kernel_samsung_a34x-permissive/drivers/tty/serial/ip22zilog.c

// SPDX-License-Identifier: GPL-2.0
/*
 * 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/module.h>
#include <linux/kernel.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"

/*
 * 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
#define IP22ZILOG_FLAG_RESET_DONE	0x00000200

	unsigned int			tty_break;

	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);
		}
	}
}

#define Rx_BRK 0x0100                   /* BREAK event software flag.  */
#define Rx_SYS 0x0200                   /* SysRq event software flag.  */

static bool ip22zilog_receive_chars(struct uart_ip22zilog_port *up,
						  struct zilog_channel *channel)
{
	unsigned char ch, flag;
	unsigned int r1;
	bool push = up->port.state != NULL;

	for (;;) {
		ch = readb(&channel->control);
		ZSDELAY();
		if (!(ch & Rx_CH_AV))
			break;

		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->data);
		ZSDELAY();

		ch &= up->parity_mask;

		/* Handle the null char got when BREAK is removed.  */
		if (!ch)
			r1 |= up->tty_break;

		/* A real serial line, record the character and status.  */
		flag = TTY_NORMAL;
		up->port.icount.rx++;
		if (r1 & (PAR_ERR | Rx_OVR | CRC_ERR | Rx_SYS | Rx_BRK)) {
			up->tty_break = 0;

			if (r1 & (Rx_SYS | Rx_BRK)) {
				up->port.icount.brk++;
				if (r1 & Rx_SYS)
					continue;
				r1 &= ~(PAR_ERR | CRC_ERR);
			}
			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 & Rx_BRK)
				flag = TTY_BREAK;
			else if (r1 & PAR_ERR)
				flag = TTY_PARITY;
			else if (r1 & CRC_ERR)
				flag = TTY_FRAME;
		}

		if (uart_handle_sysrq_char(&up->port, ch))
			continue;

		if (push)
			uart_insert_char(&up->port, r1, Rx_OVR, ch, flag);
	}
	return push;
}

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

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

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

	if (up->curregs[R15] & BRKIE) {
		if ((status & BRK_ABRT) && !(up->prev_status & BRK_ABRT)) {
			if (uart_handle_break(&up->port))
				up->tty_break = Rx_SYS;
			else
				up->tty_break = Rx_BRK;
		}
	}

	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 ^ up->prev_status) ^ DCD)
			uart_handle_dcd_change(&up->port,
					       (status & DCD));
		if ((status ^ up->prev_status) ^ CTS)
			uart_handle_cts_change(&up->port,
					       (status & CTS));

		wake_up_interruptible(&up->port.state->port.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 becoming 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.state == NULL)
		goto ack_tx_int;
	xmit = &up->port.state->xmit;
	if (uart_circ_empty(xmit))
		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 uart_ip22zilog_port *up = dev_id;

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

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

		if (push)
			tty_flip_buffer_push(&up->port.state->port);

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

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

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

		if (push)
			tty_flip_buffer_push(&up->port.state->port);

		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 char status;

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

	return status;
}

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

	spin_lock_irqsave(&port->lock, flags);

	status = ip22zilog_read_channel_status(port);

	spin_unlock_irqrestore(&port->lock, flags);

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

	return ret;
}

/* The port lock is held and interrupts are disabled.  */
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 =
		container_of(port, 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)
{
	struct uart_ip22zilog_port *up =
		container_of(port, 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)
{
	struct uart_ip22zilog_port *up =
		container_of(port, 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->state->xmit;

		if (uart_circ_empty(xmit))
			return;
		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 =
		container_of(port, 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 =
		container_of(port, 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_reset(struct uart_ip22zilog_port *up)
{
	struct zilog_channel *channel;
	int i;

	if (up->flags & IP22ZILOG_FLAG_RESET_DONE)
		return;

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

	if (!ZS_IS_CHANNEL_A(up)) {
		up++;
		channel = ZILOG_CHANNEL_FROM_PORT(&up->port);
	}
	write_zsreg(channel, R9, FHWRES);
	ZSDELAY_LONG();
	(void) read_zsreg(channel, R0);

	up->flags |= IP22ZILOG_FLAG_RESET_DONE;
	up->next->flags |= IP22ZILOG_FLAG_RESET_DONE;
}

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

	channel = ZILOG_CHANNEL_FROM_PORT(&up->port);

	__ip22zilog_reset(up);

	__load_zsregs(channel, up->curregs);
	/* set master interrupt enable */
	write_zsreg(channel, R9, up->curregs[R9]);
	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 & (IGNBRK | 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 ktermios *termios,
		      struct ktermios *old)
{
	struct uart_ip22zilog_port *up =
		container_of(port, 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;

	ip22zilog_maybe_update_regs(up, ZILOG_CHANNEL_FROM_PORT(port));
	uart_update_timeout(port, termios->c_cflag, baud);

	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 const 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)
{
	return kzalloc(size, GFP_KERNEL);
}

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 uart_port *port, int ch)
{
	struct zilog_channel *channel = ZILOG_CHANNEL_FROM_PORT(port);
	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];
	unsigned long flags;

	spin_lock_irqsave(&up->port.lock, flags);
	uart_console_write(&up->port, s, count, ip22zilog_put_char);
	udelay(2);
	spin_unlock_irqrestore(&up->port.lock, flags);
}

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 = 9600, bits = 8;
	int parity = 'n';
	int flow = 'n';

	up->flags |= IP22ZILOG_FLAG_IS_CONS;

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

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

	up->curregs[R15] |= BRKIE;

	__ip22zilog_startup(up);

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

	if (options)
		uart_parse_options(options, &baud, &parity, &bits, &flow);
	return uart_set_options(&up->port, con, baud, parity, bits, flow);
}

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",
	.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.line = (chip * 2) + 1;
		up[(chip * 2) + 1].flags |= IP22ZILOG_FLAG_IS_CHANNEL_A;
	}

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

		/* 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;
		brg = BPS_TO_BRG(9600, ZS_CLOCK / ZS_CLOCK_DIVISOR);
		up->curregs[R12] = (brg & 0xff);
		up->curregs[R13] = (brg >> 8) & 0xff;
		up->curregs[R14] = BRENAB;
	}
}

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");
	}

	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;
	struct uart_ip22zilog_port *up;

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

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

	/* Free IO mem */
	up = &ip22zilog_port_table[0];
	for (i = 0; i < NUM_IP22ZILOG; i++) {
		if (up[(i * 2) + 0].port.mapbase) {
		   iounmap((void*)up[(i * 2) + 0].port.mapbase);
		   up[(i * 2) + 0].port.mapbase = 0;
		}
		if (up[(i * 2) + 1].port.mapbase) {
			iounmap((void*)up[(i * 2) + 1].port.mapbase);
			up[(i * 2) + 1].port.mapbase = 0;
		}
	}

	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");