/*
   * (C) Copyright 2014 Red Hat Inc.
   * Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.
   * 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 cannnot 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 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 absense 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:
  
  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 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_rshould 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.
  
  pxe_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
  -------------------------
  
  <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.
  
  
  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.