[RFC/PATCH] x86: Early-boot serial I/O support

[RFC/PATCH] x86: Early-boot serial I/O support

[Pekka Enberg]

This patch adds serial I/O support to 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: Ingo Molnar <mingo@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---
 arch/x86/boot/boot.h |    1 +
 arch/x86/boot/main.c |    3 ++
 arch/x86/boot/tty.c  |   65 +++++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 63 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
index 98239d2..5b37014 100644
--- a/arch/x86/boot/boot.h
+++ b/arch/x86/boot/boot.h
@@ -333,6 +333,7 @@ size_t strnlen(const char *s, size_t maxlen);
 unsigned int atou(const char *s);
 
 /* tty.c */
+void serial_init(void);
 void puts(const char *);
 void putchar(int);
 int getchar(void);
diff --git a/arch/x86/boot/main.c b/arch/x86/boot/main.c
index 140172b..1437efe 100644
--- a/arch/x86/boot/main.c
+++ b/arch/x86/boot/main.c
@@ -127,6 +127,9 @@ static void init_heap(void)
 
 void main(void)
 {
+	/* Initialize serial console */
+	serial_init();
+
 	/* First, copy the boot header into the "zeropage" */
 	copy_boot_params();
 
diff --git a/arch/x86/boot/tty.c b/arch/x86/boot/tty.c
index 01ec69c..2f0def5 100644
--- a/arch/x86/boot/tty.c
+++ b/arch/x86/boot/tty.c
@@ -10,8 +10,7 @@
  * ----------------------------------------------------------------------- */
 
 /*
- * Very simple screen I/O
- * XXX: Probably should add very simple serial I/O?
+ * Very simple screen and serial I/O
  */
 
 #include "boot.h"
@@ -20,13 +19,58 @@
  * These functions are in .inittext so they can be used to signal
  * error during initialization.
  */
+static int early_serial_base = 0x3f8;  /* ttyS0 */
 
-void __attribute__((section(".inittext"))) putchar(int ch)
+#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
+
+void __attribute__((section(".inittext"))) serial_init(void)
 {
-	struct biosregs ireg;
+	unsigned baud = DEFAULT_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);
+}
 
-	if (ch == '\n')
-		putchar('\r');	/* \n -> \r\n */
+static void __attribute__((section(".inittext"))) serial_putchar(int ch)
+{
+	unsigned timeout = 0xffff;
+
+	while ((inb(early_serial_base + LSR) & XMTRDY) == 0 && --timeout)
+		;
+
+	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 +80,15 @@ 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);
+	serial_putchar(ch);
+}
+
 void __attribute__((section(".inittext"))) puts(const char *str)
 {
 	while (*str)
-- 
1.6.3.3

Re: [RFC/PATCH] x86: Early-boot serial I/O support

[Yinghai Lu]

On Sat, Jul 10, 2010 at 11:07 AM, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> This patch adds serial I/O support to 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: Ingo Molnar <mingo@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
> ---
>  arch/x86/boot/boot.h |    1 +
>  arch/x86/boot/main.c |    3 ++
>  arch/x86/boot/tty.c  |   65 +++++++++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 63 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index 98239d2..5b37014 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -333,6 +333,7 @@ size_t strnlen(const char *s, size_t maxlen);
>  unsigned int atou(const char *s);
>
>  /* tty.c */
> +void serial_init(void);
>  void puts(const char *);
>  void putchar(int);
>  int getchar(void);
> diff --git a/arch/x86/boot/main.c b/arch/x86/boot/main.c
> index 140172b..1437efe 100644
> --- a/arch/x86/boot/main.c
> +++ b/arch/x86/boot/main.c
> @@ -127,6 +127,9 @@ static void init_heap(void)
>
>  void main(void)
>  {
> +       /* Initialize serial console */
> +       serial_init();
> +
>        /* First, copy the boot header into the "zeropage" */
>        copy_boot_params();
>
> diff --git a/arch/x86/boot/tty.c b/arch/x86/boot/tty.c
> index 01ec69c..2f0def5 100644
> --- a/arch/x86/boot/tty.c
> +++ b/arch/x86/boot/tty.c
> @@ -10,8 +10,7 @@
>  * ----------------------------------------------------------------------- */
>
>  /*
> - * Very simple screen I/O
> - * XXX: Probably should add very simple serial I/O?
> + * Very simple screen and serial I/O
>  */
>
>  #include "boot.h"
> @@ -20,13 +19,58 @@
>  * These functions are in .inittext so they can be used to signal
>  * error during initialization.
>  */
> +static int early_serial_base = 0x3f8;  /* ttyS0 */

it seems you should analyze the command line to get io port and baud rate.

earlycon=
console=uart8250...
console=ttyS0 etc.

cmdline_find_option could be used with this.

Thanks

Yinghai

Re: [RFC/PATCH] x86: Early-boot serial I/O support

[H. Peter Anvin]

On 07/10/2010 11:07 AM, Pekka Enberg wrote:
> This patch adds serial I/O support to 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.

Yinghai is correct that this really needs to pick apart the command 
line, especially since there is already support for command-line parsing 
in the early boot code.  I suggest we just reuse the earlyprintk= 
option, especially since it's basically the same code and so should have 
the same failure profile.

> +
> +	while ((inb(early_serial_base + LSR)&  XMTRDY) == 0&&  --timeout)
> +		;
> +

cpu_relax(); here, probably.

	-hpa

Re: [RFC/PATCH] x86: Early-boot serial I/O support

[Pekka Enberg]

H. Peter Anvin wrote:
>> +
>> +    while ((inb(early_serial_base + LSR)&  XMTRDY) == 0&&  --timeout)
>> +        ;
>> +
> 
> cpu_relax(); here, probably.

Fixed.