text: #include <linux/console.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/string.h>
  #include <linux/screen_info.h>
  #include <asm/io.h>
  #include <asm/processor.h>
  #include <asm/fcntl.h>
  #include <asm/setup.h>
  #include <xen/hvc-console.h>
  
  /* Simple VGA output */
  #define VGABASE		(__ISA_IO_base + 0xb8000)
  
  static int max_ypos = 25, max_xpos = 80;
  static int current_ypos = 25, current_xpos = 0;
  
  static void early_vga_write(struct console *con, const char *str, unsigned n)
  {
  	char c;
  	int  i, k, j;
  
  	while ((c = *str++) != '\0' && n-- > 0) {
  		if (current_ypos >= max_ypos) {
  			/* scroll 1 line up */
  			for (k = 1, j = 0; k < max_ypos; k++, j++) {
  				for (i = 0; i < max_xpos; i++) {
  					writew(readw(VGABASE+2*(max_xpos*k+i)),
  					       VGABASE + 2*(max_xpos*j + i));
  				}
  			}
  			for (i = 0; i < max_xpos; i++)
  				writew(0x720, VGABASE + 2*(max_xpos*j + i));
  			current_ypos = max_ypos-1;
  		}
  		if (c == '\n') {
  			current_xpos = 0;
  			current_ypos++;
  		} else if (c != '\r')  {
  			writew(((0x7 << 8) | (unsigned short) c),
  			       VGABASE + 2*(max_xpos*current_ypos +
  						current_xpos++));
  			if (current_xpos >= max_xpos) {
  				current_xpos = 0;
  				current_ypos++;
  			}
  		}
  	}
  }
  
  static struct console early_vga_console = {
  	.name =		"earlyvga",
  	.write =	early_vga_write,
  	.flags =	CON_PRINTBUFFER,
  	.index =	-1,
  };
  
  /* Serial functions loosely based on a similar package from Klaus P. Gerlicher */
  
  static int early_serial_base = 0x3f8;  /* ttyS0 */
  
  #define XMTRDY          0x20
  
  #define DLAB		0x80
  
  #define TXR             0       /*  Transmit register (WRITE) */
  #define RXR             0       /*  Receive register  (READ)  */
  #define IER             1       /*  Interrupt Enable          */
  #define IIR             2       /*  Interrupt ID              */
  #define FCR             2       /*  FIFO control              */
  #define LCR             3       /*  Line control              */
  #define MCR             4       /*  Modem control             */
  #define LSR             5       /*  Line Status               */
  #define MSR             6       /*  Modem Status              */
  #define DLL             0       /*  Divisor Latch Low         */
  #define DLH             1       /*  Divisor latch High        */
  
  static int early_serial_putc(unsigned char ch)
  {
  	unsigned timeout = 0xffff;
  	while ((inb(early_serial_base + LSR) & XMTRDY) == 0 && --timeout)
  		cpu_relax();
  	outb(ch, early_serial_base + TXR);
  	return timeout ? 0 : -1;
  }
  
  static void early_serial_write(struct console *con, const char *s, unsigned n)
  {
  	while (*s && n-- > 0) {
  		if (*s == '\n')
  			early_serial_putc('\r');
  		early_serial_putc(*s);
  		s++;
  	}
  }
  
  #define DEFAULT_BAUD 9600
  
  static __init void early_serial_init(char *s)
  {
  	unsigned char c;
  	unsigned divisor;
  	unsigned baud = DEFAULT_BAUD;
  	char *e;
  
  	if (*s == ',')
  		++s;
  
  	if (*s) {
  		unsigned port;
  		if (!strncmp(s,"0x",2)) {
  			early_serial_base = simple_strtoul(s, &e, 16);
  		} else {
  			static int bases[] = { 0x3f8, 0x2f8 };
  
  			if (!strncmp(s,"ttyS",4))
  				s += 4;
  			port = simple_strtoul(s, &e, 10);
  			if (port > 1 || s == e)
  				port = 0;
  			early_serial_base = bases[port];
  		}
  		s += strcspn(s, ",");
  		if (*s == ',')
  			s++;
  	}
  
  	outb(0x3, early_serial_base + LCR);	/* 8n1 */
  	outb(0, early_serial_base + IER);	/* no interrupt */
  	outb(0, early_serial_base + FCR);	/* no fifo */
  	outb(0x3, early_serial_base + MCR);	/* DTR + RTS */
  
  	if (*s) {
  		baud = simple_strtoul(s, &e, 0);
  		if (baud == 0 || s == e)
  			baud = DEFAULT_BAUD;
  	}
  
  	divisor = 115200 / baud;
  	c = inb(early_serial_base + LCR);
  	outb(c | DLAB, early_serial_base + LCR);
  	outb(divisor & 0xff, early_serial_base + DLL);
  	outb((divisor >> 8) & 0xff, early_serial_base + DLH);
  	outb(c & ~DLAB, early_serial_base + LCR);
  }
  
  static struct console early_serial_console = {
  	.name =		"earlyser",
  	.write =	early_serial_write,
  	.flags =	CON_PRINTBUFFER,
  	.index =	-1,
  };
  
  /* Console interface to a host file on AMD's SimNow! */
  
  static int simnow_fd;
  
  enum {
  	MAGIC1 = 0xBACCD00A,
  	MAGIC2 = 0xCA110000,
  	XOPEN = 5,
  	XWRITE = 4,
  };
  
  static noinline long simnow(long cmd, long a, long b, long c)
  {
  	long ret;
  	asm volatile("cpuid" :
  		     "=a" (ret) :
  		     "b" (a), "c" (b), "d" (c), "0" (MAGIC1), "D" (cmd + MAGIC2));
  	return ret;
  }
  
  static void __init simnow_init(char *str)
  {
  	char *fn = "klog";
  	if (*str == '=')
  		fn = ++str;
  	/* error ignored */
  	simnow_fd = simnow(XOPEN, (unsigned long)fn, O_WRONLY|O_APPEND|O_CREAT, 0644);
  }
  
  static void simnow_write(struct console *con, const char *s, unsigned n)
  {
  	simnow(XWRITE, simnow_fd, (unsigned long)s, n);
  }
  
  static struct console simnow_console = {
  	.name =		"simnow",
  	.write =	simnow_write,
  	.flags =	CON_PRINTBUFFER,
  	.index =	-1,
  };
  
  /* Direct interface for emergencies */
  struct console *early_console = &early_vga_console;
  static int early_console_initialized = 0;
  
  void early_printk(const char *fmt, ...)
  {
  	char buf[512];
  	int n;
  	va_list ap;
  
  	va_start(ap,fmt);
  	n = vscnprintf(buf,512,fmt,ap);
  	early_console->write(early_console,buf,n);
  	va_end(ap);
  }
  
  static int __initdata keep_early;
  
  static int __init setup_early_printk(char *buf)
  {
  	if (!buf)
  		return 0;
  
  	if (early_console_initialized)
  		return 0;
  	early_console_initialized = 1;
  
  	if (strstr(buf, "keep"))
  		keep_early = 1;
  
  	if (!strncmp(buf, "serial", 6)) {
  		early_serial_init(buf + 6);
  		early_console = &early_serial_console;
  	} else if (!strncmp(buf, "ttyS", 4)) {
  		early_serial_init(buf);
  		early_console = &early_serial_console;
  	} else if (!strncmp(buf, "vga", 3)
  	           && boot_params.screen_info.orig_video_isVGA == 1) {
  		max_xpos = boot_params.screen_info.orig_video_cols;
  		max_ypos = boot_params.screen_info.orig_video_lines;
  		current_ypos = boot_params.screen_info.orig_y;
  		early_console = &early_vga_console;
   	} else if (!strncmp(buf, "simnow", 6)) {
   		simnow_init(buf + 6);
   		early_console = &simnow_console;
   		keep_early = 1;
  #ifdef CONFIG_HVC_XEN
  	} else if (!strncmp(buf, "xen", 3)) {
  		early_console = &xenboot_console;
  #endif
  	}
  
  	if (keep_early)
  		early_console->flags &= ~CON_BOOT;
  	else
  		early_console->flags |= CON_BOOT;
  	register_console(early_console);
  	return 0;
  }
  early_param("earlyprintk", setup_early_printk);
  