linux/arch/x86/include/asm/serial.h

#ifndef _ASM_X86_SERIAL_H
#define _ASM_X86_SERIAL_H

/*
 * This assumes you have a 1.8432 MHz clock for your UART.
 *
 * It'd be nice if someone built a serial card with a 24.576 MHz
 * clock, since the 16550A is capable of handling a top speed of 1.5
 * megabits/second; but this requires a faster clock.
 */
#define BASE_BAUD (1843200/16)

/* Standard COM flags (except for COM4, because of the 8514 problem) */
#ifdef CONFIG_SERIAL_DETECT_IRQ
# define STD_COMX_FLAGS	(ASYNC_BOOT_AUTOCONF |	ASYNC_SKIP_TEST	| ASYNC_AUTO_IRQ)
# define STD_COM4_FLAGS	(ASYNC_BOOT_AUTOCONF |	0		| ASYNC_AUTO_IRQ)
#else
# define STD_COMX_FLAGS	(ASYNC_BOOT_AUTOCONF |	ASYNC_SKIP_TEST	| 0		)
# define STD_COM4_FLAGS	(ASYNC_BOOT_AUTOCONF |	0		| 0		)
#endif

#define SERIAL_PORT_DFNS								\
	/* UART		CLK		PORT	IRQ	FLAGS			    */	\
	{ .uart = 0,	BASE_BAUD,	0x3F8,	4,	STD_COMX_FLAGS	}, /* ttyS0 */	\
	{ .uart = 0,	BASE_BAUD,	0x2F8,	3,	STD_COMX_FLAGS	}, /* ttyS1 */	\
	{ .uart = 0,	BASE_BAUD,	0x3E8,	4,	STD_COMX_FLAGS	}, /* ttyS2 */	\
	{ .uart = 0,	BASE_BAUD,	0x2E8,	3,	STD_COM4_FLAGS	}, /* ttyS3 */

#endif /* _ASM_X86_SERIAL_H */