start
         * Increase only after a full MSS of bytes is acked
         *
         * TCP sender SHOULD increase cwnd by the number of
         * previously unacknowledged bytes ACKed by each incoming
         * acknowledgment, provided the increase is not more than L
         */
        if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
                return;

        if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
                cnt = sysctl_tcp_max_ssthresh >> 1;     /* limited slow start */
        else
                cnt = tp->snd_cwnd;                     /* exponential increase */

        /* RFC3465: ABC
         * We MAY increase by 2 if discovered delayed ack
         */
        if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
                cnt <<= 1;
        tp->bytes_acked = 0;

        tp->snd_cwnd_cnt += cnt;
        while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
                tp->snd_cwnd_cnt -= tp->snd_cwnd;
                if (tp->snd_cwnd < tp->snd_cwnd_clamp)
                        tp->snd_cwnd++;
        }
}
EXPORT_SYMBOL_GPL(tcp_slow_start);

/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */
void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w)
{
        if (tp->snd_cwnd_cnt >= w) {
                if (tp->snd_cwnd < tp->snd_cwnd_clamp)
                        tp->snd_cwnd++;
                tp->snd_cwnd_cnt = 0;
        } else {
                tp->snd_cwnd_cnt++;
        }
}
EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

/*
 * TCP Reno congestion control
 * This is special case used for fallback as well.
 */
/* This is Jacobson's slow start and congestion avoidance.
 * SIGCOMM '88, p. 328.
 */
void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tcp_is_cwnd_limited(sk, in_flight))
                return;

        /* In "safe" area, increase. */
        if (tp->snd_cwnd <= tp->snd_ssthresh)
                tcp_slow_start(tp);

        /* In dangerous area, increase slowly. */
        else if (sysctl_tcp_abc) {
                /* RFC3465: Appropriate Byte Count
                 * increase once for each full cwnd acked
                 */
                if (tp->bytes_acked >= tp->snd_cwnd*tp->mss_cache) {
                        tp->bytes_acked -= tp->snd_cwnd*tp->mss_cache;
                        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
                                tp->snd_cwnd++;
                }
        } else {
                tcp_cong_avoid_ai(tp, tp->snd_cwnd);
        }
}
EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);

/* Slow start threshold is half the congestion window (min 2) */
u32 tcp_reno_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        return max(tp->snd_cwnd >> 1U, 2U);
}
EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);

/* Lower bound on congestion window with halving. */
u32 tcp_reno_min_cwnd(const struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        return tp->snd_ssthresh/2;
}
EXPORT_SYMBOL_GPL(tcp_reno_min_cwnd);

struct tcp_congestion_ops tcp_reno = {
        .flags          = TCP_CONG_NON_RESTRICTED,
        .name           = "reno",
        .owner          = THIS_MODULE,
        .ssthresh       = tcp_reno_ssthresh,
        .cong_avoid     = tcp_reno_cong_avoid,
        .min_cwnd       = tcp_reno_min_cwnd,
};

/* Initial congestion control used (until SYN)
 * really reno under another name so we can tell difference
 * during tcp_set_default_congestion_control
 */
struct tcp_congestion_ops tcp_init_congestion_ops  = {
        .name           = "",
        .owner          = THIS_MODULE,
        .ssthresh       = tcp_reno_ssthresh,
        .cong_avoid     = tcp_reno_cong_avoid,
        .min_cwnd       = tcp_reno_min_cwnd,
};
EXPORT_SYMBOL_GPL(tcp_init_congestion_ops);