// SPDX-License-Identifier: Apache-2.0
//
// /dev/netdev -- a network interface whose backend is a process.
// See include/netuser.h.
//
// The shape is `/dev/pci`'s, deliberately, and for the same reasons. One
// device, one owner, claimed and released, and the kernel takes it back when
// the process dies -- because the process it is handed to is by assumption one
// that may crash, which is the entire argument for it being a process.
//
// What is different is the direction. `/dev/pci` hands a device *to* a
// process; this lets a process *be* a device. The kernel is the consumer here
// and the process is the driver, so the queues run the other way and the
// vtable is ours rather than theirs.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "file.h"
#include "errno.h"
#include "memlayout.h"
#include "mmu.h"
#include "proc.h"
#include "uaccess.h"
#include "device.h"
#include "netdev.h"
#include "sbuf.h"
#include "dbg.h"
#include "netuser.h"

struct frame {
  int   len;
  uint8 data[NETUSER_MTU];
};

// Two rings. `tx` is what the stack has handed down and the process has not
// yet read; `rx` is what the process has written and the stack has not yet
// polled. Both are single-producer/single-consumer in practice -- one process
// on one side, the network thread on the other -- but the lock is taken
// anyway, because "in practice" is not a property the scheduler guarantees.
static struct {
  struct spinlock lock;
  int             inited;

  struct proc    *owner;
  struct device  *dev;
  uint8           mac[6];

  struct frame    tx[NETUSER_QLEN];
  uint            tx_head, tx_tail;      // monotonic, indexed mod QLEN

  struct frame    rx[NETUSER_QLEN];
  uint            rx_head, rx_tail;

  struct netuser_stat st;
} nu;

static void
netuser_once(void)
{
  if (!nu.inited) {
    initlock(&nu.lock, "netuser");
    nu.inited = 1;
  }
}

// --- the vtable the kernel sees ---------------------------------------------

// Queue a frame for the process to read.
//
// Returns -1 when the ring is full, which netdev.h defines as congestion
// rather than failure -- the caller is expected to come back. That matters
// here more than it does for real hardware: the "wire" is a process that may
// not have been scheduled yet, and treating a full ring as an error would drop
// packets every time the machine was busy.
static int
netuser_transmit(struct device *d, const void *frame, int len)
{
  int rv = 0;

  (void)d;
  if (len <= 0 || len > NETUSER_MTU)
    return -1;

  acquire(&nu.lock);
  if (!nu.owner) {
    release(&nu.lock);
    return -1;
  }
  if (nu.tx_head - nu.tx_tail >= NETUSER_QLEN) {
    nu.st.tx_dropped++;
    rv = -1;
  } else {
    struct frame *f = &nu.tx[nu.tx_head % NETUSER_QLEN];
    memmove(f->data, frame, len);
    f->len = len;
    nu.tx_head++;
    nu.st.tx_frames++;
    nu.st.tx_bytes += len;
  }
  release(&nu.lock);

  if (rv == 0)
    wakeup(&nu.tx_head);
  return rv;
}

// Take a frame the process wrote, if there is one. Never blocks: net_eth_poll
// calls this in a loop and decides for itself when to stop.
static int
netuser_receive(struct device *d, void *out, int cap)
{
  int len = 0;

  (void)d;
  acquire(&nu.lock);
  if (nu.rx_head != nu.rx_tail) {
    struct frame *f = &nu.rx[nu.rx_tail % NETUSER_QLEN];
    len = f->len;
    if (len > cap)
      len = cap;
    memmove(out, f->data, len);
    nu.rx_tail++;
  }
  release(&nu.lock);
  return len;
}

static const unsigned char *
netuser_mac(struct device *d)
{
  (void)d;
  return nu.mac;
}

// Up exactly while somebody is holding the descriptor.
//
// This is the honest answer and it is also the useful one: a link that stays
// up after its driver died is a link that swallows every packet sent to it.
static int
netuser_link_up(struct device *d)
{
  (void)d;
  return nu.owner != 0;
}

static const struct netdev_ops netuser_ops = {
  .transmit = netuser_transmit,
  .receive  = netuser_receive,
  .mac      = netuser_mac,
  .link_up  = netuser_link_up,
};

// --- claiming and releasing --------------------------------------------------

static void
netuser_drop(void)
{
  // The device stays registered. Removing it would mean unpicking it from
  // lwIP's interface list, which nothing in that stack supports -- and leaving
  // it with link_up() answering false is the same thing from the stack's point
  // of view, without inventing a teardown path nobody else has.
  nu.owner   = 0;
  nu.tx_head = nu.tx_tail = 0;
  nu.rx_head = nu.rx_tail = 0;
  nu.st.attached = 0;
}

static int
do_create(uint64 argp)
{
  struct netuser_create c;
  struct device *d;
  int first = 0;

  if (copy_from_user(&c, argp, sizeof(c)) < 0)
    return -EFAULT;

  netuser_once();
  acquire(&nu.lock);
  if (nu.owner) {
    release(&nu.lock);
    return -EBUSY;               // one interface, one process
  }
  nu.owner = myproc();
  for (int i = 0; i < 6; i++)
    nu.mac[i] = c.mac[i];
  nu.tx_head = nu.tx_tail = 0;
  nu.rx_head = nu.rx_tail = 0;
  nu.st.attached = 1;
  d = nu.dev;
  release(&nu.lock);

  // Registered once and reused. A process that comes back after crashing gets
  // the same interface rather than a second one, which is what makes restart
  // invisible to anything above -- the same property H2 built for the GPU.
  if (!d) {
    c.name[sizeof(c.name) - 1] = 0;
    d = device_add(c.name[0] ? c.name : "eth", DEVBUS_PLATFORM, DEVCLASS_NET,
                   "netuser");
    if (!d) {
      acquire(&nu.lock);
      netuser_drop();
      release(&nu.lock);
      return -ENOSPC;
    }
    device_set_desc(d, "a process, acting as a network interface");
    d->ops  = (void *)&netuser_ops;
    d->priv = 0;
    nu.dev  = d;
    first   = 1;
  }

  if (first) {
    // Hand it to the stack. net_attach_late() adds it to lwIP's interface list
    // *and* starts DHCP on it, which netinit()'s boot-time walk cannot do for
    // something that did not exist yet -- an interface that only appears once
    // a program has started would otherwise come up with no address and no way
    // to ever get one.
    if (net_attach_late(d) < 0) {
      cprintf("netuser: the network stack would not take the interface\n");
      acquire(&nu.lock);
      netuser_drop();
      release(&nu.lock);
      return -ENOSPC;
    }
    cprintf("netuser: %s is a process now, %x:%x:%x:%x:%x:%x\n", d->name,
            nu.mac[0], nu.mac[1], nu.mac[2], nu.mac[3], nu.mac[4], nu.mac[5]);
  }
  return 0;
}

static int
do_release(void)
{
  netuser_once();
  acquire(&nu.lock);
  if (nu.owner == myproc())
    netuser_drop();
  release(&nu.lock);
  wakeup(&nu.tx_head);
  return 0;
}

// Called from exit(), beside pciuser_release_all(), and for the same reason: a
// driver that crashed must not leave an interface that claims to be up.
void
netuser_release_all(struct proc *p)
{
  netuser_once();
  acquire(&nu.lock);
  if (nu.owner == p)
    netuser_drop();
  release(&nu.lock);
  wakeup(&nu.tx_head);
}

int
netuser_ioctl(uint64 request, uint64 argp)
{
  switch (request) {
  case NETUSERIOC_CREATE:  return do_create(argp);
  case NETUSERIOC_RELEASE: return do_release();
  case NETUSERIOC_STAT: {
    struct netuser_stat s;
    netuser_once();
    acquire(&nu.lock);
    s = nu.st;
    release(&nu.lock);
    if (copy_to_user(argp, &s, sizeof(s)) < 0)
      return -EFAULT;
    return 0;
  }
  default:
    return -ENOTTY;
  }
}

// --- the descriptor ----------------------------------------------------------

// A frame the stack wants sent. Blocks until there is one.
int
netuser_read(struct inode *ip, uint64 off, char *dst, int n)
{
  struct frame *f;
  int len;

  (void)off;
  netuser_once();

  if (nu.owner != myproc())
    return -EACCES;

  iunlock(ip);
  acquire(&nu.lock);
  while (nu.tx_head == nu.tx_tail) {
    if (myproc()->killed || nu.owner != myproc()) {
      release(&nu.lock);
      ilock(ip);
      return -1;
    }
    sleep(&nu.tx_head, &nu.lock);
  }
  f   = &nu.tx[nu.tx_tail % NETUSER_QLEN];
  len = f->len;
  if (len > n)
    len = n;
  memmove(dst, f->data, len);
  nu.tx_tail++;
  release(&nu.lock);
  ilock(ip);
  return len;
}

// A frame that arrived. Handed to the stack at its next poll.
int
netuser_write(struct inode *ip, uint64 off, char *src, int n)
{
  (void)ip; (void)off;
  netuser_once();

  if (nu.owner != myproc())
    return -EACCES;
  if (n <= 0 || n > NETUSER_MTU)
    return -EINVAL;

  acquire(&nu.lock);
  if (nu.rx_head - nu.rx_tail >= NETUSER_QLEN) {
    // Dropped, and counted. The alternative is blocking the driver until the
    // stack catches up, which on a receive path means the driver stops reading
    // its hardware and the hardware drops it instead -- the same packet lost,
    // further from anything that can say so.
    nu.st.rx_dropped++;
    release(&nu.lock);
    return n;
  }
  {
    struct frame *f = &nu.rx[nu.rx_head % NETUSER_QLEN];
    memmove(f->data, src, n);
    f->len = n;
    nu.rx_head++;
    nu.st.rx_frames++;
    nu.st.rx_bytes += n;
  }
  release(&nu.lock);
  return n;
}

// Whether a read would return without blocking, so O_NONBLOCK and poll() work
// without the driver growing a flags argument. See struct devsw.
int
netuser_ready(struct inode *ip)
{
  int r;
  (void)ip;
  netuser_once();
  acquire(&nu.lock);
  r = (nu.tx_head != nu.tx_tail);
  release(&nu.lock);
  return r;
}

// --- what the debugger can be told -------------------------------------------

static void
netuser_dump(struct sbuf *out, const char *arg)
{
  (void)arg;
  netuser_once();

  acquire(&nu.lock);
  if (!nu.dev) {
    sb_puts(out, "  (no process has ever claimed /dev/netdev)\n");
    release(&nu.lock);
    return;
  }
  sb_puts(out, "  interface ");
  sb_puts(out, nu.dev->name);
  sb_puts(out, nu.owner ? "  up, held by pid " : "  DOWN, nobody holds it");
  if (nu.owner)
    sb_putu(out, nu.owner->pid);
  sb_putc(out, '\n');

  sb_puts(out, "  mac       ");
  for (int i = 0; i < 6; i++) {
    sb_puthex(out, nu.mac[i], 2);
    if (i < 5) sb_putc(out, ':');
  }
  sb_putc(out, '\n');

  sb_puts(out, "\n  tx  "); sb_putu(out, nu.st.tx_frames);
  sb_puts(out, " frames, ");  sb_putu(out, nu.st.tx_bytes);
  sb_puts(out, " bytes, ");   sb_putu(out, nu.st.tx_dropped);
  sb_puts(out, " dropped, "); sb_putu(out, nu.tx_head - nu.tx_tail);
  sb_puts(out, " waiting to be read\n");

  sb_puts(out, "  rx  "); sb_putu(out, nu.st.rx_frames);
  sb_puts(out, " frames, ");  sb_putu(out, nu.st.rx_bytes);
  sb_puts(out, " bytes, ");   sb_putu(out, nu.st.rx_dropped);
  sb_puts(out, " dropped, "); sb_putu(out, nu.rx_head - nu.rx_tail);
  sb_puts(out, " waiting for the stack\n");
  release(&nu.lock);
}

void
netuser_init(void)
{
  netuser_once();
  dbg_register("netuser", netuser_dump,
               "the interface backed by a process, and what has moved");
}