U-Boot

.. SPDX-License-Identifier: GPL-2.0+
   Copyright 2010-2011 Calxeda, Inc.

PXE Boot and extlinux.conf
==========================

The ``pxe`` commands provide a near subset of the functionality
provided by the PXELINUX boot loader. This allows U-Boot based systems
to be controlled remotely using the same PXE based techniques that
many non U-Boot based servers use.

The ``sysboot`` command and Extlinux boot method use the same file
format as PXE boot for ``extlinux.conf``.

Commands
--------

``pxe get``
        **Syntax:** ``pxe get``

	follows PXELINUX's rules for retrieving configuration files
	from a tftp server, and supports a subset of PXELINUX's config
	file syntax. It requires certain environment variables to be
	set, see the Environment section below.

	**File Paths**

	``pxe get`` repeatedly tries to download config files until it
	either successfully downloads one or runs out of paths to
	try. The order and contents of paths it tries mirrors exactly
	that of PXELINUX - you can read in more detail about it at:

	http://syslinux.zytor.com/wiki/index.php/Doc/pxelinux

``pxe boot``
        **Syntax:** ``pxe boot [pxefile_addr_r]``

	Interprets a pxe file stored in memory.

	``pxefile_addr_r`` is an optional argument giving the location
	of the pxe file. The file must be terminated with a NUL byte.

	There are some environment variables that may need to be set,
	depending on conditions, see the Environment section below.

``sysboot``
        **Syntax:** ``sysboot [-p] <interface> <dev[:part]> <ext2|fat|any> [addr] [filename]``

	Load and boot an ``extlinux.conf`` file from a local
	filesystem. Paths in the ``extlinux.conf`` file (kernel,
	initrd, FDT and overlays) will be interpreted within that
	filesystem.

	Example:

	``sysboot mmc 0.0:2 any ${pxefile_addr_r} /boot/extlinux.conf``

Environment
-----------

``pxefile_addr_r``
        Should be set to a location in RAM large enough to hold pxe
        files while they're being processed. Up to 16 config files may
        be held in memory at once. The exact number and size of the
        files varies with how the system is being used. A typical
        config file is a few hundred bytes long. Required for ``pxe
        get``, for ``pxe boot`` if the optional argument
        ``pxefile_addr_r`` is not supplied.

``bootfile``
        Typically set in the DHCP response handler, required for ``pxe
        get``. For ``pxe boot``, this path is used to generate the
        base directory that all other paths to files retrieved by
        ``pxe boot`` will use. If no bootfile is specified, paths used
        in pxe files will be used as is.

``serverip``
        Typically set in the DHCP response handler, this is the IP
        address of the tftp server from which other files will be
        retrieved. Required for ``pxe get``.

``kernel_addr_r``, ``initrd_addr_r``
        Locations in RAM to store the kernel (or FIT image) and
        initrd. These locations will be passed to the ``bootm``
        command to boot the kernel. These environment variables are
        required to be set.

``fdt_addr_r``
        Location in RAM to store the retrieved fdt blob. Retrieval is
        possible if ``fdt`` label is defined in pxe file and
        ``fdt_addr_r`` is set. If retrieval is possible,
        ``fdt_addr_r`` will be passed to ``bootm`` command to boot the
        kernel.

``fdt_addr``
        Location of a fdt blob. ``fdt_addr`` will be passed to
        ``bootm`` command if it is set and ``fdt_addr_r`` is not
        passed to bootm command.

``fdtoverlay_addr_r``
        Location in RAM to temporarily store fdt overlay(s) before
        applying them to the fdt blob stored at
        ``fdt_addr_r``. Required to use the ``fdtoverlays`` command in
        the PXE file.

``pxe_label_override``
        Override label to be used, if exists, instead of the default
        label. This will allow consumers to choose a pxe label at
        runtime instead of having to prompt the user. If
        ``pxe_label_override`` is set but does not exist in the pxe
        menu, pxe would fallback to the default label if given, and no
        failure is returned but rather a warning message.

``ethaddr``
        This is the standard MAC address for the ethernet adapter in
        use. ``pxe get`` uses it to look for a configuration file
        specific to a system's MAC address.

``pxeuuid``
        This is a UUID in standard form using lower case hexadecimal
        digits, for example,
        550e8400-e29b-41d4-a716-446655440000. ``pxe get`` uses it to
        look for a configuration file based on the system's UUID.

pxe file format
---------------

The pxe file format is nearly a subset of the PXELINUX file format;
see http://syslinux.zytor.com/wiki/index.php/PXELINUX. It's composed
of one line commands - global commands, and commands specific to
labels. Lines beginning with # are treated as comments. White space
between and at the beginning of lines is ignored.

The size of pxe files and the number of labels is only limited by the amount
of RAM available to U-Boot. Memory for labels is dynamically allocated as
they're parsed, and memory for pxe files is statically allocated, and its
location is given by the pxefile_addr_r environment variable. The pxe code is
not aware of the size of the pxefile memory and will outgrow it if pxe files
are too large.

Supported global commands
^^^^^^^^^^^^^^^^^^^^^^^^^
Unrecognized commands are ignored.

``default <label>``
        The label named here is treated as the default and is the
	first label 'pxe boot' attempts to boot.

``fallback <label>``
        The label named here is treated as a fallback option that may
	be attempted should it be detected that booting of the default
	has failed to complete, for example via U-Boot's boot count
	limit functionality.

``menu title <string>``
        Sets a title for the menu of labels being displayed.

``menu include <path>``
        Use tftp to retrieve the pxe file at ``<path>``, which is then
        immediately parsed as if the start of its contents were the
        next line in the current file. nesting of include up to 16
        files deep is supported.

``prompt <flag>``
        If 1, always prompt the user to enter a label to boot from. If
        0, only prompt the user if timeout expires.

``timeout <num>``
        Wait for user input for <num>/10 seconds before auto-booting a
        node.

``label <name>``
        Begin a label definition. Labels continue until a command not
        recognized as a label command is seen, or EOF is reached.

Supported label commands
^^^^^^^^^^^^^^^^^^^^^^^^
Labels end when a command not recognized as a label command is reached, or EOF.

``menu default``
        set this label as the default label to boot; this is the same
        behavior as the global default command but specified in a
        different way

``kernel <path>``
        If this label is chosen, use tftp to retrieve the kernel (or
        FIT image) at ``<path>``. it will be stored at the address
        indicated in the ``kernel_addr_r`` environment variable, and
        that address will be passed to ``bootm`` to boot this
        kernel. For FIT image, the configuration specification can be
        appended to the file name, with the format:

                ``<path>#<conf>[#<extra-conf[#...]]``

        It will be passed to bootm with that address (see:
        doc/uImage.FIT/command_syntax_extensions.txt). It is useful
        for overlay selection in pxe file (see
        :doc:`./fit/overlay-fdt-boot`).

``fdtoverlays <path> [...]``
        If this label is chosen, use tftp to retrieve the DT
        overlay(s) at ``<path>``. It will be temporarily stored at the
        address indicated in the ``fdtoverlay_addr_r`` environment
        variable, and then applied in the load order to the fdt blob
        stored at the address indicated in the ``fdt_addr_r``
        environment variable.

``devicetree-overlay <path> [...]``
        if this label is chosen, use tftp to retrieve the DT
        overlay(s) at ``<path>``. It will be temporarily stored at the
        address indicated in the ``fdtoverlay_addr_r`` environment
        variable, and then applied in the load order to the fdt blob
        stored at the address indicated in the ``fdt_addr_r``
        environment variable. Alias for fdtoverlays.

``kaslrseed``
        set this label to request random number from hwrng as kaslr seed.

``append <string>``
        Use ``<string>`` as the kernel command line when booting this
        label. Environment variable references like ``${var}`` are
        substituted before boot.

``initrd <path>``
        If this label is chosen, use tftp to retrieve the initrd at
        ``<path>``. it will be stored at the address indicated in the
        ``initrd_addr_r`` environment variable, and that address will
        be passed to ``bootm``. For FIT image, the initrd can be
        provided with the same value than kernel, including
        configuration:

                ``<path>#<conf>[#<extra-conf[#...]]``

        In this case, ``kernel_addr_r`` is passed to ``bootm``.

``fdt <path>``
        If this label is chosen, use tftp to retrieve the fdt blob at
        ``<path>``. It will be stored at the address indicated in the
        ``fdt_addr_r`` environment variable, and that address will be
        passed to ``bootm``. For FIT image, the device tree can be
        provided with the same value than kernel, including
        configuration:

                ``<path>#<conf>[#<extra-conf[#...]]``

        In this case, ``kernel_addr_r`` is passed to ``bootm``.

``devicetree <path>``
        If this label is chosen, use tftp to retrieve the fdt blob at
        ``<path>``. it will be stored at the address indicated in the
        ``fdt_addr_r`` environment variable, and that address will be
        passed to ``bootm``. Alias for fdt.

``fdtdir <path>``
        If this label is chosen, use tftp to retrieve a fdt blob
        relative to ``<path>``. If the ``fdtfile`` environment
        variable is set, ``<path>/<fdtfile>`` is retrieved. Otherwise,
        the filename is generated from the ``soc`` and ``board``
        environment variables, i.e. ``<path>/<soc>-<board>.dtb`` is
        retrieved. If the ``fdt`` command is specified, ``fdtdir`` is
        ignored.

``localboot <flag>``
        Run the command defined by ``localcmd`` in the
        environment. ``<flag>`` is ignored and is only here to match
        the syntax of PXELINUX config files.

Example
-------
Here's a couple of example files to show how this works.

.. code-block::
   :caption: /tftpboot/pxelinux.cfg/menus/base.menu

   menu title Linux selections

   # This is the default label
   label install
       menu label Default Install Image
       kernel kernels/install.bin
       append console=ttyAMA0,38400 debug earlyprintk
       initrd initrds/uzInitrdDebInstall

   # Just another label
   label linux-2.6.38
       kernel kernels/linux-2.6.38.bin
       append root=/dev/sdb1

   # The locally installed kernel
   label local
       menu label Locally installed kernel
       append root=/dev/sdb1
       localboot 1

.. code-block::
   :caption: /tftpboot/pxelinux.cfg/default

   menu include pxelinux.cfg/menus/base.menu
   timeout 500

   default linux-2.6.38

When a pxe client retrieves and boots the default pxe file, ``pxe
boot`` will wait for user input for 5 seconds before booting the
``linux-2.6.38`` label, which will cause
``/tftpboot/kernels/linux-2.6.38.bin`` to be downloaded, and boot with
the command line ``root=/dev/sdb1``

Differences with PXELINUX
-------------------------

The biggest difference between U-Boot's pxe and PXELINUX is that since
U-Boot's pxe support is written entirely in C, it can run on any platform
with network support in U-Boot. Here are some other differences between
PXELINUX and U-Boot's pxe support.

- U-Boot's pxe does not support the PXELINUX DHCP option codes specified
  in RFC 5071, but could be extended to do so.

- when U-Boot's pxe fails to boot, it will return control to U-Boot,
  allowing another command to run, other U-Boot command, instead of resetting
  the machine like PXELINUX.

- U-Boot's pxe doesn't rely on or provide an UNDI/PXE stack in memory, it
  only uses U-Boot.

- U-Boot's pxe doesn't provide the full menu implementation that PXELINUX
  does, only a simple text based menu using the commands described in
  this README. With PXELINUX, it's possible to have a graphical boot
  menu, submenus, passwords, etc. U-Boot's pxe could be extended to support
  a more robust menuing system like that of PXELINUX's.

- U-Boot's pxe expects U-Boot uimg's as kernels.  Anything that would work
  with the 'bootm' command in U-Boot could work with the 'pxe boot' command.

- U-Boot's pxe only recognizes a single file on the initrd command line.  It
  could be extended to support multiple.

- in U-Boot's pxe, the localboot command doesn't necessarily cause a local
  disk boot - it will do whatever is defined in the 'localcmd' env
  variable. And since it doesn't support a full UNDI/PXE stack, the
  type field is ignored.

- the interactive prompt in U-Boot's pxe only allows you to choose a label
  from the menu.  If you want to boot something not listed, you can ctrl+c
  out of 'pxe boot' and use existing U-Boot commands to accomplish it.