x86, setup: Early-boot serial I/O support

This patch adds serial I/O support to the real-mode setup (very early
boot) printf(). It's useful for debugging boot code when running Linux
under KVM, for example. The actual code was lifted from early printk.

Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1278835617-11368-1-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 105 insertions, 6 deletions

diff --git a/arch/x86/boot/tty.c b/arch/x86/boot/tty.c
index 01ec69c..f3ceee2 100644
--- a/arch/x86/boot/tty.c
+++ b/arch/x86/boot/tty.c
@@ -10,23 +10,51 @@
  * ----------------------------------------------------------------------- */
 
 /*
- * Very simple screen I/O
- * XXX: Probably should add very simple serial I/O?
+ * Very simple screen and serial I/O
  */
 
 #include "boot.h"
 
+#define DEFAULT_SERIAL_PORT 0x3f8 /* ttyS0 */
+
+static int	early_serial_base;
+
+#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        */
+
+#define DEFAULT_BAUD 9600
+
 /*
  * These functions are in .inittext so they can be used to signal
  * error during initialization.
  */
 
-void __attribute__((section(".inittext"))) putchar(int ch)
+static void __attribute__((section(".inittext"))) serial_putchar(int ch)
 {
-	struct biosregs ireg;
+	unsigned timeout = 0xffff;
 
-	if (ch == '\n')
-		putchar('\r');	/* \n -> \r\n */
+	while ((inb(early_serial_base + LSR) & XMTRDY) == 0 && --timeout)
+		cpu_relax();
+
+	outb(ch, early_serial_base + TXR);
+}
+
+static void __attribute__((section(".inittext"))) bios_putchar(int ch)
+{
+	struct biosregs ireg;
 
 	initregs(&ireg);
 	ireg.bx = 0x0007;
@@ -36,6 +64,17 @@ void __attribute__((section(".inittext"))) putchar(int ch)
 	intcall(0x10, &ireg, NULL);
 }
 
+void __attribute__((section(".inittext"))) putchar(int ch)
+{
+	if (ch == '\n')
+		putchar('\r');	/* \n -> \r\n */
+
+	bios_putchar(ch);
+
+	if (early_serial_base != 0)
+		serial_putchar(ch);
+}
+
 void __attribute__((section(".inittext"))) puts(const char *str)
 {
 	while (*str)
@@ -112,3 +151,63 @@ int getchar_timeout(void)
 
 	return 0;		/* Timeout! */
 }
+
+static void early_serial_init(int baud)
+{
+	unsigned char c;
+	unsigned divisor;
+
+	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 */
+
+	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);
+}
+
+void console_init(void)
+{
+	int baud = DEFAULT_BAUD;
+	char arg[32];
+	int pos = 0;
+
+	if (cmdline_find_option("earlyprintk", arg, sizeof arg) > 0) {
+		char *e;
+
+		if (!strncmp(arg, "serial", 6)) {
+			early_serial_base = DEFAULT_SERIAL_PORT;
+			pos += 6;
+		}
+
+		if (arg[pos] == ',')
+			pos++;
+
+		if (!strncmp(arg, "ttyS", 4)) {
+			static const int bases[] = { 0x3f8, 0x2f8 };
+			int port = 0;
+
+			if (!strncmp(arg + pos, "ttyS", 4))
+				pos += 4;
+
+			if (arg[pos++] == '1')
+				port = 1;
+
+			early_serial_base = bases[port];
+		}
+
+		if (arg[pos] == ',')
+			pos++;
+
+		baud = simple_strtoull(arg + pos, &e, 0);
+		if (baud == 0 || arg + pos == e)
+			baud = DEFAULT_BAUD;
+	}
+
+	if (early_serial_base != 0)
+		early_serial_init(baud);
+}