add README.distro file

Add documentation on how to setup a system to use the generic distro
configs and boot commands. This spells out what is needed to make a
system conformant, but does not limit the board to only the defaults.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
[swarren, added concept, user config, BOOT_TARGET_DEVICES sections.
edited the rest]
Signed-off-by: Stephen Warren <swarren@nvidia.com>

doc/README.distro

+/*
+ * (C) Copyright 2014 Red Hat Inc.
+ * Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.
+ *
+ * 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.