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How-to loader.o to preform a kexec like operations

loader.o allow you to run a custom kernel, without the danger of having to reprogram your Flash ROM. It loads a new kernel into memory and jumps to it directly.

Compiling loader.o

  1. Get loader.o original source and optionally patch for initrd problem.
  2. Build a toolchain for cross compiling.
  3. In Makefile point INCLUDE = -isystem parameter to a 2.4.21-pre4 kernel header files!
  4. Fix gcc and ld entry in Makefile to meet your cross compiled toolchain.
  5. Apply initrd patch if you like.
  6. Run make

Use pre-compiled loader.o

If you think that compiling loader.o by yourself is too much hassle then you can use a pre-compiled loader.o

Preparing proper kernel image for loading

For kernel building instructions look kernel building wiki page.

Before you start compiling kernel you should apply this patch, which is going to fix memory detection problem. 

--- arch/ppc/platforms/sandpoint_setup.c.orig	2007-08-09 18:02:58.000000000 -0400
+++ arch/ppc/platforms/sandpoint_setup.c	2007-08-09 18:03:41.000000000 -0400
@@ -451,8 +451,8 @@
 
 	/* This might be fixed in DINK32 12.4, or we'll have another
 	 * way to determine the correct memory size anyhow. */
-	/* return mpc10x_get_mem_size(MPC10X_MEM_MAP_B); */
-	return 16*1024*1024;
+	return mpc10x_get_mem_size(MPC10X_MEM_MAP_B);
+	/*return 16*1024*1024;*/
 }
 
 static void __init

If you would like to use custom cmdline parameter for your kernel, for example “root=/dev/sda3” then you will also need this patch:

  1. Search for ”#endif /* CONFIG_BLK_DEV_INITRD */” in arch/ppc/platform/sandpoint_setup.c
  2. Add following lines after ”#endif /* CONFIG_BLK_DEV_INITRD */” line:
        if ( r6 )
        {
               *(char *)(r7+KERNELBASE) = 0;
               strcpy(cmd_line, (char *)(r6+KERNELBASE));
        }

After you have successfully built a kernel image you should note that the loader need the kernel to be in binary format. It can be done with powerpc-linux-objcopy command. 

powerpc-linux-objcopy -O binary vmlinux

Using loader.o to load a kernel image

sync;insmod /mnt/HD_a2/loader.o kernel=/mnt/HD_a2/vmlinux

or 

sync;insmod /mnt/HD_a2/loader.o kernel=/mnt/HD_a2/vmlinux cmdline="root=/dev/sda3"

Use pre-compiled vmlinux

Pre-compiled vmlinux is available at here. It does support custom cmdline parameter. I does have usb and usb storage modules built in, so it is possible to load operating system from usb stick. This kernel image is optimized to boot Gentoo linux. .config is available at here.
To load from usb stick: 

sync;insmod /mnt/HD_a2/loader.o kernel=/mnt/HD_a2/vmlinux cmdline="root=/dev/sdb1"

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