0c817d3ffa
The old directory layout is gone, remove documentation about it Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
184 lines
6.8 KiB
Text
184 lines
6.8 KiB
Text
U2Boot
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------
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This is u2boot, our proposal for a next generation of the famous U-Boot
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bootloader. U-Boot offers an excellent choice as a bootloader for
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today's embedded systems, seen from a user's point of view.
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Nevertheless, there are quite some design flaws which turned out over
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the last years and we think that they cannot be solved in a production
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tree. So this tree tries to do several things right - without caring
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about losing support for old boards.
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General features include:
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- A posix based file API
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inside U-Boot the usual open/close/read/write/lseek functions are used.
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This makes it familiar to everyone who has programmed under unix systems.
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- usual shell commands like ls/cd/mkdir/echo/cat,...
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- The environment is not a variable store anymore, but a file store. It has
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currently some limitations, of course. The environment is not a real
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read/write filesystem, it is more like a tar archive, or even more like
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an ar archive, because it cannot handle directories. The saveenv command
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saves the files under a certain directory (by default /env) in persistent
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storage (by default /dev/env0). There is a counterpart called loadenv, too.
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- Real filesystem support
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The loader starts up with mounting a ramdisk on /. Then a devfs is mounted
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on /dev allowing the user (or shell commands) to access devices. Apart from
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these two filesystems there is currently one filesystem ported: cramfs. One
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can mount it with the usual mount command.
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- device/driver model
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Devices are no longer described by defines in the config file. Instead
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there are devices which can be registered in the board .c file or
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dynamically allocated. Drivers will match upon the devices automatically.
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- clocksource support
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Timekeeping has been simplified by the use of the Linux clocksource API.
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Only one function is needed for a new board, no [gs]et_timer[masked]() or
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reset_timer[masked]() functions.
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- Kconfig and Kernel build system
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Only targets which are really needed get recompiled. Parallel builds are
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no problem anymore. This also removes the need for many many ifdefs in
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the code.
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- simulation target
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U-Boot can be compiled to run under Linux. While this is rather useless
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in real world this is a great debugging and development aid. New features
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can be easily developped and tested on long train journeys and started
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under gdb. There is a console driver for linux which emulates a serial
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device and a tap based ethernet driver. Linux files can be mapped to
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devices under U-Boot to emulate storage devices.
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- device parameter support
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Each device can have a unlimited number of parameters. They can be accessed
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on the command line with <devid>.<param>="...", for example
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'eth0.ip=192.168.0.7' or 'echo $eth0.ip'
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- initcalls
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hooks in the startup process can be archieved with *_initcall() directives
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in each file.
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- getopt
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There is a small getopt implementation. Some commands got really
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complicated (both in code and in usage) due to the fact that U-Boot only
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allowed positional parameters.
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- editor
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Scripts can be edited with a small editor. This editor has no features
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except the ones really needed: moving the cursor and typing characters.
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Building U-Boot
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---------------
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U-Boot uses the Linux kernel's build system. It consists of two parts:
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the makefile infrastructure (kbuild), plus a configuration system
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(kconfig). So building U-Boot is very similar to building the Linux
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kernel.
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For the examples below, we use the User Mode U-Boot implementation, which
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is a port of U-Boot to the Linux userspace. This makes it possible to
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test drive the code without having real hardware. So for this test
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scenario, ARCH=sandbox is the valid architecture selection. This currently
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only works on ia32 hosts and partly on x86-64.
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Selection of the architecture and the cross compiler can be done by using
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the environment variables ARCH and CROSS_COMPILE.
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In order to configure the various aspects of U-Boot, start the U-Boot
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configuration system:
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# make menuconfig
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This command starts a menu box and lets you select all the different
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options available for your architecture. Once the configuration was
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finished (you can simulate this by using the standard demo config file
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with 'make sandbox_defconfig'), there is a .config file in the toplevel
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directory of the sourcode.
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Once U-Boot is configured, we can start the compilation
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# make
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If everything goes well, the result is a file called uboot:
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# ls -l uboot
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-rwxr-xr-x 1 rsc ptx 114073 Jun 26 22:34 uboot
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U-Boot usually needs an environment for storing the configuation data.
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You can generate an environment using the example environment contained
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in examples/environment:
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# ./scripts/ubootenv -s -p 0x10000 examples/environment/ env.bin
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To get some files to play with you can generate a cramfs image:
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# mkcramfs somedir/ cramfs.bin
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The U-Boot image is a normal Linux executable, so it can be started
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just like every other program:
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# ./uboot -e env.bin -i cramfs.bin
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U-Boot 2.0.0-trunk (Jun 26 2007 - 22:34:38)
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loading environment from /dev/env0
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uboot> /
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Specifying -[ie] <file> tells U-Boot to map the file as a device
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under /dev. Files given with '-e' will appear as /dev/env[n]. Files
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given with '-i' will appear as /dev/fd[n].
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If U-Boot finds a valid configuration sector on /dev/env0 it will
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load it to /env. It then executes /env/init if it exists. If you have
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loaded the example environment U-Boot will show you a menu asking for
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your settings.
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If you have started U-Boot as root you will find a new tap device on your
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host which you can configure using ifconfig. Once you configured U-Boots
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network settings accordingly you can do a ping or tftpboot.
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If you have mapped a cramfs image try mounting it with
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# mkdir /cram
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# mount /dev/fd0 cramfs /cram
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Memory can be examined as usual using md/mw commands. They both understand
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the -f <file> option to tell the commands that they should work on the
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specified files instead of /dev/mem which holds the complete address space.
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Note that if you call 'md /dev/fd0' (without -f) U-Boot will segfault on
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the host, because it will interpret /dev/fd0 as a number.
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Directory layout
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----------------
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Most of the directory layout is based upon the Linux Kernel:
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arch/*/ -> contains architecture specific parts
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arch/*/mach-*/ -> SoC specific code
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drivers/serial -> drivers
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drivers/net
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drivers/...
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include/asm-* -> architecture specific includes
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include/asm-*/arch-* -> SoC specific includes
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fs/ -> filesystem support and filesystem drivers
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lib/ -> generic library functions (getopt, readline and the
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like)
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common/ -> common stuff
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commands/ -> many things previously in common/cmd_*, one command
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per file
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net/ -> Networking stuff
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scripts/ -> Kconfig system
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Documentation/ ->
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