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    /*
     * (C) Copyright 2014 Red Hat Inc.
     * Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.
    
     * Copyright (C) 2015 K. Merker <merker@debian.org>
    
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     *
     * 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
    
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    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.
    
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    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,
    
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    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:
    
    http://www.freedesktop.org/wiki/Specifications/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 "default y if ARCH_FOO" to the DISTRO_DEFAULTS section of
    the Kconfig file in the root of the u-boot sources.
    
    
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    In your board configuration file, include the following:
    
    ------------------------------------------------------------
    #ifndef CONFIG_SPL_BUILD
    #include <config_distro_defaults.h>
    #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.h> 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.
    
    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_,
      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
    
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      have to copy itself somewhere else first before decompression.
    
      A size of 16MB for the kernel is likely adequate.
    
    
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      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
    -------------------------
    
    <config_distro_bootcmd.h> 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 (MMC, SATA, SCSI, IDE, USB, DHCP, PXE).
    - 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
    
    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.