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From: Keith Owens <kaos@ocs.com.au>
To: Adrian Bunk <bunk@fs.tum.de>
Cc: Marcelo Tosatti <marcelo@conectiva.com.br>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: Linux 2.4.18pre3 
In-Reply-To: Your message of "Fri, 11 Jan 2002 00:44:30 BST."
             <Pine.NEB.4.44.0201110040420.13965-100000@mimas.fachschaften.tu-muenchen.de> 
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Date: Fri, 11 Jan 2002 16:50:42 +1100
Message-ID: <3419.1010728242@kao2.melbourne.sgi.com>
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On Fri, 11 Jan 2002 00:44:30 +0100 (CET), 
Adrian Bunk <bunk@fs.tum.de> wrote:
>something changed between pre2 and pre3 that broke the booting of
>non-modular kernels. After LILO's "Loading Linux" message nothing else
>happens. When I enable CONFIG_MODULES my kernel boots.

Works for me.
# uname -a
Linux snowy 2.4.18-pre3 #2 SMP Fri Jan 11 16:34:08 EST 2002 i686 unknown
# depmod
depmod: QM_MODULES: Function not implemented
# lilo -V
LILO version 21

Time for another version of my VIDEO_CHAR debugging patch.

If a kernel hangs early in the boot process (before the console has
been initialized) then printk is no use because you never see the
output.  There is a technique for using the video display to indicate
boot progress so you can localize the problem.  Reporting "my kernel
hangs during boot at line nnn in routine xyz" is a lot better than "my
kernel hangs during boot".

The idea is to write characters direct to the video screen during
booting using a macro called VIDEO_CHAR.  This macro takes a character
position and a single character value to be displayed.  Use different
positions on the screen for different levels of code and use different
characters in one position to indicate which stage that level is up to.
For example, with the patch below, the string EAC at hang indicates
parse_options(), checksetup().

Other people have patches for early printk output and those patches
might be better for your needs.  The beauty of VIDEO_CHAR is that it
does not require any registers on ix86, so it can be used in Assembler
without save/restore hassles.

The patch below is generic, except for the definition of VIDEO_CHAR
which is ix86 specific.  If this patch ever becomes part of the main
kernel then VIDEO_CHAR needs to be moved to an arch specific header.
If any arch other than ix86 uses this technique, please mail
kaos@ocs.com.au with your definition of VIDEO_CHAR.

You can plant VIDEO_CHAR calls anywhere you like, up to the call to
mem_init().  After mem_init has done its work and memory has been
remapped, VIDEO_CHAR cannot write to video memory, it will oops.
However by then the console has been initialized so you can use printk.

Demonstration patch against 2.4.18-pre3.  This only uses screen
positions 0, 1, 2.  If you want to drill down into lower level
routines, just use screen positions 3 onwards.  To activate the
debugging, add
  #define DEBUG_VIDEO_CHAR
to the start of init/main.c.  If the machine is hanging then a zero
delay is fine, if it is rebooting then you need a delay to note the
characters in the top left hand corner of the screen before it reboots.
  #define VIDEO_CHAR_DELAY_COUNT 100000000
A value around your clock speed gives a delay of approx. 1 second.

Index: 18-pre3.1/init/main.c
--- 18-pre3.1/init/main.c Sat, 01 Dec 2001 11:29:21 +1100 kaos (linux-2.4/k/11_main.c 1.1.5.1.1.8.1.3.1.8 644)
+++ 18-pre3.1(w)/init/main.c Fri, 11 Jan 2002 16:46:41 +1100 kaos (linux-2.4/k/11_main.c 1.1.5.1.1.8.1.3.1.8 644)
@@ -79,6 +79,16 @@ extern int irda_device_init(void);
 #error Sorry, your GCC is too old. It builds incorrect kernels.
 #endif
 
+#ifdef DEBUG_VIDEO_CHAR
+#ifndef VIDEO_CHAR_DELAY_COUNT
+#define VIDEO_CHAR_DELAY_COUNT 0
+#endif
+/* ix86 specific */
+#define VIDEO_CHAR(c, v) { int i; *((volatile char *)(0xb8000 + 2*(c))) = (v); for (i = 0; i < VIDEO_CHAR_DELAY_COUNT; ++i) ; }
+#else
+#define VIDEO_CHAR(c, v)
+#endif
+
 extern char _stext, _etext;
 extern char *linux_banner;
 
@@ -425,12 +435,14 @@ static void __init parse_options(char *l
 	char *next,*quote;
 	int args, envs;
 
+	VIDEO_CHAR(1, 'A');
 	if (!*line)
 		return;
 	args = 0;
 	envs = 1;	/* TERM is set to 'linux' by default */
 	next = line;
 	while ((line = next) != NULL) {
+		VIDEO_CHAR(2, 'A');
                 quote = strchr(line,'"');
                 next = strchr(line, ' ');
                 while (next != NULL && quote != NULL && quote < next) {
@@ -443,9 +455,11 @@ static void __init parse_options(char *l
                                 next = strchr(next+1, ' ');
                         }
                 }
+		VIDEO_CHAR(2, 'B');
                 if (next != NULL)
                         *next++ = 0;
 		if (!strncmp(line,"init=",5)) {
+			VIDEO_CHAR(3, 'A');
 			line += 5;
 			execute_command = line;
 			/* In case LILO is going to boot us with default command line,
@@ -456,8 +470,10 @@ static void __init parse_options(char *l
 			args = 0;
 			continue;
 		}
+		VIDEO_CHAR(2, 'C');
 		if (checksetup(line))
 			continue;
+		VIDEO_CHAR(2, 'D');
 		
 		/*
 		 * Then check if it's an environment variable or
@@ -473,9 +489,12 @@ static void __init parse_options(char *l
 			if (*line)
 				argv_init[++args] = line;
 		}
+		VIDEO_CHAR(2, 'E');
 	}
+	VIDEO_CHAR(1, 'B');
 	argv_init[args+1] = NULL;
 	envp_init[envs+1] = NULL;
+	VIDEO_CHAR(1, 'C');
 }
 
 
@@ -548,16 +567,27 @@ asmlinkage void __init start_kernel(void
  * Interrupts are still disabled. Do necessary setups, then
  * enable them
  */
+	VIDEO_CHAR(0, 'A');
 	lock_kernel();
+	VIDEO_CHAR(0, 'B');
 	printk(linux_banner);
+	VIDEO_CHAR(0, 'C');
 	setup_arch(&command_line);
+	VIDEO_CHAR(0, 'D');
 	printk("Kernel command line: %s\n", saved_command_line);
+	VIDEO_CHAR(0, 'E');
 	parse_options(command_line);
+	VIDEO_CHAR(0, 'F');
 	trap_init();
+	VIDEO_CHAR(0, 'G');
 	init_IRQ();
+	VIDEO_CHAR(0, 'H');
 	sched_init();
+	VIDEO_CHAR(0, 'I');
 	softirq_init();
+	VIDEO_CHAR(0, 'J');
 	time_init();
+	VIDEO_CHAR(0, 'K');
 
 	/*
 	 * HACK ALERT! This is early. We're enabling the console before
@@ -565,8 +595,10 @@ asmlinkage void __init start_kernel(void
 	 * this. But we do want output early, in case something goes wrong.
 	 */
 	console_init();
+	VIDEO_CHAR(0, 'L');
 #ifdef CONFIG_MODULES
 	init_modules();
+	VIDEO_CHAR(0, 'M');
 #endif
 	if (prof_shift) {
 		unsigned int size;
@@ -577,10 +609,14 @@ asmlinkage void __init start_kernel(void
 		size = prof_len * sizeof(unsigned int) + PAGE_SIZE-1;
 		prof_buffer = (unsigned int *) alloc_bootmem(size);
 	}
+	VIDEO_CHAR(0, 'N');
 
 	kmem_cache_init();
+	VIDEO_CHAR(0, 'O');
 	sti();
+	VIDEO_CHAR(0, 'P');
 	calibrate_delay();
+	VIDEO_CHAR(0, 'Q');
 #ifdef CONFIG_BLK_DEV_INITRD
 	if (initrd_start && !initrd_below_start_ok &&
 			initrd_start < min_low_pfn << PAGE_SHIFT) {
@@ -588,6 +624,7 @@ asmlinkage void __init start_kernel(void
 		    "disabling it.\n",initrd_start,min_low_pfn << PAGE_SHIFT);
 		initrd_start = 0;
 	}
+	VIDEO_CHAR(0, 'R');
 #endif
 	mem_init();
 	kmem_cache_sizes_init();

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