/** @mainpage Universal Bootloader
@section uboot_v2_intro Introduction
This is u2boot, our proposal for a next generation of the famous U-Boot
bootloader. U-Boot offers an excellent choice as a bootloader for
today's embedded systems, seen from a user's point of view.
Nevertheless, there are quite some design flaws which turned out over
the last years and we think that they cannot be solved in a production
tree. So this tree tries to do several things right - without caring
about losing support for old boards.
@par General features include:
- A posix based file API
- inside U-Boot the usual open/close/read/write/lseek functions are used.
This makes it familiar to everyone who has programmed under unix systems.
- usual shell commands like ls/cd/mkdir/echo/cat,...
- The environment is not a variable store anymore, but a file store. It has
currently some limitations, of course. The environment is not a real
read/write filesystem, it is more like a tar archive, or even more like
an ar archive, because it cannot handle directories. The saveenv command
saves the files under a certain directory (by default /env) in persistent
storage (by default /dev/env0). There is a counterpart called loadenv, too.
- Real filesystem support
- The loader starts up with mounting a ramdisk on /. Then a devfs is mounted
on /dev allowing the user (or shell commands) to access devices. Apart from
these two filesystems there is currently one filesystem ported: cramfs. One
can mount it with the usual mount command.
- device/driver model
- Devices are no longer described by defines in the config file. Instead
there are devices which can be registered in the board .c file or
dynamically allocated. Drivers will match upon the devices automatically.
- clocksource support
- Timekeeping has been simplified by the use of the Linux clocksource API.
Only one function is needed for a new board, no [gs]et_timer[masked]() or
- Kconfig and Kernel build system
- Only targets which are really needed get recompiled. Parallel builds are
no problem anymore. This also removes the need for many many ifdefs in
- simulation target
- U-Boot can be compiled to run under Linux. While this is rather useless
in real world this is a great debugging and development aid. New features
can be easily developped and tested on long train journeys and started
under gdb. There is a console driver for linux which emulates a serial
device and a tap based ethernet driver. Linux files can be mapped to
devices under U-Boot to emulate storage devices.
- device parameter support
- Each device can have a unlimited number of parameters. They can be accessed
on the command line with <devid>.<param>="...", for example
'eth0.ip=192.168.0.7' or 'echo $eth0.ip'
- hooks in the startup process can be archieved with *_initcall() directives
in each file.
- There is a small getopt implementation. Some commands got really
complicated (both in code and in usage) due to the fact that U-Boot only
allowed positional parameters.
- Scripts can be edited with a small editor. This editor has no features
except the ones really needed: moving the cursor and typing characters.
@par Building U-Boot
U-Boot uses the Linux kernel's build system. It consists of two parts:
the makefile infrastructure (kbuild), plus a configuration system
(kconfig). So building U-Boot is very similar to building the Linux
For the examples below, we use the User Mode U-Boot implementation, which
is a port of U-Boot to the Linux userspace. This makes it possible to
test drive the code without having real hardware. So for this test
scenario, ARCH=sandbox is the valid architecture selection. This currently
only works on ia32 hosts and partly on x86-64.
Selection of the architecture and the cross compiler can be done in two
ways. You can either specify it using the environment variables ARCH
and CROSS_COMPILE, or you can create the soft links cross_arch and
cross_compile pointing to your architecture and compiler. For ARCH=sandbox
we do not need a cross compiler so it is sufficient to specify the
@code # ln -s sandbox cross_arch @endcode
In order to configure the various aspects of U-Boot, start the U-Boot
@code # make menuconfig @endcode
This command starts a menu box and lets you select all the different
options available for your architecture. Once the configuration was
finished (you can simulate this by using the standard demo config file
with 'make sandbox_defconfig'), there is a .config file in the toplevel
directory of the sourcode.
Once U-Boot is configured, we can start the compilation
@code # make @endcode
If everything goes well, the result is a file called uboot:
# ls -l uboot
-rwxr-xr-x 1 rsc ptx 114073 Jun 26 22:34 uboot
U-Boot usually needs an environment for storing the configuation data.
You can generate an environment using the example environment contained
@code # ./scripts/ubootenv -s -p 0x10000 examples/environment/ env.bin @endcode
To get some files to play with you can generate a cramfs image:
@code # mkcramfs somedir/ cramfs.bin @endcode
The U-Boot image is a normal Linux executable, so it can be started
just like every other program:
# ./uboot -e env.bin -i cramfs.bin
U-Boot 2.0.0-trunk (Jun 26 2007 - 22:34:38)
loading environment from /dev/env0
Specifying -[ie] <file> tells U-Boot to map the file as a device
under /dev. Files given with '-e' will appear as /dev/env[n]. Files
given with '-i' will appear as /dev/fd[n].
If U-Boot finds a valid configuration sector on /dev/env0 it will
load it to /env. It then executes /env/init if it exists. If you have
loaded the example environment U-Boot will show you a menu asking for
If you have started U-Boot as root you will find a new tap device on your
host which you can configure using ifconfig. Once you configured U-Boots
network settings accordingly you can do a ping or tftpboot.
If you have mapped a cramfs image try mounting it with
# mkdir /cram
# mount /dev/fd0 cramfs /cram
Memory can be examined as usual using md/mw commands. They both understand
the -f <file> option to tell the commands that they should work on the
specified files instead of /dev/mem which holds the complete address space.
Note that if you call 'md /dev/fd0' (without -f) U-Boot will segfault on
the host, because it will interpret /dev/fd0 as a number.
@par Directory layout
Most of the directory layout is based upon the Linux Kernel:
arch / * / -> contains architecture specific parts
arch / * / mach-* / -> SoC specific code
drivers / serial -> drivers
drivers / net
drivers / ...
include / asm-* -> architecture specific includes
include / asm-* / arch-* -> SoC specific includes
fs / -> filesystem support and filesystem drivers
lib / -> generic library functions (getopt, readline and the
common / -> common stuff
commands / -> many things previously in common/cmd_*, one command
net / -> Networking stuff
scripts / -> Kconfig system
Documentation / ->
@section license U-Boot's License
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA