/*
 * Driver for Zilog serial chips found on SGI workstations and
 * servers.  This driver could actually be made more generic.
 *
 * This is based on the drivers/serial/sunzilog.c code as of 2.6.0-test7 and the
 * old drivers/sgi/char/sgiserial.c code which itself is based of the original
 * drivers/sbus/char/zs.c code.  A lot of code has been simply moved over
 * directly from there but much has been rewritten.  Credits therefore go out
 * to David S. Miller, Eddie C. Dost, Pete Zaitcev, Ted Ts'o and Alex Buell
 * for their work there.
 *
 *  Copyright (C) 2002 Ralf Baechle (ralf@linux-mips.org)
 *  Copyright (C) 2002 David S. Miller (davem@redhat.com)
 */
#include <linux/config.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/errno.h>
#include <linux/delay.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
#include <linux/major.h>
#include <linux/string.h>
#include <linux/ptrace.h>
#include <linux/ioport.h>
#include <linux/slab.h>
#include <linux/circ_buf.h>
#include <linux/serial.h>
#include <linux/sysrq.h>
#include <linux/console.h>
#include <linux/spinlock.h>
#include <linux/init.h>

#include <asm/io.h>
#include <asm/irq.h>
#include <asm/sgialib.h>
#include <asm/sgi/ioc.h>
#include <asm/sgi/hpc3.h>
#include <asm/sgi/ip22.h>

#if defined(CONFIG_SERIAL_IP22_ZILOG_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
#define SUPPORT_SYSRQ
#endif

#include <linux/serial_core.h>

#include "ip22zilog.h"

void ip22_do_break(void);

/*
 * On IP22 we need to delay after register accesses but we do not need to
 * flush writes.
 */
#define ZSDELAY()               udelay(5)
#define ZSDELAY_LONG()          udelay(20)
#define ZS_WSYNC(channel)       do { } while (0)

#define NUM_IP22ZILOG           1
#define NUM_CHANNELS            (NUM_IP22ZILOG * 2)

#define ZS_CLOCK                3672000 /* Zilog input clock rate. */
#define ZS_CLOCK_DIVISOR        16      /* Divisor this driver uses. */

/*
 * We wrap our port structure around the generic uart_port.
 */
struct uart_ip22zilog_port {
        struct uart_port                port;

        /* IRQ servicing chain.  */
        struct uart_ip22zilog_port      *next;

        /* Current values of Zilog write registers.  */
        unsigned char                   curregs[NUM_ZSREGS];

        unsigned int                    flags;
#define IP22ZILOG_FLAG_IS_CONS          0x00000004
#define IP22ZILOG_FLAG_IS_KGDB          0x00000008
#define IP22ZILOG_FLAG_MODEM_STATUS     0x00000010
#define IP22ZILOG_FLAG_IS_CHANNEL_A     0x00000020
#define IP22ZILOG_FLAG_REGS_HELD        0x00000040
#define IP22ZILOG_FLAG_TX_STOPPED       0x00000080
#define IP22ZILOG_FLAG_TX_ACTIVE        0x00000100

        unsigned int                    cflag;

        /* L1-A keyboard break state.  */
        int                             kbd_id;
        int                             l1_down;

        unsigned char                   parity_mask;
        unsigned char                   prev_status;
};

#define ZILOG_CHANNEL_FROM_PORT(PORT)   ((struct zilog_channel *)((PORT)->membase))
#define UART_ZILOG(PORT)                ((struct uart_ip22zilog_port *)(PORT))
#define IP22ZILOG_GET_CURR_REG(PORT, REGNUM)            \
        (UART_ZILOG(PORT)->curregs[REGNUM])
#define IP22ZILOG_SET_CURR_REG(PORT, REGNUM, REGVAL)    \
        ((UART_ZILOG(PORT)->curregs[REGNUM]) = (REGVAL))
#define ZS_IS_CONS(UP)  ((UP)->flags & IP22ZILOG_FLAG_IS_CONS)
#define ZS_IS_KGDB(UP)  ((UP)->flags & IP22ZILOG_FLAG_IS_KGDB)
#define ZS_WANTS_MODEM_STATUS(UP)       ((UP)->flags & IP22ZILOG_FLAG_MODEM_STATUS)
#define ZS_IS_CHANNEL_A(UP)     ((UP)->flags & IP22ZILOG_FLAG_IS_CHANNEL_A)
#define ZS_REGS_HELD(UP)        ((UP)->flags & IP22ZILOG_FLAG_REGS_HELD)
#define ZS_TX_STOPPED(UP)       ((UP)->flags & IP22ZILOG_FLAG_TX_STOPPED)
#define ZS_TX_ACTIVE(UP)        ((UP)->flags & IP22ZILOG_FLAG_TX_ACTIVE)

/* Reading and writing Zilog8530 registers.  The delays are to make this
 * driver work on the IP22 which needs a settling delay after each chip
 * register access, other machines handle this in hardware via auxiliary
 * flip-flops which implement the settle time we do in software.
 *
 * The port lock must be held and local IRQs must be disabled
 * when {read,write}_zsreg is invoked.
 */
static unsigned char read_zsreg(struct zilog_channel *channel,
                                unsigned char reg)
{
        unsigned char retval;

        writeb(reg, &channel->control);
        ZSDELAY();
        retval = readb(&channel->control);
        ZSDELAY();

        return retval;
}

static void write_zsreg(struct zilog_channel *channel,
                        unsigned char reg, unsigned char value)
{
        writeb(reg, &channel->control);
        ZSDELAY();
        writeb(value, &channel->control);
        ZSDELAY();
}

static void ip22zilog_clear_fifo(struct zilog_channel *channel)
{
        int i;

        for (i = 0; i < 32; i++) {
                unsigned char regval;

                regval = readb(&channel->control);
                ZSDELAY();
                if (regval & Rx_CH_AV)
                        break;

                regval = read_zsreg(channel, R1);
                readb(&channel->data);
                ZSDELAY();

                if (regval & (PAR_ERR | Rx_OVR | CRC_ERR)) {
                        writeb(ERR_RES, &channel->control);
                        ZSDELAY();
                        ZS_WSYNC(channel);
                }
        }
}

/* This function must only be called when the TX is not busy.  The UART
 * port lock must be held and local interrupts disabled.
 */
static void __load_zsregs(struct zilog_channel *channel, unsigned char *regs)
{
        int i;

        /* Let pending transmits finish.  */
        for (i = 0; i < 1000; i++) {
                unsigned char stat = read_zsreg(channel, R1);
                if (stat & ALL_SNT)
                        break;
                udelay(100);
        }

        writeb(ERR_RES, &channel->control);
        ZSDELAY();
        ZS_WSYNC(channel);

        ip22zilog_clear_fifo(channel);

        /* Disable all interrupts.  */
        write_zsreg(channel, R1,
                    regs[R1] & ~(RxINT_MASK | TxINT_ENAB | EXT_INT_ENAB));

        /* Set parity, sync config, stop bits, and clock divisor.  */
        write_zsreg(channel, R4, regs[R4]);

        /* Set misc. TX/RX control bits.  */
        write_zsreg(channel, R10, regs[R10]);

        /* Set TX/RX controls sans the enable bits.  */
        write_zsreg(channel, R3, regs[R3] & ~RxENAB);
        write_zsreg(channel, R5, regs[R5] & ~TxENAB);

        /* Synchronous mode config.  */
        write_zsreg(channel, R6, regs[R6]);
        write_zsreg(channel, R7, regs[R7]);

        /* Don't mess with the interrupt vector (R2, unused by us) and
         * master interrupt control (R9).  We make sure this is setup
         * properly at probe time then never touch it again.
         */

        /* Disable baud generator.  */
        write_zsreg(channel, R14, regs[R14] & ~BRENAB);

        /* Clock mode control.  */
        write_zsreg(channel, R11, regs[R11]);

        /* Lower and upper byte of baud rate generator divisor.  */
        write_zsreg(channel, R12, regs[R12]);
        write_zsreg(channel, R13, regs[R13]);
        
        /* Now rewrite R14, with BRENAB (if set).  */
        write_zsreg(channel, R14, regs[R14]);

        /* External status interrupt control.  */
        write_zsreg(channel, R15, regs[R15]);

        /* Reset external status interrupts.  */
        write_zsreg(channel, R0, RES_EXT_INT);
        write_zsreg(channel, R0, RES_EXT_INT);

        /* Rewrite R3/R5, this time without enables masked.  */
        write_zsreg(channel, R3, regs[R3]);
        write_zsreg(channel, R5, regs[R5]);

        /* Rewrite R1, this time without IRQ enabled masked.  */
        write_zsreg(channel, R1, regs[R1]);
}

/* Reprogram the Zilog channel HW registers with the copies found in the
 * software state struct.  If the transmitter is busy, we defer this update
 * until the next TX complete interrupt.  Else, we do it right now.
 *
 * The UART port lock must be held and local interrupts disabled.
 */
static void ip22zilog_maybe_update_regs(struct uart_ip22zilog_port *up,
                                       struct zilog_channel *channel)
{
        if (!ZS_REGS_HELD(up)) {
                if (ZS_TX_ACTIVE(up)) {
                        up->flags |= IP22ZILOG_FLAG_REGS_HELD;
                } else {
                        __load_zsregs(channel, up->curregs);
                }
        }
}

static void ip22zilog_receive_chars(struct uart_ip22zilog_port *up,
                                   struct zilog_channel *channel,
                                   struct pt_regs *regs)
{
        struct tty_struct *tty = up->port.info->tty;    /* XXX info==NULL? */

        while (1) {
                unsigned char ch, r1;

                if (unlikely(tty->flip.count >= TTY_FLIPBUF_SIZE)) {
                        tty->flip.work.func((void *)tty);
                        if (tty->flip.count >= TTY_FLIPBUF_SIZE)
                                return;         /* XXX Ignores SysRq when we need it most. Fix. */
                }

                r1 = read_zsreg(channel, R1);
                if (r1 & (PAR_ERR | Rx_OVR | CRC_ERR)) {
                        writeb(ERR_RES, &channel->control);
                        ZSDELAY();
                        ZS_WSYNC(channel);
                }

                ch = readb(&channel->control);
                ZSDELAY();

                /* This funny hack depends upon BRK_ABRT not interfering
                 * with the other bits we care about in R1.
                 */
                if (ch & BRK_ABRT)
                        r1 |= BRK_ABRT;

                ch = readb(&channel->data);
                ZSDELAY();

                ch &= up->parity_mask;

                if (ZS_IS_CONS(up) && (r1 & BRK_ABRT)) {
                        /* Wait for BREAK to deassert to avoid potentially
                         * confusing the PROM.
                         */
                        while (1) {
                                ch = readb(&channel->control);
                                ZSDELAY();
                                if (!(ch & BRK_ABRT))
                                        break;
                        }
                        ip22_do_break();
                        return;
                }

                /* A real serial line, record the character and status.  */
                *tty->flip.char_buf_ptr = ch;
                *tty->flip.flag_buf_ptr = TTY_NORMAL;
                up->port.icount.rx++;
                if (r1 & (BRK_ABRT | PAR_ERR | Rx_OVR | CRC_ERR)) {
                        if (r1 & BRK_ABRT) {
                                r1 &= ~(PAR_ERR | CRC_ERR);
                                up->port.icount.brk++;
                                if (uart_handle_break(&up->port))
                                        goto next_char;
                        }
                        else if (r1 & PAR_ERR)
                                up->port.icount.parity++;
                        else if (r1 & CRC_ERR)
                                up->port.icount.frame++;
                        if (r1 & Rx_OVR)
                                up->port.icount.overrun++;
                        r1 &= up->port.read_status_mask;
                        if (r1 & BRK_ABRT)
                                *tty->flip.flag_buf_ptr = TTY_BREAK;
                        else if (r1 & PAR_ERR)
                                *tty->flip.flag_buf_ptr = TTY_PARITY;
                        else if (r1 & CRC_ERR)
                                *tty->flip.flag_buf_ptr = TTY_FRAME;
                }
                if (uart_handle_sysrq_char(&up->port, ch, regs))
                        goto next_char;

                if (up->port.ignore_status_mask == 0xff ||
                    (r1 & up->port.ignore_status_mask) == 0) {
                        tty->flip.flag_buf_ptr++;
                        tty->flip.char_buf_ptr++;
                        tty->flip.count++;
                }
                if ((r1 & Rx_OVR) &&
                    tty->flip.count < TTY_FLIPBUF_SIZE) {
                        *tty->flip.flag_buf_ptr = TTY_OVERRUN;
                        tty->flip.flag_buf_ptr++;
                        tty->flip.char_buf_ptr++;
                        tty->flip.count++;
                }
        next_char:
                ch = readb(&channel->control);
                ZSDELAY();
                if (!(ch & Rx_CH_AV))
                        break;
        }

        tty_flip_buffer_push(tty);
}

static void ip22zilog_status_handle(struct uart_ip22zilog_port *up,
                                   struct zilog_channel *channel,
                                   struct pt_regs *regs)
{
        unsigned char status;

        status = readb(&channel->control);
        ZSDELAY();

        writeb(RES_EXT_INT, &channel->control);
        ZSDELAY();
        ZS_WSYNC(channel);

        if (ZS_WANTS_MODEM_STATUS(up)) {
                if (status & SYNC)
                        up->port.icount.dsr++;

                /* The Zilog just gives us an interrupt when DCD/CTS/etc. change.
                 * But it does not tell us which bit has changed, we have to keep
                 * track of this ourselves.
                 */
                if ((status & DCD) ^ up->prev_status)
                        uart_handle_dcd_change(&up->port,
                                               (status & DCD));
                if ((status & CTS) ^ up->prev_status)
                        uart_handle_cts_change(&up->port,
                                               (status & CTS));

                wake_up_interruptible(&up->port.info->delta_msr_wait);
        }

        up->prev_status = status;
}

static void ip22zilog_transmit_chars(struct uart_ip22zilog_port *up,
                                    struct zilog_channel *channel)
{
        struct circ_buf *xmit;

        if (ZS_IS_CONS(up)) {
                unsigned char status = readb(&channel->control);
                ZSDELAY();

                /* TX still busy?  Just wait for the next TX done interrupt.
                 *
                 * It can occur because of how we do serial console writes.  It would
                 * be nice to transmit console writes just like we normally would for
                 * a TTY line. (ie. buffered and TX interrupt driven).  That is not
                 * easy because console writes cannot sleep.  One solution might be
                 * to poll on enough port->xmit space becomming free.  -DaveM
                 */
                if (!(status & Tx_BUF_EMP))
                        return;
        }

        up->flags &= ~IP22ZILOG_FLAG_TX_ACTIVE;

        if (ZS_REGS_HELD(up)) {
                __load_zsregs(channel, up->curregs);
                up->flags &= ~IP22ZILOG_FLAG_REGS_HELD;
        }

        if (ZS_TX_STOPPED(up)) {
                up->flags &= ~IP22ZILOG_FLAG_TX_STOPPED;
                goto ack_tx_int;
        }

        if (up->port.x_char) {
                up->flags |= IP22ZILOG_FLAG_TX_ACTIVE;
                writeb(up->port.x_char, &channel->data);
                ZSDELAY();
                ZS_WSYNC(channel);

                up->port.icount.tx++;
                up->port.x_char = 0;
                return;
        }

        if (up->port.info == NULL)
                goto ack_tx_int;
        xmit = &up->port.info->xmit;
        if (uart_circ_empty(xmit)) {
                uart_write_wakeup(&up->port);
                goto ack_tx_int;
        }
        if (uart_tx_stopped(&up->port))
                goto ack_tx_int;

        up->flags |= IP22ZILOG_FLAG_TX_ACTIVE;
        writeb(xmit->buf[xmit->tail], &channel->data);
        ZSDELAY();
        ZS_WSYNC(channel);

        xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
        up->port.icount.tx++;

        if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
                uart_write_wakeup(&up->port);

        return;

ack_tx_int:
        writeb(RES_Tx_P, &channel->control);
        ZSDELAY();
        ZS_WSYNC(channel);
}

static irqreturn_t ip22zilog_interrupt(int irq, void *dev_id, struct pt_regs *regs)
{
        struct uart_ip22zilog_port *up = dev_id;

        while (up) {
                struct zilog_channel *channel
                        = ZILOG_CHANNEL_FROM_PORT(&up->port);
                unsigned char r3;

                spin_lock(&up->port.lock);
                r3 = read_zsreg(channel, R3);

                /* Channel A */
                if (r3 & (CHAEXT | CHATxIP | CHARxIP)) {
                        writeb(RES_H_IUS, &channel->control);
                        ZSDELAY();
                        ZS_WSYNC(channel);

                        if (r3 & CHARxIP)
                                ip22zilog_receive_chars(up, channel, regs);
                        if (r3 & CHAEXT)
                                ip22zilog_status_handle(up, channel, regs);
                        if (r3 & CHATxIP)
                                ip22zilog_transmit_chars(up, channel);
                }
                spin_unlock(&up->port.lock);

                /* Channel B */
                up = up->next;
                channel = ZILOG_CHANNEL_FROM_PORT(&up->port);

                spin_lock(&up->port.lock);
                if (r3 & (CHBEXT | CHBTxIP | CHBRxIP)) {
                        writeb(RES_H_IUS, &channel->control);
                        ZSDELAY();
                        ZS_WSYNC(channel);

                        if (r3 & CHBRxIP)
                                ip22zilog_receive_chars(up, channel, regs);
                        if (r3 & CHBEXT)
                                ip22zilog_status_handle(up, channel, regs);
                        if (r3 & CHBTxIP)
                                ip22zilog_transmit_chars(up, channel);
                }
                spin_unlock(&up->port.lock);

                up = up->next;
        }

        return IRQ_HANDLED;
}

/* A convenient way to quickly get R0 status.  The caller must _not_ hold the
 * port lock, it is acquired here.
 */
static __inline__ unsigned char ip22zilog_read_channel_status(struct uart_port *port)
{
        struct zilog_channel *channel;
        unsigned long flags;
        unsigned char status;

        spin_lock_irqsave(&port->lock, flags);

        channel = ZILOG_CHANNEL_FROM_PORT(port);
        status = readb(&channel->control);
        ZSDELAY();

        spin_unlock_irqrestore(&port->lock, flags);

        return status;
}

/* The port lock is not held.  */
static unsigned int ip22zilog_tx_empty(struct uart_port *port)
{
        unsigned char status;
        unsigned int ret;

        status = ip22zilog_read_channel_status(port);
        if (status & Tx_BUF_EMP)
                ret = TIOCSER_TEMT;
        else
                ret = 0;

        return ret;
}

/* The port lock is not held.  */
static unsigned int ip22zilog_get_mctrl(struct uart_port *port)
{
        unsigned char status;
        unsigned int ret;

        status = ip22zilog_read_channel_status(port);

        ret = 0;
        if (status & DCD)
                ret |= TIOCM_CAR;
        if (status & SYNC)
                ret |= TIOCM_DSR;
        if (status & CTS)
                ret |= TIOCM_CTS;

        return ret;
}

/* The port lock is held and interrupts are disabled.  */
static void ip22zilog_set_mctrl(struct uart_port *port, unsigned int mctrl)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;
        struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(port);
        unsigned char set_bits, clear_bits;

        set_bits = clear_bits = 0;

        if (mctrl & TIOCM_RTS)
                set_bits |= RTS;
        else
                clear_bits |= RTS;
        if (mctrl & TIOCM_DTR)
                set_bits |= DTR;
        else
                clear_bits |= DTR;

        /* NOTE: Not subject to 'transmitter active' rule.  */ 
        up->curregs[R5] |= set_bits;
        up->curregs[R5] &= ~clear_bits;
        write_zsreg(channel, R5, up->curregs[R5]);
}

/* The port lock is held and interrupts are disabled.  */
static void ip22zilog_stop_tx(struct uart_port *port, unsigned int tty_stop)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;

        up->flags |= IP22ZILOG_FLAG_TX_STOPPED;
}

/* The port lock is held and interrupts are disabled.  */
static void ip22zilog_start_tx(struct uart_port *port, unsigned int tty_start)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;
        struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(port);
        unsigned char status;

        up->flags |= IP22ZILOG_FLAG_TX_ACTIVE;
        up->flags &= ~IP22ZILOG_FLAG_TX_STOPPED;

        status = readb(&channel->control);
        ZSDELAY();

        /* TX busy?  Just wait for the TX done interrupt.  */
        if (!(status & Tx_BUF_EMP))
                return;

        /* Send the first character to jump-start the TX done
         * IRQ sending engine.
         */
        if (port->x_char) {
                writeb(port->x_char, &channel->data);
                ZSDELAY();
                ZS_WSYNC(channel);

                port->icount.tx++;
                port->x_char = 0;
        } else {
                struct circ_buf *xmit = &port->info->xmit;

                writeb(xmit->buf[xmit->tail], &channel->data);
                ZSDELAY();
                ZS_WSYNC(channel);

                xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
                port->icount.tx++;

                if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
                        uart_write_wakeup(&up->port);
        }
}

/* The port lock is held and interrupts are disabled.  */
static void ip22zilog_stop_rx(struct uart_port *port)
{
        struct uart_ip22zilog_port *up = UART_ZILOG(port);
        struct zilog_channel *channel;

        if (ZS_IS_CONS(up))
                return;

        channel = ZILOG_CHANNEL_FROM_PORT(port);

        /* Disable all RX interrupts.  */
        up->curregs[R1] &= ~RxINT_MASK;
        ip22zilog_maybe_update_regs(up, channel);
}

/* The port lock is held.  */
static void ip22zilog_enable_ms(struct uart_port *port)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;
        struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(port);
        unsigned char new_reg;

        new_reg = up->curregs[R15] | (DCDIE | SYNCIE | CTSIE);
        if (new_reg != up->curregs[R15]) {
                up->curregs[R15] = new_reg;

                /* NOTE: Not subject to 'transmitter active' rule.  */ 
                write_zsreg(channel, R15, up->curregs[R15]);
        }
}

/* The port lock is not held.  */
static void ip22zilog_break_ctl(struct uart_port *port, int break_state)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;
        struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(port);
        unsigned char set_bits, clear_bits, new_reg;
        unsigned long flags;

        set_bits = clear_bits = 0;

        if (break_state)
                set_bits |= SND_BRK;
        else
                clear_bits |= SND_BRK;

        spin_lock_irqsave(&port->lock, flags);

        new_reg = (up->curregs[R5] | set_bits) & ~clear_bits;
        if (new_reg != up->curregs[R5]) {
                up->curregs[R5] = new_reg;

                /* NOTE: Not subject to 'transmitter active' rule.  */ 
                write_zsreg(channel, R5, up->curregs[R5]);
        }

        spin_unlock_irqrestore(&port->lock, flags);
}

static void __ip22zilog_startup(struct uart_ip22zilog_port *up)
{
        struct zilog_channel *channel;

        channel = ZILOG_CHANNEL_FROM_PORT(&up->port);
        up->prev_status = readb(&channel->control);

        /* Enable receiver and transmitter.  */
        up->curregs[R3] |= RxENAB;
        up->curregs[R5] |= TxENAB;

        up->curregs[R1] |= EXT_INT_ENAB | INT_ALL_Rx | TxINT_ENAB;
        ip22zilog_maybe_update_regs(up, channel);
}

static int ip22zilog_startup(struct uart_port *port)
{
        struct uart_ip22zilog_port *up = UART_ZILOG(port);
        unsigned long flags;

        if (ZS_IS_CONS(up))
                return 0;

        spin_lock_irqsave(&port->lock, flags);
        __ip22zilog_startup(up);
        spin_unlock_irqrestore(&port->lock, flags);
        return 0;
}

/*
 * The test for ZS_IS_CONS is explained by the following e-mail:
 *****
 * From: Russell King <rmk@arm.linux.org.uk>
 * Date: Sun, 8 Dec 2002 10:18:38 +0000
 *
 * On Sun, Dec 08, 2002 at 02:43:36AM -0500, Pete Zaitcev wrote:
 * > I boot my 2.5 boxes using "console=ttyS0,9600" argument,
 * > and I noticed that something is not right with reference
 * > counting in this case. It seems that when the console
 * > is open by kernel initially, this is not accounted
 * > as an open, and uart_startup is not called.
 *
 * That is correct.  We are unable to call uart_startup when the serial
 * console is initialised because it may need to allocate memory (as
 * request_irq does) and the memory allocators may not have been
 * initialised.
 *
 * 1. initialise the port into a state where it can send characters in the
 *    console write method.
 *
 * 2. don't do the actual hardware shutdown in your shutdown() method (but
 *    do the normal software shutdown - ie, free irqs etc)
 *****
 */
static void ip22zilog_shutdown(struct uart_port *port)
{
        struct uart_ip22zilog_port *up = UART_ZILOG(port);
        struct zilog_channel *channel;
        unsigned long flags;

        if (ZS_IS_CONS(up))
                return;

        spin_lock_irqsave(&port->lock, flags);

        channel = ZILOG_CHANNEL_FROM_PORT(port);

        /* Disable receiver and transmitter.  */
        up->curregs[R3] &= ~RxENAB;
        up->curregs[R5] &= ~TxENAB;

        /* Disable all interrupts and BRK assertion.  */
        up->curregs[R1] &= ~(EXT_INT_ENAB | TxINT_ENAB | RxINT_MASK);
        up->curregs[R5] &= ~SND_BRK;
        ip22zilog_maybe_update_regs(up, channel);

        spin_unlock_irqrestore(&port->lock, flags);
}

/* Shared by TTY driver and serial console setup.  The port lock is held
 * and local interrupts are disabled.
 */
static void
ip22zilog_convert_to_zs(struct uart_ip22zilog_port *up, unsigned int cflag,
                       unsigned int iflag, int brg)
{

        up->curregs[R10] = NRZ;
        up->curregs[R11] = TCBR | RCBR;

        /* Program BAUD and clock source. */
        up->curregs[R4] &= ~XCLK_MASK;
        up->curregs[R4] |= X16CLK;
        up->curregs[R12] = brg & 0xff;
        up->curregs[R13] = (brg >> 8) & 0xff;
        up->curregs[R14] = BRENAB;

        /* Character size, stop bits, and parity. */
        up->curregs[3] &= ~RxN_MASK;
        up->curregs[5] &= ~TxN_MASK;
        switch (cflag & CSIZE) {
        case CS5:
                up->curregs[3] |= Rx5;
                up->curregs[5] |= Tx5;
                up->parity_mask = 0x1f;
                break;
        case CS6:
                up->curregs[3] |= Rx6;
                up->curregs[5] |= Tx6;
                up->parity_mask = 0x3f;
                break;
        case CS7:
                up->curregs[3] |= Rx7;
                up->curregs[5] |= Tx7;
                up->parity_mask = 0x7f;
                break;
        case CS8:
        default:
                up->curregs[3] |= Rx8;
                up->curregs[5] |= Tx8;
                up->parity_mask = 0xff;
                break;
        };
        up->curregs[4] &= ~0x0c;
        if (cflag & CSTOPB)
                up->curregs[4] |= SB2;
        else
                up->curregs[4] |= SB1;
        if (cflag & PARENB)
                up->curregs[4] |= PAR_ENAB;
        else
                up->curregs[4] &= ~PAR_ENAB;
        if (!(cflag & PARODD))
                up->curregs[4] |= PAR_EVEN;
        else
                up->curregs[4] &= ~PAR_EVEN;

        up->port.read_status_mask = Rx_OVR;
        if (iflag & INPCK)
                up->port.read_status_mask |= CRC_ERR | PAR_ERR;
        if (iflag & (BRKINT | PARMRK))
                up->port.read_status_mask |= BRK_ABRT;

        up->port.ignore_status_mask = 0;
        if (iflag & IGNPAR)
                up->port.ignore_status_mask |= CRC_ERR | PAR_ERR;
        if (iflag & IGNBRK) {
                up->port.ignore_status_mask |= BRK_ABRT;
                if (iflag & IGNPAR)
                        up->port.ignore_status_mask |= Rx_OVR;
        }

        if ((cflag & CREAD) == 0)
                up->port.ignore_status_mask = 0xff;
}

/* The port lock is not held.  */
static void
ip22zilog_set_termios(struct uart_port *port, struct termios *termios,
                      struct termios *old)
{
        struct uart_ip22zilog_port *up = (struct uart_ip22zilog_port *) port;
        unsigned long flags;
        int baud, brg;

        baud = uart_get_baud_rate(port, termios, old, 1200, 76800);

        spin_lock_irqsave(&up->port.lock, flags);

        brg = BPS_TO_BRG(baud, ZS_CLOCK / ZS_CLOCK_DIVISOR);

        ip22zilog_convert_to_zs(up, termios->c_cflag, termios->c_iflag, brg);

        if (UART_ENABLE_MS(&up->port, termios->c_cflag))
                up->flags |= IP22ZILOG_FLAG_MODEM_STATUS;
        else
                up->flags &= ~IP22ZILOG_FLAG_MODEM_STATUS;

        up->cflag = termios->c_cflag;

        ip22zilog_maybe_update_regs(up, ZILOG_CHANNEL_FROM_PORT(port));

        spin_unlock_irqrestore(&up->port.lock, flags);
}

static const char *ip22zilog_type(struct uart_port *port)
{
        return "IP22-Zilog";
}

/* We do not request/release mappings of the registers here, this
 * happens at early serial probe time.
 */
static void ip22zilog_release_port(struct uart_port *port)
{
}

static int ip22zilog_request_port(struct uart_port *port)
{
        return 0;
}

/* These do not need to do anything interesting either.  */
static void ip22zilog_config_port(struct uart_port *port, int flags)
{
}

/* We do not support letting the user mess with the divisor, IRQ, etc. */
static int ip22zilog_verify_port(struct uart_port *port, struct serial_struct *ser)
{
        return -EINVAL;
}

static struct uart_ops ip22zilog_pops = {
        .tx_empty       =       ip22zilog_tx_empty,
        .set_mctrl      =       ip22zilog_set_mctrl,
        .get_mctrl      =       ip22zilog_get_mctrl,
        .stop_tx        =       ip22zilog_stop_tx,
        .start_tx       =       ip22zilog_start_tx,
        .stop_rx        =       ip22zilog_stop_rx,
        .enable_ms      =       ip22zilog_enable_ms,
        .break_ctl      =       ip22zilog_break_ctl,
        .startup        =       ip22zilog_startup,
        .shutdown       =       ip22zilog_shutdown,
        .set_termios    =       ip22zilog_set_termios,
        .type           =       ip22zilog_type,
        .release_port   =       ip22zilog_release_port,
        .request_port   =       ip22zilog_request_port,
        .config_port    =       ip22zilog_config_port,
        .verify_port    =       ip22zilog_verify_port,
};

static struct uart_ip22zilog_port *ip22zilog_port_table;
static struct zilog_layout **ip22zilog_chip_regs;

static struct uart_ip22zilog_port *ip22zilog_irq_chain;
static int zilog_irq = -1;

static void * __init alloc_one_table(unsigned long size)
{
        void *ret;

        ret = kmalloc(size, GFP_KERNEL);
        if (ret != NULL)
                memset(ret, 0, size);

        return ret;
}

static void __init ip22zilog_alloc_tables(void)
{
        ip22zilog_port_table = (struct uart_ip22zilog_port *)
                alloc_one_table(NUM_CHANNELS * sizeof(struct uart_ip22zilog_port));
        ip22zilog_chip_regs = (struct zilog_layout **)
                alloc_one_table(NUM_IP22ZILOG * sizeof(struct zilog_layout *));

        if (ip22zilog_port_table == NULL || ip22zilog_chip_regs == NULL) {
                panic("IP22-Zilog: Cannot allocate IP22-Zilog tables.");
        }
}

/* Get the address of the registers for IP22-Zilog instance CHIP.  */
static struct zilog_layout * __init get_zs(int chip)
{
        unsigned long base;

        if (chip < 0 || chip >= NUM_IP22ZILOG) {
                panic("IP22-Zilog: Illegal chip number %d in get_zs.", chip);
        }

        /* Not probe-able, hard code it. */
        base = (unsigned long) &sgioc->uart;

        zilog_irq = SGI_SERIAL_IRQ;
        request_mem_region(base, 8, "IP22-Zilog");

        return (struct zilog_layout *) base;
}

#define ZS_PUT_CHAR_MAX_DELAY   2000    /* 10 ms */

#ifdef CONFIG_SERIAL_IP22_ZILOG_CONSOLE
static void ip22zilog_put_char(struct zilog_channel *channel, unsigned char ch)
{
        int loops = ZS_PUT_CHAR_MAX_DELAY;

        /* This is a timed polling loop so do not switch the explicit
         * udelay with ZSDELAY as that is a NOP on some platforms.  -DaveM
         */
        do {
                unsigned char val = readb(&channel->control);
                if (val & Tx_BUF_EMP) {
                        ZSDELAY();
                        break;
                }
                udelay(5);
        } while (--loops);

        writeb(ch, &channel->data);
        ZSDELAY();
        ZS_WSYNC(channel);
}

static void
ip22zilog_console_write(struct console *con, const char *s, unsigned int count)
{
        struct uart_ip22zilog_port *up = &ip22zilog_port_table[con->index];
        struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(&up->port);
        unsigned long flags;
        int i;

        spin_lock_irqsave(&up->port.lock, flags);
        for (i = 0; i < count; i++, s++) {
                ip22zilog_put_char(channel, *s);
                if (*s == 10)
                        ip22zilog_put_char(channel, 13);
        }
        udelay(2);
        spin_unlock_irqrestore(&up->port.lock, flags);
}

void
ip22serial_console_termios(struct console *con, char *options)
{
        int baud = 9600, bits = 8, cflag;
        int parity = 'n';
        int flow = 'n';

        if (options)
                uart_parse_options(options, &baud, &parity, &bits, &flow);

        cflag = CREAD | HUPCL | CLOCAL;

        switch (baud) {
                case 150: cflag |= B150; break;
                case 300: cflag |= B300; break;
                case 600: cflag |= B600; break;
                case 1200: cflag |= B1200; break;
                case 2400: cflag |= B2400; break;
                case 4800: cflag |= B4800; break;
                case 9600: cflag |= B9600; break;
                case 19200: cflag |= B19200; break;
                case 38400: cflag |= B38400; break;
                default: baud = 9600; cflag |= B9600; break;
        }

        con->cflag = cflag | CS8;                       /* 8N1 */
}

static int __init ip22zilog_console_setup(struct console *con, char *options)
{
        struct uart_ip22zilog_port *up = &ip22zilog_port_table[con->index];
        unsigned long flags;
        int baud, brg;

        printk("Console: ttyS%d (IP22-Zilog)\n", con->index);

        /* Get firmware console settings.  */
        ip22serial_console_termios(con, options);

        /* Firmware console speed is limited to 150-->38400 baud so
         * this hackish cflag thing is OK.
         */
        switch (con->cflag & CBAUD) {
        case B150: baud = 150; break;
        case B300: baud = 300; break;
        case B600: baud = 600; break;
        case B1200: baud = 1200; break;
        case B2400: baud = 2400; break;
        case B4800: baud = 4800; break;
        default: case B9600: baud = 9600; break;
        case B19200: baud = 19200; break;
        case B38400: baud = 38400; break;
        };

        brg = BPS_TO_BRG(baud, ZS_CLOCK / ZS_CLOCK_DIVISOR);

        spin_lock_irqsave(&up->port.lock, flags);

        up->curregs[R15] = BRKIE;
        ip22zilog_convert_to_zs(up, con->cflag, 0, brg);

        __ip22zilog_startup(up);

        spin_unlock_irqrestore(&up->port.lock, flags);

        return 0;
}

static struct uart_driver ip22zilog_reg;

static struct console ip22zilog_console = {
        .name   =       "ttyS",
        .write  =       ip22zilog_console_write,
        .device =       uart_console_device,
        .setup  =       ip22zilog_console_setup,
        .flags  =       CON_PRINTBUFFER,
        .index  =       -1,
        .data   =       &ip22zilog_reg,
};
#endif /* CONFIG_SERIAL_IP22_ZILOG_CONSOLE */

static struct uart_driver ip22zilog_reg = {
        .owner          = THIS_MODULE,
        .driver_name    = "serial",
        .devfs_name     = "tts/",
        .dev_name       = "ttyS",
        .major          = TTY_MAJOR,
        .minor          = 64,
        .nr             = NUM_CHANNELS,
#ifdef CONFIG_SERIAL_IP22_ZILOG_CONSOLE
        .cons           = &ip22zilog_console,
#endif
};

static void __init ip22zilog_prepare(void)
{
        struct uart_ip22zilog_port *up;
        struct zilog_layout *rp;
        int channel, chip;

        /*
         * Temporary fix.
         */
        for (channel = 0; channel < NUM_CHANNELS; channel++)
                spin_lock_init(&ip22zilog_port_table[channel].port.lock);

        ip22zilog_irq_chain = &ip22zilog_port_table[NUM_CHANNELS - 1];
        up = &ip22zilog_port_table[0];
        for (channel = NUM_CHANNELS - 1 ; channel > 0; channel--)
                up[channel].next = &up[channel - 1];
        up[channel].next = NULL;

        for (chip = 0; chip < NUM_IP22ZILOG; chip++) {
                if (!ip22zilog_chip_regs[chip]) {
                        ip22zilog_chip_regs[chip] = rp = get_zs(chip);

                        up[(chip * 2) + 0].port.membase = (char *) &rp->channelB;
                        up[(chip * 2) + 1].port.membase = (char *) &rp->channelA;

                        /* In theory mapbase is the physical address ...  */
                        up[(chip * 2) + 0].port.mapbase =
                                (unsigned long) ioremap((unsigned long) &rp->channelB, 8);
                        up[(chip * 2) + 1].port.mapbase =
                                (unsigned long) ioremap((unsigned long) &rp->channelA, 8);
                }

                /* Channel A */
                up[(chip * 2) + 0].port.iotype = UPIO_MEM;
                up[(chip * 2) + 0].port.irq = zilog_irq;
                up[(chip * 2) + 0].port.uartclk = ZS_CLOCK;
                up[(chip * 2) + 0].port.fifosize = 1;
                up[(chip * 2) + 0].port.ops = &ip22zilog_pops;
                up[(chip * 2) + 0].port.type = PORT_IP22ZILOG;
                up[(chip * 2) + 0].port.flags = 0;
                up[(chip * 2) + 0].port.line = (chip * 2) + 0;
                up[(chip * 2) + 0].flags = 0;

                /* Channel B */
                up[(chip * 2) + 1].port.iotype = UPIO_MEM;
                up[(chip * 2) + 1].port.irq = zilog_irq;
                up[(chip * 2) + 1].port.uartclk = ZS_CLOCK;
                up[(chip * 2) + 1].port.fifosize = 1;
                up[(chip * 2) + 1].port.ops = &ip22zilog_pops;
                up[(chip * 2) + 1].port.type = PORT_IP22ZILOG;
                up[(chip * 2) + 1].port.flags |= IP22ZILOG_FLAG_IS_CHANNEL_A;
                up[(chip * 2) + 1].port.line = (chip * 2) + 1;
                up[(chip * 2) + 1].flags = 0;
        }
}

static void __init ip22zilog_init_hw(void)
{
        int i;

        for (i = 0; i < NUM_CHANNELS; i++) {
                struct uart_ip22zilog_port *up = &ip22zilog_port_table[i];
                struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(&up->port);
                unsigned long flags;
                int baud, brg;

                spin_lock_irqsave(&up->port.lock, flags);

                if (ZS_IS_CHANNEL_A(up)) {
                        write_zsreg(channel, R9, FHWRES);
                        ZSDELAY_LONG();
                        (void) read_zsreg(channel, R0);
                }

                /* Normal serial TTY. */
                up->parity_mask = 0xff;
                up->curregs[R1] = EXT_INT_ENAB | INT_ALL_Rx | TxINT_ENAB;
                up->curregs[R4] = PAR_EVEN | X16CLK | SB1;
                up->curregs[R3] = RxENAB | Rx8;
                up->curregs[R5] = TxENAB | Tx8;
                up->curregs[R9] = NV | MIE;
                up->curregs[R10] = NRZ;
                up->curregs[R11] = TCBR | RCBR;
                baud = 9600;
                brg = BPS_TO_BRG(baud, ZS_CLOCK / ZS_CLOCK_DIVISOR);
                up->curregs[R12] = (brg & 0xff);
                up->curregs[R13] = (brg >> 8) & 0xff;
                up->curregs[R14] = BRENAB;
                __load_zsregs(channel, up->curregs);
                /* set master interrupt enable */
                write_zsreg(channel, R9, up->curregs[R9]);

                spin_unlock_irqrestore(&up->port.lock, flags);
        }
}

static int __init ip22zilog_ports_init(void)
{
        int ret;

        printk(KERN_INFO "Serial: IP22 Zilog driver (%d chips).\n", NUM_IP22ZILOG);

        ip22zilog_prepare();

        if (request_irq(zilog_irq, ip22zilog_interrupt, 0,
                        "IP22-Zilog", ip22zilog_irq_chain)) {
                panic("IP22-Zilog: Unable to register zs interrupt handler.\n");
        }

        ip22zilog_init_hw();

        ret = uart_register_driver(&ip22zilog_reg);
        if (ret == 0) {
                int i;

                for (i = 0; i < NUM_CHANNELS; i++) {
                        struct uart_ip22zilog_port *up = &ip22zilog_port_table[i];

                        uart_add_one_port(&ip22zilog_reg, &up->port);
                }
        }

        return ret;
}

static int __init ip22zilog_init(void)
{
        /* IP22 Zilog setup is hard coded, no probing to do.  */
        ip22zilog_alloc_tables();
        ip22zilog_ports_init();

        return 0;
}

static void __exit ip22zilog_exit(void)
{
        int i;

        for (i = 0; i < NUM_CHANNELS; i++) {
                struct uart_ip22zilog_port *up = &ip22zilog_port_table[i];

                uart_remove_one_port(&ip22zilog_reg, &up->port);
        }

        uart_unregister_driver(&ip22zilog_reg);
}

module_init(ip22zilog_init);
module_exit(ip22zilog_exit);

/* David wrote it but I'm to blame for the bugs ...  */
MODULE_AUTHOR("Ralf Baechle <ralf@linux-mips.org>");
MODULE_DESCRIPTION("SGI Zilog serial port driver");
MODULE_LICENSE("GPL");