rpi-boot
--------

A simple second stage boot loader for the Raspberry Pi to assist in the
development of hobby kernels.


Features
--------

* Supports booting flat binary files, ELF executables and ELF and a.out
executables with a Multiboot header.  Linux kernels are currently not
supported (but support is planned for a future version).

* Supports FAT(16/32) and ext2 filesystems.

* Supports loading additional modules when in Multiboot mode.

* Proviedes functions to the loaded kernel to allow it to easily
access the framebuffer (via a printf() interface) and the filesystem
(via fopen/fread/fclose/opendir/readdir/closedir).


Installation
------------

Edit config.h to your needs to enable/disable certain functionality.

Run make in the source directory and copy the kernel.img file to the SD card
containing your Raspian distribution.  You are recommended to backup your
original kernel.img file first.


Usage
-----

The rpi-boot bootloader will initially search for files called
/boot/rpi_boot.cfg, /boot/rpi-boot.cfg and /boot/grub/grub.cfg (in that order)
on the first partition of the SD card.  If one is not found, it searches for
those files on each filesystem known to the system.

It understands the following commands:

multiboot <kernel> [cmdline]
	- Load a multiboot compliant kernel

module <file> [name]
	- Load an additional Multiboot module

kernel <kernel>
	- Load a non-multiboot compliant kernel

boot
	- Boot the kernel

binary_load_addr <address>
	- Set the load address for a flat binary file subsequently loaded by the
		'kernel' command

entry_addr <address>
	- Set the entry point that is jumped to by a subsequent 'boot' command

console_log <file>[+] [buffer_size]
	- Copy the console log to a file.  If '+' is specified after the file name
		then append to the file instead of overwriting it.  If 'buffer_size'
		is specified then it is the number of bytes to buffer before writing
		to the file.  This prevents the log quickly using up all the write
		cycles on solid state media, but runs the risk of some messages being
		lost on a kernel crash.  Call fflush(get_log_file()) from the guest
		OS to manually flush the buffer.  Support for console_log requires
		that ENABLE_CONSOLE_LOGFILE be enabled in config.h


System state on kernel start
----------------------------

For Multiboot kernels, the system state is defined in MULTIBOOT-ARM, otherwise:

r0 will be 0
r1 will be the ARM machine type
r2 will be the address of the ATAGs

(the above 3 are identical to the ARM Linux boot protocol)

r3 will be the address of the multiboot_arm_functions structure as defined in
MULTIBOOT-ARM.

See the test-kernel directory for an example.


Caveats
-------

rpi-boot occupies the first 1 MiB of memory for itself.  If you wish to make
use of the multiboot_arm_functions functions you need to preserve this.  If
not you can overwrite it from within your kernel.  If you do overwrite it,
you should make sure you have already obtained all the information you need
from the various Multiboot structures (or copied them elsewhere).

Because of this, rpi-boot will not boot kernels which are linked below 1 MiB.