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/* 3c507.c: An EtherLink16 device driver for Linux. */

/*

        Written 1993,1994 by Donald Becker.

        Copyright 1993 United States Government as represented by the

        Director, National Security Agency.

        This software may be used and distributed according to the terms

        of the GNU General Public License, incorporated herein by reference.

        The author may be reached as becker@scyld.com, or C/O

        Scyld Computing Corporation

        410 Severn Ave., Suite 210

        Annapolis MD 21403

        Thanks go to jennings@Montrouge.SMR.slb.com ( Patrick Jennings)

        and jrs@world.std.com (Rick Sladkey) for testing and bugfixes.

        Mark Salazar <leslie@access.digex.net> made the changes for cards with

        only 16K packet buffers.

        Things remaining to do:

        Verify that the tx and rx buffers don't have fencepost errors.

        Move the theory of operation and memory map documentation.

        The statistics need to be updated correctly.

*/

#define DRV_NAME                "3c507"

#define DRV_VERSION             "1.10a"

#define DRV_RELDATE             "11/17/2001"

static const char version[] =

        DRV_NAME ".c:v" DRV_VERSION " " DRV_RELDATE " Donald Becker (becker@scyld.com)\n";

/*

  Sources:

        This driver wouldn't have been written with the availability of the

        Crynwr driver source code.      It provided a known-working implementation

        that filled in the gaping holes of the Intel documentation.  Three cheers

        for Russ Nelson.

        Intel Microcommunications Databook, Vol. 1, 1990.  It provides just enough

        info that the casual reader might think that it documents the i82586 :-<.

*/

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/types.h>

#include <linux/fcntl.h>

#include <linux/interrupt.h>

#include <linux/ioport.h>

#include <linux/in.h>

#include <linux/string.h>

#include <linux/spinlock.h>

#include <linux/ethtool.h>

#include <linux/errno.h>

#include <linux/netdevice.h>

#include <linux/etherdevice.h>

#include <linux/skbuff.h>

#include <linux/slab.h>

#include <linux/init.h>

#include <linux/bitops.h>

#include <asm/dma.h>

#include <asm/io.h>

#include <asm/system.h>

#include <asm/uaccess.h>

/* use 0 for production, 1 for verification, 2..7 for debug */

#ifndef NET_DEBUG

#define NET_DEBUG 1

#endif

static unsigned int net_debug = NET_DEBUG;

#define debug net_debug

/*

                        Details of the i82586.

   You'll really need the databook to understand the details of this part,

   but the outline is that the i82586 has two separate processing units.

   Both are started from a list of three configuration tables, of which only

   the last, the System Control Block (SCB), is used after reset-time.  The SCB

   has the following fields:

                Status word

                Command word

                Tx/Command block addr.

                Rx block addr.

   The command word accepts the following controls for the Tx and Rx units:

  */

#define  CUC_START       0x0100

#define  CUC_RESUME      0x0200

#define  CUC_SUSPEND 0x0300

#define  RX_START        0x0010

#define  RX_RESUME       0x0020

#define  RX_SUSPEND      0x0030

/* The Rx unit uses a list of frame descriptors and a list of data buffer

   descriptors.  We use full-sized (1518 byte) data buffers, so there is

   a one-to-one pairing of frame descriptors to buffer descriptors.

   The Tx ("command") unit executes a list of commands that look like:

                Status word             Written by the 82586 when the command is done.

                Command word    Command in lower 3 bits, post-command action in upper 3

                Link word               The address of the next command.

                Parameters              (as needed).

        Some definitions related to the Command Word are:

 */

#define CMD_EOL         0x8000                  /* The last command of the list, stop. */

#define CMD_SUSP        0x4000                  /* Suspend after doing cmd. */

#define CMD_INTR        0x2000                  /* Interrupt after doing cmd. */

enum commands {

        CmdNOp = 0, CmdSASetup = 1, CmdConfigure = 2, CmdMulticastList = 3,

        CmdTx = 4, CmdTDR = 5, CmdDump = 6, CmdDiagnose = 7};

/* Information that need to be kept for each board. */

struct net_local {

        int last_restart;

        ushort rx_head;

        ushort rx_tail;

        ushort tx_head;

        ushort tx_cmd_link;

        ushort tx_reap;

        ushort tx_pkts_in_ring;

        spinlock_t lock;

        void __iomem *base;

};

/*

                Details of the EtherLink16 Implementation

  The 3c507 is a generic shared-memory i82586 implementation.

  The host can map 16K, 32K, 48K, or 64K of the 64K memory into

  0x0[CD][08]0000, or all 64K into 0xF[02468]0000.

  */

/* Offsets from the base I/O address. */

#define SA_DATA         0        /* Station address data, or 3Com signature. */

#define MISC_CTRL       6       /* Switch the SA_DATA banks, and bus config bits. */

#define RESET_IRQ       10      /* Reset the latched IRQ line. */

#define SIGNAL_CA       11      /* Frob the 82586 Channel Attention line. */

#define ROM_CONFIG      13

#define MEM_CONFIG      14

#define IRQ_CONFIG      15

#define EL16_IO_EXTENT 16

/* The ID port is used at boot-time to locate the ethercard. */

#define ID_PORT         0x100

/* Offsets to registers in the mailbox (SCB). */

#define iSCB_STATUS     0x8

#define iSCB_CMD                0xA

#define iSCB_CBL                0xC     /* Command BLock offset. */

#define iSCB_RFA                0xE     /* Rx Frame Area offset. */

/*  Since the 3c507 maps the shared memory window so that the last byte is

        at 82586 address FFFF, the first byte is at 82586 address 0, 16K, 32K, or

        48K corresponding to window sizes of 64K, 48K, 32K and 16K respectively.

        We can account for this be setting the 'SBC Base' entry in the ISCP table

        below for all the 16 bit offset addresses, and also adding the 'SCB Base'

        value to all 24 bit physical addresses (in the SCP table and the TX and RX

        Buffer Descriptors).

                                        -Mark

        */

#define SCB_BASE                ((unsigned)64*1024 - (dev->mem_end - dev->mem_start))

/*

  What follows in 'init_words[]' is the "program" that is downloaded to the

  82586 memory.  It's mostly tables and command blocks, and starts at the

  reset address 0xfffff6.  This is designed to be similar to the EtherExpress,

  thus the unusual location of the SCB at 0x0008.

  Even with the additional "don't care" values, doing it this way takes less

  program space than initializing the individual tables, and I feel it's much

  cleaner.

  The databook is particularly useless for the first two structures, I had

  to use the Crynwr driver as an example.

   The memory setup is as follows:

   */

#define CONFIG_CMD      0x0018

#define SET_SA_CMD      0x0024

#define SA_OFFSET       0x002A

#define IDLELOOP        0x30

#define TDR_CMD         0x38

#define TDR_TIME        0x3C

#define DUMP_CMD        0x40

#define DIAG_CMD        0x48

#define SET_MC_CMD      0x4E

#define DUMP_DATA       0x56    /* A 170 byte buffer for dump and Set-MC into. */

#define TX_BUF_START    0x0100

#define NUM_TX_BUFS     5

#define TX_BUF_SIZE     (1518+14+20+16) /* packet+header+TBD */

#define RX_BUF_START    0x2000

#define RX_BUF_SIZE     (1518+14+18)    /* packet+header+RBD */

#define RX_BUF_END              (dev->mem_end - dev->mem_start)

#define TX_TIMEOUT 5

/*

  That's it: only 86 bytes to set up the beast, including every extra

  command available.  The 170 byte buffer at DUMP_DATA is shared between the

  Dump command (called only by the diagnostic program) and the SetMulticastList

  command.

  To complete the memory setup you only have to write the station address at

  SA_OFFSET and create the Tx & Rx buffer lists.

  The Tx command chain and buffer list is setup as follows:

  A Tx command table, with the data buffer pointing to...

  A Tx data buffer descriptor.  The packet is in a single buffer, rather than

        chaining together several smaller buffers.

  A NoOp command, which initially points to itself,

  And the packet data.

  A transmit is done by filling in the Tx command table and data buffer,

  re-writing the NoOp command, and finally changing the offset of the last

  command to point to the current Tx command.  When the Tx command is finished,

  it jumps to the NoOp, when it loops until the next Tx command changes the

  "link offset" in the NoOp.  This way the 82586 never has to go through the

  slow restart sequence.

  The Rx buffer list is set up in the obvious ring structure.  We have enough

  memory (and low enough interrupt latency) that we can avoid the complicated

  Rx buffer linked lists by alway associating a full-size Rx data buffer with

  each Rx data frame.

  I current use four transmit buffers starting at TX_BUF_START (0x0100), and

  use the rest of memory, from RX_BUF_START to RX_BUF_END, for Rx buffers.

  */

static unsigned short init_words[] = {

        /*      System Configuration Pointer (SCP). */

        0x0000,                                 /* Set bus size to 16 bits. */

        0,0,                                      /* pad words. */

        0x0000,0x0000,                  /* ISCP phys addr, set in init_82586_mem(). */

        /*      Intermediate System Configuration Pointer (ISCP). */

        0x0001,                                 /* Status word that's cleared when init is done. */

        0x0008,0,0,                               /* SCB offset, (skip, skip) */

        /* System Control Block (SCB). */

        0,0xf000|RX_START|CUC_START,     /* SCB status and cmd. */

        CONFIG_CMD,                             /* Command list pointer, points to Configure. */

        RX_BUF_START,                           /* Rx block list. */

        0,0,0,0,                            /* Error count: CRC, align, buffer, overrun. */

        /* 0x0018: Configure command.  Change to put MAC data with packet. */

        0, CmdConfigure,         /* Status, command.             */

        SET_SA_CMD,                             /* Next command is Set Station Addr. */

        0x0804,                                 /* "4" bytes of config data, 8 byte FIFO. */

        0x2e40,                                 /* Magic values, including MAC data location. */

        0,                                               /* Unused pad word. */

        /* 0x0024: Setup station address command. */

        0, CmdSASetup,

        SET_MC_CMD,                             /* Next command. */

        0xaa00,0xb000,0x0bad,   /* Station address (to be filled in) */

        /* 0x0030: NOP, looping back to itself.  Point to first Tx buffer to Tx. */

        0, CmdNOp, IDLELOOP, 0 /* pad */,

        /* 0x0038: A unused Time-Domain Reflectometer command. */

        0, CmdTDR, IDLELOOP, 0,

        /* 0x0040: An unused Dump State command. */

        0, CmdDump, IDLELOOP, DUMP_DATA,

        /* 0x0048: An unused Diagnose command. */

        0, CmdDiagnose, IDLELOOP,

        /* 0x004E: An empty set-multicast-list command. */

        0, CmdMulticastList, IDLELOOP, 0,

};

/* Index to functions, as function prototypes. */

static int      el16_probe1(struct net_device *dev, int ioaddr);

static int      el16_open(struct net_device *dev);

static int      el16_send_packet(struct sk_buff *skb, struct net_device *dev);

static irqreturn_t el16_interrupt(int irq, void *dev_id);

static void el16_rx(struct net_device *dev);

static int      el16_close(struct net_device *dev);

static void el16_tx_timeout (struct net_device *dev);

static void hardware_send_packet(struct net_device *dev, void *buf, short length, short pad);

static void init_82586_mem(struct net_device *dev);

static const struct ethtool_ops netdev_ethtool_ops;

static void init_rx_bufs(struct net_device *);

static int io = 0x300;

static int irq;

static int mem_start;

/* Check for a network adaptor of this type, and return '0' iff one exists.

        If dev->base_addr == 0, probe all likely locations.

        If dev->base_addr == 1, always return failure.

        If dev->base_addr == 2, (detachable devices only) allocate space for the

        device and return success.

        */

struct net_device * __init el16_probe(int unit)

{

        struct net_device *dev = alloc_etherdev(sizeof(struct net_local));

        static unsigned ports[] = { 0x300, 0x320, 0x340, 0x280, 0};

        unsigned *port;

        int err = -ENODEV;

        if (!dev)

                return ERR_PTR(-ENODEV);

        if (unit >= 0) {

                sprintf(dev->name, "eth%d", unit);

                netdev_boot_setup_check(dev);

                io = dev->base_addr;

                irq = dev->irq;

                mem_start = dev->mem_start & 15;

        }

        if (io > 0x1ff)         /* Check a single specified location. */

                err = el16_probe1(dev, io);

        else if (io != 0)

                err = -ENXIO;           /* Don't probe at all. */

        else {

                for (port = ports; *port; port++) {

                        err = el16_probe1(dev, *port);

                        if (!err)

                                break;

                }

        }

        if (err)

                goto out;

        err = register_netdev(dev);

        if (err)

                goto out1;

        return dev;

out1:

        free_irq(dev->irq, dev);

        iounmap(((struct net_local *)netdev_priv(dev))->base);

        release_region(dev->base_addr, EL16_IO_EXTENT);

out:

        free_netdev(dev);

        return ERR_PTR(err);

}

static int __init el16_probe1(struct net_device *dev, int ioaddr)

{

        static unsigned char init_ID_done, version_printed;

        int i, irq, irqval, retval;

        struct net_local *lp;

        DECLARE_MAC_BUF(mac);

        if (init_ID_done == 0) {

                ushort lrs_state = 0xff;

                /* Send the ID sequence to the ID_PORT to enable the board(s). */

                outb(0x00, ID_PORT);

                for(i = 0; i < 255; i++) {

                        outb(lrs_state, ID_PORT);

                        lrs_state <<= 1;

                        if (lrs_state & 0x100)

                                lrs_state ^= 0xe7;

                }

                outb(0x00, ID_PORT);

                init_ID_done = 1;

        }

        if (!request_region(ioaddr, EL16_IO_EXTENT, DRV_NAME))

                return -ENODEV;

        if ((inb(ioaddr) != '*') || (inb(ioaddr + 1) != '3') ||

            (inb(ioaddr + 2) != 'C') || (inb(ioaddr + 3) != 'O')) {

                retval = -ENODEV;

                goto out;

        }

        if (net_debug  &&  version_printed++ == 0)

                printk(version);

        printk("%s: 3c507 at %#x,", dev->name, ioaddr);

        /* We should make a few more checks here, like the first three octets of

           the S.A. for the manufacturer's code. */

        irq = inb(ioaddr + IRQ_CONFIG) & 0x0f;

        irqval = request_irq(irq, &el16_interrupt, 0, DRV_NAME, dev);

        if (irqval) {

                printk(KERN_ERR "3c507: unable to get IRQ %d (irqval=%d).\n", irq, irqval);

                retval = -EAGAIN;

                goto out;

        }

        /* We've committed to using the board, and can start filling in *dev. */

        dev->base_addr = ioaddr;

        outb(0x01, ioaddr + MISC_CTRL);

        for (i = 0; i < 6; i++)

                dev->dev_addr[i] = inb(ioaddr + i);

        printk(" %s", print_mac(mac, dev->dev_addr));

        if (mem_start)

                net_debug = mem_start & 7;

#ifdef MEM_BASE

        dev->mem_start = MEM_BASE;

        dev->mem_end = dev->mem_start + 0x10000;

#else

        {

                int base;

                int size;

                char mem_config = inb(ioaddr + MEM_CONFIG);

                if (mem_config & 0x20) {

                        size = 64*1024;

                        base = 0xf00000 + (mem_config & 0x08 ? 0x080000

                                                           : ((mem_config & 3) << 17));

                } else {

                        size = ((mem_config & 3) + 1) << 14;

                        base = 0x0c0000 + ( (mem_config & 0x18) << 12);

                }

                dev->mem_start = base;

                dev->mem_end = base + size;

        }

#endif

        dev->if_port = (inb(ioaddr + ROM_CONFIG) & 0x80) ? 1 : 0;

        dev->irq = inb(ioaddr + IRQ_CONFIG) & 0x0f;

        printk(", IRQ %d, %sternal xcvr, memory %#lx-%#lx.\n", dev->irq,

                   dev->if_port ? "ex" : "in", dev->mem_start, dev->mem_end-1);

        if (net_debug)

                printk(version);

        lp = netdev_priv(dev);

        memset(lp, 0, sizeof(*lp));

        spin_lock_init(&lp->lock);

        lp->base = ioremap(dev->mem_start, RX_BUF_END);

        if (!lp->base) {

                printk(KERN_ERR "3c507: unable to remap memory\n");

                retval = -EAGAIN;

                goto out1;

        }

        dev->open = el16_open;

        dev->stop = el16_close;

        dev->hard_start_xmit = el16_send_packet;

        dev->tx_timeout = el16_tx_timeout;

        dev->watchdog_timeo = TX_TIMEOUT;

        dev->ethtool_ops = &netdev_ethtool_ops;

        dev->flags &= ~IFF_MULTICAST;   /* Multicast doesn't work */

        return 0;

out1:

        free_irq(dev->irq, dev);

out:

        release_region(ioaddr, EL16_IO_EXTENT);

        return retval;

}

static int el16_open(struct net_device *dev)

{

        /* Initialize the 82586 memory and start it. */

        init_82586_mem(dev);

        netif_start_queue(dev);

        return 0;

}

static void el16_tx_timeout (struct net_device *dev)

{

        struct net_local *lp = netdev_priv(dev);

        int ioaddr = dev->base_addr;

        void __iomem *shmem = lp->base;

        if (net_debug > 1)

                printk ("%s: transmit timed out, %s?  ", dev->name,

                        readw(shmem + iSCB_STATUS) & 0x8000 ? "IRQ conflict" :

                        "network cable problem");

        /* Try to restart the adaptor. */

        if (lp->last_restart == dev->stats.tx_packets) {

                if (net_debug > 1)

                        printk ("Resetting board.\n");

                /* Completely reset the adaptor. */

                init_82586_mem (dev);

                lp->tx_pkts_in_ring = 0;

        } else {

                /* Issue the channel attention signal and hope it "gets better". */

                if (net_debug > 1)

                        printk ("Kicking board.\n");

                writew(0xf000 | CUC_START | RX_START, shmem + iSCB_CMD);

                outb (0, ioaddr + SIGNAL_CA);    /* Issue channel-attn. */

                lp->last_restart = dev->stats.tx_packets;

        }

        dev->trans_start = jiffies;

        netif_wake_queue (dev);

}

static int el16_send_packet (struct sk_buff *skb, struct net_device *dev)

{

        struct net_local *lp = netdev_priv(dev);

        int ioaddr = dev->base_addr;

        unsigned long flags;

        short length = ETH_ZLEN < skb->len ? skb->len : ETH_ZLEN;

        unsigned char *buf = skb->data;

        netif_stop_queue (dev);

        spin_lock_irqsave (&lp->lock, flags);

        dev->stats.tx_bytes += length;

        /* Disable the 82586's input to the interrupt line. */

        outb (0x80, ioaddr + MISC_CTRL);

        hardware_send_packet (dev, buf, skb->len, length - skb->len);

        dev->trans_start = jiffies;

        /* Enable the 82586 interrupt input. */

        outb (0x84, ioaddr + MISC_CTRL);

        spin_unlock_irqrestore (&lp->lock, flags);

        dev_kfree_skb (skb);

        /* You might need to clean up and record Tx statistics here. */

        return 0;

}

/*      The typical workload of the driver:

        Handle the network interface interrupts. */

static irqreturn_t el16_interrupt(int irq, void *dev_id)

{

        struct net_device *dev = dev_id;

        struct net_local *lp;

        int ioaddr, status, boguscount = 0;

        ushort ack_cmd = 0;

        void __iomem *shmem;

        if (dev == NULL) {

                printk ("net_interrupt(): irq %d for unknown device.\n", irq);

                return IRQ_NONE;

        }

        ioaddr = dev->base_addr;

        lp = netdev_priv(dev);

        shmem = lp->base;

        spin_lock(&lp->lock);

        status = readw(shmem+iSCB_STATUS);

        if (net_debug > 4) {

                printk("%s: 3c507 interrupt, status %4.4x.\n", dev->name, status);

        }

        /* Disable the 82586's input to the interrupt line. */

        outb(0x80, ioaddr + MISC_CTRL);

        /* Reap the Tx packet buffers. */

        while (lp->tx_pkts_in_ring) {

          unsigned short tx_status = readw(shmem+lp->tx_reap);

          if (!(tx_status & 0x8000)) {

                if (net_debug > 5)

                        printk("Tx command incomplete (%#x).\n", lp->tx_reap);

                break;

          }

          /* Tx unsuccessful or some interesting status bit set. */

          if (!(tx_status & 0x2000) || (tx_status & 0x0f3f)) {

                dev->stats.tx_errors++;

                if (tx_status & 0x0600)  dev->stats.tx_carrier_errors++;

                if (tx_status & 0x0100)  dev->stats.tx_fifo_errors++;

                if (!(tx_status & 0x0040))  dev->stats.tx_heartbeat_errors++;

                if (tx_status & 0x0020)  dev->stats.tx_aborted_errors++;

                dev->stats.collisions += tx_status & 0xf;

          }

          dev->stats.tx_packets++;

          if (net_debug > 5)

                  printk("Reaped %x, Tx status %04x.\n" , lp->tx_reap, tx_status);

          lp->tx_reap += TX_BUF_SIZE;

          if (lp->tx_reap > RX_BUF_START - TX_BUF_SIZE)

                lp->tx_reap = TX_BUF_START;

          lp->tx_pkts_in_ring--;

          /* There is always more space in the Tx ring buffer now. */

          netif_wake_queue(dev);

          if (++boguscount > 10)

                break;

        }

        if (status & 0x4000) { /* Packet received. */

                if (net_debug > 5)

                        printk("Received packet, rx_head %04x.\n", lp->rx_head);

                el16_rx(dev);

        }

        /* Acknowledge the interrupt sources. */

        ack_cmd = status & 0xf000;

        if ((status & 0x0700) != 0x0200 && netif_running(dev)) {

                if (net_debug)

                        printk("%s: Command unit stopped, status %04x, restarting.\n",

                                   dev->name, status);

                /* If this ever occurs we should really re-write the idle loop, reset

                   the Tx list, and do a complete restart of the command unit.

                   For now we rely on the Tx timeout if the resume doesn't work. */

                ack_cmd |= CUC_RESUME;

        }

        if ((status & 0x0070) != 0x0040 && netif_running(dev)) {

                /* The Rx unit is not ready, it must be hung.  Restart the receiver by

                   initializing the rx buffers, and issuing an Rx start command. */

                if (net_debug)

                        printk("%s: Rx unit stopped, status %04x, restarting.\n",

                                   dev->name, status);

                init_rx_bufs(dev);

                writew(RX_BUF_START,shmem+iSCB_RFA);

                ack_cmd |= RX_START;

        }

        writew(ack_cmd,shmem+iSCB_CMD);

        outb(0, ioaddr + SIGNAL_CA);                     /* Issue channel-attn. */

        /* Clear the latched interrupt. */

        outb(0, ioaddr + RESET_IRQ);

        /* Enable the 82586's interrupt input. */

        outb(0x84, ioaddr + MISC_CTRL);

        spin_unlock(&lp->lock);

        return IRQ_HANDLED;

}

static int el16_close(struct net_device *dev)

{

        struct net_local *lp = netdev_priv(dev);

        int ioaddr = dev->base_addr;

        void __iomem *shmem = lp->base;

        netif_stop_queue(dev);

        /* Flush the Tx and disable Rx. */

        writew(RX_SUSPEND | CUC_SUSPEND,shmem+iSCB_CMD);

        outb(0, ioaddr + SIGNAL_CA);

        /* Disable the 82586's input to the interrupt line. */

        outb(0x80, ioaddr + MISC_CTRL);

        /* We always physically use the IRQ line, so we don't do free_irq(). */

        /* Update the statistics here. */

        return 0;

}

/* Initialize the Rx-block list. */

static void init_rx_bufs(struct net_device *dev)

{

        struct net_local *lp = netdev_priv(dev);

        void __iomem *write_ptr;

        unsigned short SCB_base = SCB_BASE;

        int cur_rxbuf = lp->rx_head = RX_BUF_START;

        /* Initialize each Rx frame + data buffer. */

        do {    /* While there is room for one more. */

                write_ptr = lp->base + cur_rxbuf;

                writew(0x0000,write_ptr);                       /* Status */

                writew(0x0000,write_ptr+=2);                    /* Command */

                writew(cur_rxbuf + RX_BUF_SIZE,write_ptr+=2);   /* Link */

                writew(cur_rxbuf + 22,write_ptr+=2);            /* Buffer offset */

                writew(0x0000,write_ptr+=2);                    /* Pad for dest addr. */

                writew(0x0000,write_ptr+=2);

                writew(0x0000,write_ptr+=2);

                writew(0x0000,write_ptr+=2);                    /* Pad for source addr. */

                writew(0x0000,write_ptr+=2);

                writew(0x0000,write_ptr+=2);

                writew(0x0000,write_ptr+=2);                    /* Pad for protocol. */

                writew(0x0000,write_ptr+=2);                    /* Buffer: Actual count */

                writew(-1,write_ptr+=2);                        /* Buffer: Next (none). */