doc/develop/distro.rst - filogic/uboot - Gitiles

 .. SPDX-License-Identifier: GPL-2.0+

 Generic Distro Configuration Concept
 ====================================

 Linux distributions are faced with supporting a variety of boot mechanisms,
 environments or bootloaders (PC BIOS, EFI, U-Boot, Barebox, ...). This makes
 life complicated. Worse, bootloaders such as U-Boot have a configurable set
 of features, and each board chooses to enable a different set of features.
 Hence, distros typically need to have board-specific knowledge in order to
 set up a bootable system.

 This document defines a common set of U-Boot features that are required for
 a distro to support the board in a generic fashion. Any board wishing to
 allow distros to install and boot in an out-of-the-box fashion should enable
 all these features. Linux distros can then create a single set of boot
 support/install logic that targets these features. This will allow distros
 to install on many boards without the need for board-specific logic.

 In fact, some of these features can be implemented by any bootloader, thus
 decoupling distro install/boot logic from any knowledge of the bootloader.

 This model assumes that boards will load boot configuration files from a
 regular storage mechanism (eMMC, SD card, USB Disk, SATA disk, etc.) with
 a standard partitioning scheme (MBR, GPT). Boards that cannot support this
 storage model are outside the scope of this document, and may still need
 board-specific installer/boot-configuration support in a distro.

 To some extent, this model assumes that a board has a separate boot flash
 that contains U-Boot, and that the user has somehow installed U-Boot to this
 flash before running the distro installer. Even on boards that do not conform
 to this aspect of the model, the extent of the board-specific support in the
 distro installer logic would be to install a board-specific U-Boot package to
 the boot partition during installation. This distro-supplied U-Boot can still
 implement the same features as on any other board, and hence the distro's boot
 configuration file generation logic can still be board-agnostic.

 Locating Bootable Disks
 -----------------------

 Typical desktop/server PCs search all (or a user-defined subset of) attached
 storage devices for a bootable partition, then load the bootloader or boot
 configuration files from there. A U-Boot board port that enables the features
 mentioned in this document will search for boot configuration files in the
 same way.

 Thus, distros do not need to manipulate any kind of bootloader-specific
 configuration data to indicate which storage device the system should boot
 from.

 Distros simply need to install the boot configuration files (see next
 section) in an ext2/3/4 or FAT partition, mark the partition bootable (via
 the MBR bootable flag, or GPT legacy_bios_bootable attribute), and U-Boot (or
 any other bootloader) will find those boot files and execute them. This is
 conceptually identical to creating a grub2 configuration file on a desktop
 PC.

 Note that in the absence of any partition that is explicitly marked bootable,
 U-Boot falls back to searching the first valid partition of a disk for boot
 configuration files. Other bootloaders are recommended to do the same, since
 I believe that partition table bootable flags aren't so commonly used outside
 the realm of x86 PCs.

 U-Boot can also search for boot configuration files from a TFTP server.

 Boot Configuration Files
 ------------------------

 The standard format for boot configuration files is that of extlinux.conf, as
 handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This is roughly
 as specified at BootLoaderSpec_:


 ... with the exceptions that the BootLoaderSpec document:

 * Prescribes a separate configuration per boot menu option, whereas U-Boot
   lumps all options into a single extlinux.conf file. Hence, U-Boot searches
   for /extlinux/extlinux.conf then /boot/extlinux/extlinux.conf on disk, or
   pxelinux.cfg/default over the network.

 * Does not document the fdtdir option, which automatically selects the DTB to
   pass to the kernel.

 One example extlinux.conf generated by the Fedora installer is::

     # extlinux.conf generated by anaconda

     ui menu.c32

     menu autoboot Welcome to Fedora. Automatic boot in # second{,s}. Press a key for options.
     menu title Fedora Boot Options.
     menu hidden

     timeout 50
     #totaltimeout 9000

     default Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)

     label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl) 22 (Rawhide)
         kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl
         append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
         fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl
         initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl.img

     label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)
         kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
         append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
         fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
         initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae.img

     label Fedora-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc (0-rescue-8f6ba7b039524e0eb957d2c9203f04bc)
         kernel /boot/vmlinuz-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc
         initrd /boot/initramfs-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc.img
         append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8
         fdtdir /boot/dtb-3.16.0-0.rc6.git1.1.fc22.armv7hl+lpae


 Another hand-crafted network boot configuration file is::

     TIMEOUT 100

     MENU TITLE TFTP boot options

     LABEL jetson-tk1-emmc
             MENU LABEL ../zImage root on Jetson TK1 eMMC
             LINUX ../zImage
             FDTDIR ../
             APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=80a5a8e9-c744-491a-93c1-4f4194fd690b

     LABEL venice2-emmc
             MENU LABEL ../zImage root on Venice2 eMMC
             LINUX ../zImage
             FDTDIR ../
             APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=5f71e06f-be08-48ed-b1ef-ee4800cc860f

     LABEL sdcard
             MENU LABEL ../zImage, root on 2GB sdcard
             LINUX ../zImage
             FDTDIR ../
             APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=b2f82cda-2535-4779-b467-094a210fbae7

     LABEL fedora-installer-fk
             MENU LABEL Fedora installer w/ Fedora kernel
             LINUX fedora-installer/vmlinuz
             INITRD fedora-installer/initrd.img.orig
             FDTDIR fedora-installer/dtb
             APPEND loglevel=8 ip=dhcp inst.repo=http://10.0.0.2/mirrors/fedora/linux/development/rawhide/armhfp/os/ rd.shell cma=64M

 U-Boot Implementation
 =====================

 Enabling the distro options
 ---------------------------

 In your board's defconfig, enable the DISTRO_DEFAULTS option by adding
 a line with "CONFIG_DISTRO_DEFAULTS=y". If you want to enable this
 from Kconfig itself, for e.g. all boards using a specific SoC then
 add a "imply DISTRO_DEFAULTS" to your SoC CONFIG option.

 In your board configuration file, include the following::

     #ifndef CONFIG_SPL_BUILD
     #include <config_distro_bootcmd.h>
     #endif

 The first of those headers primarily enables a core set of U-Boot features,
 such as support for MBR and GPT partitions, ext* and FAT filesystems, booting
 raw zImage and initrd (rather than FIT- or uImage-wrapped files), etc. Network
 boot support is also enabled here, which is useful in order to boot distro
 installers given that distros do not commonly distribute bootable install
 media for non-PC targets at present.

 Finally, a few options that are mostly relevant only when using U-Boot-
 specific boot.scr scripts are enabled. This enables distros to generate a
 U-Boot-specific boot.scr script rather than extlinux.conf as the boot
 configuration file. While doing so is fully supported, and
 CONFIG_DISTRO_DEFAULTS exposes enough parameterization to boot.scr to
 allow for board-agnostic boot.scr content, this document recommends that
 distros generate extlinux.conf rather than boot.scr. extlinux.conf is intended
 to work across multiple bootloaders, whereas boot.scr will only work with
 U-Boot. TODO: document the contract between U-Boot and boot.scr re: which
 environment variables a generic boot.scr may rely upon.

 The second of those headers sets up the default environment so that $bootcmd
 is defined in a way that searches attached disks for boot configuration files,
 and executes them if found.

 Required Environment Variables
 ------------------------------

 The U-Boot "syslinux" and "pxe boot" commands require a number of environment
 variables be set. Default values for these variables are often hard-coded into
 CONFIG_EXTRA_ENV_SETTINGS in the board's U-Boot configuration file, so that
 the user doesn't have to configure them.

 fdt_addr:
   Mandatory for any system that provides the DTB in HW (e.g. ROM) and wishes
   to pass that DTB to Linux, rather than loading a DTB from the boot
   filesystem. Prohibited for any other system.

   If specified a DTB to boot the system must be available at the given
   address.

 fdt_addr_r:
   Mandatory. The location in RAM where the DTB will be loaded or copied to when
   processing the fdtdir/devicetreedir or fdt/devicetree options in
   extlinux.conf.

   This is mandatory even when fdt_addr is provided, since extlinux.conf must
   always be able to provide a DTB which overrides any copy provided by the HW.

   A size of 1MB for the FDT/DTB seems reasonable.

 fdtfile:
   Mandatory. the name of the DTB file for the specific board for instance
   the espressobin v5 board the value is "marvell/armada-3720-espressobin.dtb"
   while on a clearfog pro it is "armada-388-clearfog-pro.dtb" in the case of
   a board providing its firmware based DTB this value can be used to override
   the DTB with a different DTB. fdtfile will automatically be set for you if
   it matches the format ${soc}-${board}.dtb which covers most 32 bit use cases.
   AArch64 generally does not match as the Linux kernel put the dtb files under
   SoC vendor directories.

 ramdisk_addr_r:
   Mandatory. The location in RAM where the initial ramdisk will be loaded to
   when processing the initrd option in extlinux.conf.

   It is recommended that this location be highest in RAM out of fdt_addr_r,
   kernel_addr_r, and ramdisk_addr_r, so that the RAM disk can vary in size
   and use any available RAM.

 kernel_addr_r:
   Mandatory. The location in RAM where the kernel will be loaded to when
   processing the kernel option in the extlinux.conf.

   The kernel should be located within the first 128M of RAM in order for the
   kernel CONFIG_AUTO_ZRELADDR option to work, which is likely enabled on any
   distro kernel. Since the kernel will decompress itself to 0x8000 after the
   start of RAM, kernel_addr_r should not overlap that area, or the kernel will
   have to copy itself somewhere else first before decompression.

   A size of 16MB for the kernel is likely adequate.

 kernel_comp_addr_r:
   Optional. This is only required if user wants to boot Linux from a compressed
   Image(.gz, .bz2, .lzma, .lzo) using the booti command. It represents the
   location in RAM where the compressed Image will be decompressed temporarily.
   Once the decompression is complete, the decompressed data will be moved to
   kernel_addr_r for booting.

 kernel_comp_size:
   Optional. This is only required if user wants to boot Linux from a compressed
   Image using booti command. It represents the size of the compressed file. The
   size has to at least the size of loaded image for decompression to succeed.

 pxefile_addr_r:
   Mandatory. The location in RAM where extlinux.conf will be loaded to prior
   to processing.

   A size of 1MB for extlinux.conf is more than adequate.

 scriptaddr:
   Mandatory, if the boot script is boot.scr rather than extlinux.conf. The
   location in RAM where boot.scr will be loaded to prior to execution.

   A size of 1MB for extlinux.conf is more than adequate.

 For suggestions on memory locations for ARM systems, you must follow the
 guidelines specified in Documentation/arm/Booting in the Linux kernel tree.

 For a commented example of setting these values, please see the definition of
 MEM_LAYOUT_ENV_SETTINGS in include/configs/tegra124-common.h.

 Boot Target Configuration
 -------------------------

 The `config_distro_bootcmd.h` file defines $bootcmd and many helper command
 variables that automatically search attached disks for boot configuration files
 and execute them. Boards must provide configure <config_distro_bootcmd.h> so
 that it supports the correct set of possible boot device types. To provide this
 configuration, simply define macro BOOT_TARGET_DEVICES prior to including
 <config_distro_bootcmd.h>. For example::

     #ifndef CONFIG_SPL_BUILD
     #define BOOT_TARGET_DEVICES(func) \
             func(MMC, mmc, 1) \
             func(MMC, mmc, 0) \
             func(USB, usb, 0) \
             func(PXE, pxe, na) \
             func(DHCP, dhcp, na)
     #include <config_distro_bootcmd.h>
     #endif

 Each entry in the macro defines a single boot device (e.g. a specific eMMC
 device or SD card) or type of boot device (e.g. USB disk). The parameters to
 the func macro (passed in by the internal implementation of the header) are:

 - Upper-case disk type (DHCP, HOST, IDE, MMC, NVME, PXE, SATA, SCSI, UBIFS, USB,
   VIRTIO).
 - Lower-case disk type (same options as above).
 - ID of the specific disk (MMC only) or ignored for other types.

 User Configuration
 ==================

 Once the user has installed U-Boot, it is expected that the environment will
 be reset to the default values in order to enable $bootcmd and friends, as set
 up by <config_distro_bootcmd.h>. After this, various environment variables may
 be altered to influence the boot process:

 boot_targets:
   The list of boot locations searched.

   Example: mmc0, mmc1, usb, pxe

   Entries may be removed or re-ordered in this list to affect the boot order.

 boot_prefixes:
   For disk-based booting, the list of directories within a partition that are
   searched for boot configuration files (extlinux.conf, boot.scr).

   Example: / /boot/

   Entries may be removed or re-ordered in this list to affect the set of
   directories which are searched.

 boot_scripts:
   The name of U-Boot style boot.scr files that $bootcmd searches for.

   Example: boot.scr.uimg boot.scr

   (Typically we expect extlinux.conf to be used, but execution of boot.scr is
   maintained for backwards-compatibility.)

   Entries may be removed or re-ordered in this list to affect the set of
   filenames which are supported.

 scan_dev_for_extlinux:
   If you want to disable extlinux.conf on all disks, set the value to something
   innocuous, e.g. setenv scan_dev_for_extlinux true.

 scan_dev_for_scripts:
   If you want to disable boot.scr on all disks, set the value to something
   innocuous, e.g. setenv scan_dev_for_scripts true.

 boot_net_usb_start:
   If you want to prevent USB enumeration by distro boot commands which execute
   network operations, set the value to something innocuous, e.g. setenv
   boot_net_usb_start true. This would be useful if you know your Ethernet
   device is not attached to USB, and you wish to increase boot speed by
   avoiding unnecessary actions.

 boot_net_pci_enum:
   If you want to prevent PCI enumeration by distro boot commands which execute
   network operations, set the value to something innocuous, e.g. setenv
   boot_net_pci_enum true. This would be useful if you know your Ethernet
   device is not attached to PCI, and you wish to increase boot speed by
   avoiding unnecessary actions.

 Interactively booting from a specific device at the u-boot prompt
 =================================================================

 For interactively booting from a user-selected device at the u-boot command
 prompt, the environment provides predefined bootcmd_<target> variables for
 every target defined in boot_targets, which can be run be the user.

 If the target is a storage device, the format of the target is always
 <device type><device number>, e.g. mmc0.  Specifying the device number is
 mandatory for storage devices, even if only support for a single instance
 of the storage device is actually implemented.

 For network targets (dhcp, pxe), only the device type gets specified;
 they do not have a device number.

 Examples:

  - run bootcmd_usb0
    boots from the first USB mass storage device

  - run bootcmd_mmc1
    boots from the second MMC device

  - run bootcmd_pxe
    boots by tftp using a pxelinux.cfg

 The list of possible targets consists of:

 - network targets

   * dhcp
   * pxe

 - storage targets (to which a device number must be appended)

   * mmc
   * sata
   * scsi
   * ide
   * usb
   * virtio

 Other *boot* variables than the ones defined above are only for internal use
 of the boot environment and are not guaranteed to exist or work in the same
 way in future u-boot versions.  In particular the <device type>_boot
 variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
 detail and must not be used as a public interface.

 .. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/

 .. sectionauthor:: (C) Copyright 2014 Red Hat Inc.
 .. sectionauthor:: Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.
 .. sectionauthor:: Copyright (C) 2015 K. Merker <merker@debian.org>
	.. SPDX-License-Identifier: GPL-2.0+

	Generic Distro Configuration Concept
	====================================

	Linux distributions are faced with supporting a variety of boot mechanisms,
	environments or bootloaders (PC BIOS, EFI, U-Boot, Barebox, ...). This makes
	life complicated. Worse, bootloaders such as U-Boot have a configurable set
	of features, and each board chooses to enable a different set of features.
	Hence, distros typically need to have board-specific knowledge in order to
	set up a bootable system.

	This document defines a common set of U-Boot features that are required for
	a distro to support the board in a generic fashion. Any board wishing to
	allow distros to install and boot in an out-of-the-box fashion should enable
	all these features. Linux distros can then create a single set of boot
	support/install logic that targets these features. This will allow distros
	to install on many boards without the need for board-specific logic.

	In fact, some of these features can be implemented by any bootloader, thus
	decoupling distro install/boot logic from any knowledge of the bootloader.

	This model assumes that boards will load boot configuration files from a
	regular storage mechanism (eMMC, SD card, USB Disk, SATA disk, etc.) with
	a standard partitioning scheme (MBR, GPT). Boards that cannot support this
	storage model are outside the scope of this document, and may still need
	board-specific installer/boot-configuration support in a distro.

	To some extent, this model assumes that a board has a separate boot flash
	that contains U-Boot, and that the user has somehow installed U-Boot to this
	flash before running the distro installer. Even on boards that do not conform
	to this aspect of the model, the extent of the board-specific support in the
	distro installer logic would be to install a board-specific U-Boot package to
	the boot partition during installation. This distro-supplied U-Boot can still
	implement the same features as on any other board, and hence the distro's boot
	configuration file generation logic can still be board-agnostic.

	Locating Bootable Disks
	-----------------------

	Typical desktop/server PCs search all (or a user-defined subset of) attached
	storage devices for a bootable partition, then load the bootloader or boot
	configuration files from there. A U-Boot board port that enables the features
	mentioned in this document will search for boot configuration files in the
	same way.

	Thus, distros do not need to manipulate any kind of bootloader-specific
	configuration data to indicate which storage device the system should boot
	from.

	Distros simply need to install the boot configuration files (see next
	section) in an ext2/3/4 or FAT partition, mark the partition bootable (via
	the MBR bootable flag, or GPT legacy_bios_bootable attribute), and U-Boot (or
	any other bootloader) will find those boot files and execute them. This is
	conceptually identical to creating a grub2 configuration file on a desktop
	PC.

	Note that in the absence of any partition that is explicitly marked bootable,
	U-Boot falls back to searching the first valid partition of a disk for boot
	configuration files. Other bootloaders are recommended to do the same, since
	I believe that partition table bootable flags aren't so commonly used outside
	the realm of x86 PCs.

	U-Boot can also search for boot configuration files from a TFTP server.

	Boot Configuration Files
	------------------------

	The standard format for boot configuration files is that of extlinux.conf, as
	handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This is roughly
	as specified at BootLoaderSpec_:


	... with the exceptions that the BootLoaderSpec document:

	* Prescribes a separate configuration per boot menu option, whereas U-Boot
	lumps all options into a single extlinux.conf file. Hence, U-Boot searches
	for /extlinux/extlinux.conf then /boot/extlinux/extlinux.conf on disk, or
	pxelinux.cfg/default over the network.

	* Does not document the fdtdir option, which automatically selects the DTB to
	pass to the kernel.

	One example extlinux.conf generated by the Fedora installer is::

	# extlinux.conf generated by anaconda

	ui menu.c32

	menu autoboot Welcome to Fedora. Automatic boot in # second{,s}. Press a key for options.
	menu title Fedora Boot Options.
	menu hidden

	timeout 50
	#totaltimeout 9000

	default Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)

	label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl) 22 (Rawhide)
	kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl
	append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
	fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl
	initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl.img

	label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)
	kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
	append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
	fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
	initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae.img

	label Fedora-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc (0-rescue-8f6ba7b039524e0eb957d2c9203f04bc)
	kernel /boot/vmlinuz-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc
	initrd /boot/initramfs-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc.img
	append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8
	fdtdir /boot/dtb-3.16.0-0.rc6.git1.1.fc22.armv7hl+lpae


	Another hand-crafted network boot configuration file is::

	TIMEOUT 100

	MENU TITLE TFTP boot options

	LABEL jetson-tk1-emmc
	MENU LABEL ../zImage root on Jetson TK1 eMMC
	LINUX ../zImage
	FDTDIR ../
	APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=80a5a8e9-c744-491a-93c1-4f4194fd690b

	LABEL venice2-emmc
	MENU LABEL ../zImage root on Venice2 eMMC
	LINUX ../zImage
	FDTDIR ../
	APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=5f71e06f-be08-48ed-b1ef-ee4800cc860f

	LABEL sdcard
	MENU LABEL ../zImage, root on 2GB sdcard
	LINUX ../zImage
	FDTDIR ../
	APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=b2f82cda-2535-4779-b467-094a210fbae7

	LABEL fedora-installer-fk
	MENU LABEL Fedora installer w/ Fedora kernel
	LINUX fedora-installer/vmlinuz
	INITRD fedora-installer/initrd.img.orig
	FDTDIR fedora-installer/dtb
	APPEND loglevel=8 ip=dhcp inst.repo=http://10.0.0.2/mirrors/fedora/linux/development/rawhide/armhfp/os/ rd.shell cma=64M

	U-Boot Implementation
	=====================

	Enabling the distro options
	---------------------------

	In your board's defconfig, enable the DISTRO_DEFAULTS option by adding
	a line with "CONFIG_DISTRO_DEFAULTS=y". If you want to enable this
	from Kconfig itself, for e.g. all boards using a specific SoC then
	add a "imply DISTRO_DEFAULTS" to your SoC CONFIG option.

	In your board configuration file, include the following::

	#ifndef CONFIG_SPL_BUILD
	#include <config_distro_bootcmd.h>
	#endif

	The first of those headers primarily enables a core set of U-Boot features,
	such as support for MBR and GPT partitions, ext* and FAT filesystems, booting
	raw zImage and initrd (rather than FIT- or uImage-wrapped files), etc. Network
	boot support is also enabled here, which is useful in order to boot distro
	installers given that distros do not commonly distribute bootable install
	media for non-PC targets at present.

	Finally, a few options that are mostly relevant only when using U-Boot-
	specific boot.scr scripts are enabled. This enables distros to generate a
	U-Boot-specific boot.scr script rather than extlinux.conf as the boot
	configuration file. While doing so is fully supported, and
	CONFIG_DISTRO_DEFAULTS exposes enough parameterization to boot.scr to
	allow for board-agnostic boot.scr content, this document recommends that
	distros generate extlinux.conf rather than boot.scr. extlinux.conf is intended
	to work across multiple bootloaders, whereas boot.scr will only work with
	U-Boot. TODO: document the contract between U-Boot and boot.scr re: which
	environment variables a generic boot.scr may rely upon.

	The second of those headers sets up the default environment so that $bootcmd
	is defined in a way that searches attached disks for boot configuration files,
	and executes them if found.

	Required Environment Variables
	------------------------------

	The U-Boot "syslinux" and "pxe boot" commands require a number of environment
	variables be set. Default values for these variables are often hard-coded into
	CONFIG_EXTRA_ENV_SETTINGS in the board's U-Boot configuration file, so that
	the user doesn't have to configure them.

	fdt_addr:
	Mandatory for any system that provides the DTB in HW (e.g. ROM) and wishes
	to pass that DTB to Linux, rather than loading a DTB from the boot
	filesystem. Prohibited for any other system.

	If specified a DTB to boot the system must be available at the given
	address.

	fdt_addr_r:
	Mandatory. The location in RAM where the DTB will be loaded or copied to when
	processing the fdtdir/devicetreedir or fdt/devicetree options in
	extlinux.conf.

	This is mandatory even when fdt_addr is provided, since extlinux.conf must
	always be able to provide a DTB which overrides any copy provided by the HW.

	A size of 1MB for the FDT/DTB seems reasonable.

	fdtfile:
	Mandatory. the name of the DTB file for the specific board for instance
	the espressobin v5 board the value is "marvell/armada-3720-espressobin.dtb"
	while on a clearfog pro it is "armada-388-clearfog-pro.dtb" in the case of
	a board providing its firmware based DTB this value can be used to override
	the DTB with a different DTB. fdtfile will automatically be set for you if
	it matches the format ${soc}-${board}.dtb which covers most 32 bit use cases.
	AArch64 generally does not match as the Linux kernel put the dtb files under
	SoC vendor directories.

	ramdisk_addr_r:
	Mandatory. The location in RAM where the initial ramdisk will be loaded to
	when processing the initrd option in extlinux.conf.

	It is recommended that this location be highest in RAM out of fdt_addr_r,
	kernel_addr_r, and ramdisk_addr_r, so that the RAM disk can vary in size
	and use any available RAM.

	kernel_addr_r:
	Mandatory. The location in RAM where the kernel will be loaded to when
	processing the kernel option in the extlinux.conf.

	The kernel should be located within the first 128M of RAM in order for the
	kernel CONFIG_AUTO_ZRELADDR option to work, which is likely enabled on any
	distro kernel. Since the kernel will decompress itself to 0x8000 after the
	start of RAM, kernel_addr_r should not overlap that area, or the kernel will
	have to copy itself somewhere else first before decompression.

	A size of 16MB for the kernel is likely adequate.

	kernel_comp_addr_r:
	Optional. This is only required if user wants to boot Linux from a compressed
	Image(.gz, .bz2, .lzma, .lzo) using the booti command. It represents the
	location in RAM where the compressed Image will be decompressed temporarily.
	Once the decompression is complete, the decompressed data will be moved to
	kernel_addr_r for booting.

	kernel_comp_size:
	Optional. This is only required if user wants to boot Linux from a compressed
	Image using booti command. It represents the size of the compressed file. The
	size has to at least the size of loaded image for decompression to succeed.

	pxefile_addr_r:
	Mandatory. The location in RAM where extlinux.conf will be loaded to prior
	to processing.

	A size of 1MB for extlinux.conf is more than adequate.

	scriptaddr:
	Mandatory, if the boot script is boot.scr rather than extlinux.conf. The
	location in RAM where boot.scr will be loaded to prior to execution.

	A size of 1MB for extlinux.conf is more than adequate.

	For suggestions on memory locations for ARM systems, you must follow the
	guidelines specified in Documentation/arm/Booting in the Linux kernel tree.

	For a commented example of setting these values, please see the definition of
	MEM_LAYOUT_ENV_SETTINGS in include/configs/tegra124-common.h.

	Boot Target Configuration
	-------------------------

	The `config_distro_bootcmd.h` file defines $bootcmd and many helper command
	variables that automatically search attached disks for boot configuration files
	and execute them. Boards must provide configure <config_distro_bootcmd.h> so
	that it supports the correct set of possible boot device types. To provide this
	configuration, simply define macro BOOT_TARGET_DEVICES prior to including
	<config_distro_bootcmd.h>. For example::

	#ifndef CONFIG_SPL_BUILD
	#define BOOT_TARGET_DEVICES(func) \
	func(MMC, mmc, 1) \
	func(MMC, mmc, 0) \
	func(USB, usb, 0) \
	func(PXE, pxe, na) \
	func(DHCP, dhcp, na)
	#include <config_distro_bootcmd.h>
	#endif

	Each entry in the macro defines a single boot device (e.g. a specific eMMC
	device or SD card) or type of boot device (e.g. USB disk). The parameters to
	the func macro (passed in by the internal implementation of the header) are:

	- Upper-case disk type (DHCP, HOST, IDE, MMC, NVME, PXE, SATA, SCSI, UBIFS, USB,
	VIRTIO).
	- Lower-case disk type (same options as above).
	- ID of the specific disk (MMC only) or ignored for other types.

	User Configuration
	==================

	Once the user has installed U-Boot, it is expected that the environment will
	be reset to the default values in order to enable $bootcmd and friends, as set
	up by <config_distro_bootcmd.h>. After this, various environment variables may
	be altered to influence the boot process:

	boot_targets:
	The list of boot locations searched.

	Example: mmc0, mmc1, usb, pxe

	Entries may be removed or re-ordered in this list to affect the boot order.

	boot_prefixes:
	For disk-based booting, the list of directories within a partition that are
	searched for boot configuration files (extlinux.conf, boot.scr).

	Example: / /boot/

	Entries may be removed or re-ordered in this list to affect the set of
	directories which are searched.

	boot_scripts:
	The name of U-Boot style boot.scr files that $bootcmd searches for.

	Example: boot.scr.uimg boot.scr

	(Typically we expect extlinux.conf to be used, but execution of boot.scr is
	maintained for backwards-compatibility.)

	Entries may be removed or re-ordered in this list to affect the set of
	filenames which are supported.

	scan_dev_for_extlinux:
	If you want to disable extlinux.conf on all disks, set the value to something
	innocuous, e.g. setenv scan_dev_for_extlinux true.

	scan_dev_for_scripts:
	If you want to disable boot.scr on all disks, set the value to something
	innocuous, e.g. setenv scan_dev_for_scripts true.

	boot_net_usb_start:
	If you want to prevent USB enumeration by distro boot commands which execute
	network operations, set the value to something innocuous, e.g. setenv
	boot_net_usb_start true. This would be useful if you know your Ethernet
	device is not attached to USB, and you wish to increase boot speed by
	avoiding unnecessary actions.

	boot_net_pci_enum:
	If you want to prevent PCI enumeration by distro boot commands which execute
	network operations, set the value to something innocuous, e.g. setenv
	boot_net_pci_enum true. This would be useful if you know your Ethernet
	device is not attached to PCI, and you wish to increase boot speed by
	avoiding unnecessary actions.

	Interactively booting from a specific device at the u-boot prompt
	=================================================================

	For interactively booting from a user-selected device at the u-boot command
	prompt, the environment provides predefined bootcmd_<target> variables for
	every target defined in boot_targets, which can be run be the user.

	If the target is a storage device, the format of the target is always
	<device type><device number>, e.g. mmc0. Specifying the device number is
	mandatory for storage devices, even if only support for a single instance
	of the storage device is actually implemented.

	For network targets (dhcp, pxe), only the device type gets specified;
	they do not have a device number.

	Examples:

	- run bootcmd_usb0
	boots from the first USB mass storage device

	- run bootcmd_mmc1
	boots from the second MMC device

	- run bootcmd_pxe
	boots by tftp using a pxelinux.cfg

	The list of possible targets consists of:

	- network targets

	* dhcp
	* pxe

	- storage targets (to which a device number must be appended)

	* mmc
	* sata
	* scsi
	* ide
	* usb
	* virtio

	Other boot variables than the ones defined above are only for internal use
	of the boot environment and are not guaranteed to exist or work in the same
	way in future u-boot versions. In particular the <device type>_boot
	variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
	detail and must not be used as a public interface.

	.. _BootLoaderSpec: http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/

	.. sectionauthor:: (C) Copyright 2014 Red Hat Inc.
	.. sectionauthor:: Copyright (c) 2014-2015, NVIDIA CORPORATION. All rights reserved.
	.. sectionauthor:: Copyright (C) 2015 K. Merker <merker@debian.org>