linux-arkmicro/linux/arch/mips/kernel/early_printk_8250.c

/*
 *  8250/16550-type serial ports prom_putchar()
 *
 *  Copyright (C) 2010  Yoichi Yuasa <yuasa@linux-mips.org>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */
#include <linux/io.h>
#include <linux/serial_core.h>
#include <linux/serial_reg.h>
#include <asm/setup.h>

static void __iomem *serial8250_base;
static unsigned int serial8250_reg_shift;
static unsigned int serial8250_tx_timeout;

void setup_8250_early_printk_port(unsigned long base, unsigned int reg_shift,
				  unsigned int timeout)
{
	serial8250_base = (void __iomem *)base;
	serial8250_reg_shift = reg_shift;
	serial8250_tx_timeout = timeout;
}

static inline u8 serial_in(int offset)
{
	return readb(serial8250_base + (offset << serial8250_reg_shift));
}

static inline void serial_out(int offset, char value)
{
	writeb(value, serial8250_base + (offset << serial8250_reg_shift));
}

void prom_putchar(char c)
{
	unsigned int timeout;
	int status, bits;

	if (!serial8250_base)
		return;

	timeout = serial8250_tx_timeout;
	bits = UART_LSR_TEMT | UART_LSR_THRE;

	do {
		status = serial_in(UART_LSR);

		if (--timeout == 0)
			break;
	} while ((status & bits) != bits);

	if (timeout)
		serial_out(UART_TX, c);
}