// SPDX-License-Identifier: Apache-2.0
//
// The lwIP network interface backed by the e1000 driver.
//
// Two directions and almost nothing else: a linkoutput that flattens a pbuf
// chain into a frame and hands it to the driver, and a poll that takes frames
// off the driver and hands them up. Everything about IP, ARP and TCP lives in
// lwIP; everything about descriptors and registers lives in the driver; this
// file is the seam.

#include "types.h"
#include "defs.h"
#include "param.h"

#include "lwip/opt.h"
#include "lwip/netif.h"
#include "lwip/pbuf.h"
#include "lwip/etharp.h"
#include "lwip/ethip6.h"
#include "netif/ethernet.h"

#define ETH_MTU     1500
#define ETH_FRAMESZ 1518        // 1500 payload + 14 header + 4 FCS allowance

static struct netif e1000_netif;

// One staging buffer, not one per call: a frame is 1518 bytes and the kernel
// stack is 4096 in total, so a local of this size on a path that is several
// frames deep is exactly the overflow the kstack canary exists to catch.
// Safe as a single static because every caller holds the lwIP lock.
static char txframe[ETH_FRAMESZ];
static char rxframe[ETH_FRAMESZ];

// Send one packet. lwIP hands over a pbuf chain; the driver wants one flat
// buffer, so this walks the chain into txframe.
static err_t
e1000_linkoutput(struct netif *netif, struct pbuf *p)
{
    (void)netif;

    if (p->tot_len > sizeof(txframe))
        return ERR_IF;

    u16_t off = 0;
    for (struct pbuf *q = p; q != NULL; q = q->next) {
        if (off + q->len > sizeof(txframe))
            return ERR_IF;
        memmove(txframe + off, q->payload, q->len);
        off += q->len;
    }

    // A full transmit ring is congestion, not failure. Reporting ERR_IF would
    // make lwIP drop the segment and wait for a retransmission timer; ERR_WOULDBLOCK
    // lets it try again, which for TCP is the difference between a stall of
    // milliseconds and one of seconds.
    if (e1000_transmit(txframe, off) != 0)
        return ERR_WOULDBLOCK;

    return ERR_OK;
}

// Bring the interface up. Called once, by the network thread.
static err_t
e1000_netif_init(struct netif *netif)
{
    const unsigned char *mac = e1000_mac();

    netif->name[0] = 'e';
    netif->name[1] = 'n';
    netif->output     = etharp_output;   // IP -> ARP -> linkoutput
    netif->linkoutput = e1000_linkoutput;
    netif->mtu        = ETH_MTU;
    netif->hwaddr_len = 6;
    for (int i = 0; i < 6; i++)
        netif->hwaddr[i] = mac[i];
    netif->flags = NETIF_FLAG_BROADCAST | NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;
    return ERR_OK;
}

struct netif *
net_e1000_netif(void)
{
    return &e1000_netif;
}

// Attach the interface with no address yet -- DHCP fills it in.
int
net_e1000_attach(void)
{
    ip4_addr_t any;
    IP4_ADDR(&any, 0, 0, 0, 0);

    if (netif_add(&e1000_netif, &any, &any, &any, NULL,
                  e1000_netif_init, netif_input) == NULL)
        return -1;

    netif_set_default(&e1000_netif);
    netif_set_up(&e1000_netif);
    netif_set_link_up(&e1000_netif);
    return 0;
}

// Take whatever the driver has and push it into lwIP. Returns how many frames
// moved, so the caller can tell a busy poll from an idle one.
//
// Bounded rather than draining: a receive loop with no limit starves everything
// else on a busy network, and the caller comes straight back anyway.
int
net_e1000_poll(void)
{
    int moved = 0;

    for (int i = 0; i < 32; i++) {
        int len = e1000_receive(rxframe, sizeof(rxframe));
        if (len <= 0)
            break;

        // PBUF_POOL, so this comes from lwIP's own pool rather than a heap it
        // does not have. A null return means the pool is exhausted, which is a
        // dropped packet -- entirely normal under load, and what the protocol
        // above is designed to survive.
        struct pbuf *p = pbuf_alloc(PBUF_RAW, (u16_t)len, PBUF_POOL);
        if (p == NULL)
            break;

        pbuf_take(p, rxframe, (u16_t)len);

        // netif->input is netif_input, set at netif_add: it dispatches on the
        // Ethernet type. Ownership of the pbuf passes with the call, including
        // when it fails, so there is nothing to free here.
        if (e1000_netif.input(p, &e1000_netif) != ERR_OK)
            pbuf_free(p);

        moved++;
    }
    return moved;
}