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Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -07001/*
2 * (C) Copyright 2014 Red Hat Inc.
3 * Copyright (c) 2014-2015, NVIDIA CORPORATION. All rights reserved.
Karsten Merker8c249292015-03-21 14:15:38 +01004 * Copyright (C) 2015 K. Merker <merker@debian.org>
Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -07005 *
6 * SPDX-License-Identifier: GPL-2.0+
7 */
8
9Generic Distro Configuration Concept
10====================================
11
12Linux distributions are faced with supporting a variety of boot mechanisms,
13environments or bootloaders (PC BIOS, EFI, U-Boot, Barebox, ...). This makes
14life complicated. Worse, bootloaders such as U-Boot have a configurable set
15of features, and each board chooses to enable a different set of features.
16Hence, distros typically need to have board-specific knowledge in order to
17set up a bootable system.
18
19This document defines a common set of U-Boot features that are required for
20a distro to support the board in a generic fashion. Any board wishing to
21allow distros to install and boot in an out-of-the-box fashion should enable
22all these features. Linux distros can then create a single set of boot
23support/install logic that targets these features. This will allow distros
24to install on many boards without the need for board-specific logic.
25
26In fact, some of these features can be implemented by any bootloader, thus
27decoupling distro install/boot logic from any knowledge of the bootloader.
28
29This model assumes that boards will load boot configuration files from a
30regular storage mechanism (eMMC, SD card, USB Disk, SATA disk, etc.) with
Masahiro Yamada28fd00b2015-07-07 18:47:17 +090031a standard partitioning scheme (MBR, GPT). Boards that cannot support this
Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -070032storage model are outside the scope of this document, and may still need
33board-specific installer/boot-configuration support in a distro.
34
35To some extent, this model assumes that a board has a separate boot flash
36that contains U-Boot, and that the user has somehow installed U-Boot to this
37flash before running the distro installer. Even on boards that do not conform
38to this aspect of the model, the extent of the board-specific support in the
39distro installer logic would be to install a board-specific U-Boot package to
Masahiro Yamada28fd00b2015-07-07 18:47:17 +090040the boot partition during installation. This distro-supplied U-Boot can still
41implement the same features as on any other board, and hence the distro's boot
42configuration file generation logic can still be board-agnostic.
Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -070043
44Locating Bootable Disks
45-----------------------
46
47Typical desktop/server PCs search all (or a user-defined subset of) attached
48storage devices for a bootable partition, then load the bootloader or boot
49configuration files from there. A U-Boot board port that enables the features
50mentioned in this document will search for boot configuration files in the
51same way.
52
53Thus, distros do not need to manipulate any kind of bootloader-specific
54configuration data to indicate which storage device the system should boot
55from.
56
57Distros simply need to install the boot configuration files (see next
58section) in an ext2/3/4 or FAT partition, mark the partition bootable (via
59the MBR bootable flag, or GPT legacy_bios_bootable attribute), and U-Boot (or
60any other bootloader) will find those boot files and execute them. This is
61conceptually identical to creating a grub2 configuration file on a desktop
62PC.
63
Masahiro Yamada28fd00b2015-07-07 18:47:17 +090064Note that in the absence of any partition that is explicitly marked bootable,
Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -070065U-Boot falls back to searching the first valid partition of a disk for boot
66configuration files. Other bootloaders are recommended to do the same, since
67I believe that partition table bootable flags aren't so commonly used outside
68the realm of x86 PCs.
69
70U-Boot can also search for boot configuration files from a TFTP server.
71
72Boot Configuration Files
73------------------------
74
75The standard format for boot configuration files is that of extlinux.conf, as
76handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This is roughly
77as specified at:
78
79http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
80
81... with the exceptions that the BootLoaderSpec document:
82
83* Prescribes a separate configuration per boot menu option, whereas U-Boot
84 lumps all options into a single extlinux.conf file. Hence, U-Boot searches
85 for /extlinux/extlinux.conf then /boot/extlinux/extlinux.conf on disk, or
86 pxelinux.cfg/default over the network.
87
88* Does not document the fdtdir option, which automatically selects the DTB to
89 pass to the kernel.
90
91One example extlinux.conf generated by the Fedora installer is:
92
93------------------------------------------------------------
94# extlinux.conf generated by anaconda
95
96ui menu.c32
97
98menu autoboot Welcome to Fedora. Automatic boot in # second{,s}. Press a key for options.
99menu title Fedora Boot Options.
100menu hidden
101
102timeout 50
103#totaltimeout 9000
104
105default Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)
106
107label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl) 22 (Rawhide)
108 kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl
109 append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
110 fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl
111 initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl.img
112
113label Fedora (3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae) 22 (Rawhide)
114 kernel /boot/vmlinuz-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
115 append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8 LANG=en_US.UTF-8 drm.debug=0xf
116 fdtdir /boot/dtb-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae
117 initrd /boot/initramfs-3.17.0-0.rc4.git2.1.fc22.armv7hl+lpae.img
118
119label Fedora-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc (0-rescue-8f6ba7b039524e0eb957d2c9203f04bc)
120 kernel /boot/vmlinuz-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc
121 initrd /boot/initramfs-0-rescue-8f6ba7b039524e0eb957d2c9203f04bc.img
122 append ro root=UUID=8eac677f-8ea8-4270-8479-d5ddbb797450 console=ttyS0,115200n8
123 fdtdir /boot/dtb-3.16.0-0.rc6.git1.1.fc22.armv7hl+lpae
124------------------------------------------------------------
125
126Another hand-crafted network boot configuration file is:
127
128------------------------------------------------------------
129TIMEOUT 100
130
131MENU TITLE TFTP boot options
132
133LABEL jetson-tk1-emmc
134 MENU LABEL ../zImage root on Jetson TK1 eMMC
135 LINUX ../zImage
136 FDTDIR ../
137 APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=80a5a8e9-c744-491a-93c1-4f4194fd690b
138
139LABEL venice2-emmc
140 MENU LABEL ../zImage root on Venice2 eMMC
141 LINUX ../zImage
142 FDTDIR ../
143 APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=5f71e06f-be08-48ed-b1ef-ee4800cc860f
144
145LABEL sdcard
146 MENU LABEL ../zImage, root on 2GB sdcard
147 LINUX ../zImage
148 FDTDIR ../
149 APPEND console=ttyS0,115200n8 console=tty1 loglevel=8 rootwait rw earlyprintk root=PARTUUID=b2f82cda-2535-4779-b467-094a210fbae7
150
151LABEL fedora-installer-fk
152 MENU LABEL Fedora installer w/ Fedora kernel
153 LINUX fedora-installer/vmlinuz
154 INITRD fedora-installer/initrd.img.orig
155 FDTDIR fedora-installer/dtb
156 APPEND loglevel=8 ip=dhcp inst.repo=http://10.0.0.2/mirrors/fedora/linux/development/rawhide/armhfp/os/ rd.shell cma=64M
157------------------------------------------------------------
158
159U-Boot Implementation
160=====================
161
162Enabling the distro options
163---------------------------
164
165In your board configuration file, include the following:
166
167------------------------------------------------------------
168#ifndef CONFIG_SPL_BUILD
169#include <config_distro_defaults.h>
170#include <config_distro_bootcmd.h>
171#endif
172------------------------------------------------------------
173
174The first of those headers primarily enables a core set of U-Boot features,
175such as support for MBR and GPT partitions, ext* and FAT filesystems, booting
176raw zImage and initrd (rather than FIT- or uImage-wrapped files), etc. Network
177boot support is also enabled here, which is useful in order to boot distro
178installers given that distros do not commonly distribute bootable install
179media for non-PC targets at present.
180
181Finally, a few options that are mostly relevant only when using U-Boot-
182specific boot.scr scripts are enabled. This enables distros to generate a
183U-Boot-specific boot.scr script rather than extlinux.conf as the boot
184configuration file. While doing so is fully supported, and
185<config_distro_defaults.h> exposes enough parameterization to boot.scr to
186allow for board-agnostic boot.scr content, this document recommends that
187distros generate extlinux.conf rather than boot.scr. extlinux.conf is intended
188to work across multiple bootloaders, whereas boot.scr will only work with
189U-Boot. TODO: document the contract between U-Boot and boot.scr re: which
190environment variables a generic boot.scr may rely upon.
191
192The second of those headers sets up the default environment so that $bootcmd
193is defined in a way that searches attached disks for boot configuration files,
194and executes them if found.
195
196Required Environment Variables
197------------------------------
198
199The U-Boot "syslinux" and "pxe boot" commands require a number of environment
200variables be set. Default values for these variables are often hard-coded into
201CONFIG_EXTRA_ENV_SETTINGS in the board's U-Boot configuration file, so that
202the user doesn't have to configure them.
203
204fdt_addr:
205
206 Mandatory for any system that provides the DTB in HW (e.g. ROM) and wishes
207 to pass that DTB to Linux, rather than loading a DTB from the boot
208 filesystem. Prohibited for any other system.
209
210 If specified a DTB to boot the system must be available at the given
211 address.
212
213fdt_addr_r:
214
215 Mandatory. The location in RAM where the DTB will be loaded or copied to when
216 processing the fdtdir/devicetreedir or fdt/devicetree options in
217 extlinux.conf.
218
219 This is mandatory even when fdt_addr is provided, since extlinux.conf must
220 always be able to provide a DTB which overrides any copy provided by the HW.
221
222 A size of 1MB for the FDT/DTB seems reasonable.
223
224ramdisk_addr_r:
225
226 Mandatory. The location in RAM where the initial ramdisk will be loaded to
227 when processing the initrd option in extlinux.conf.
228
229 It is recommended that this location be highest in RAM out of fdt_addr_,
230 kernel_addr_r, and ramdisk_addr_r, so that the RAM disk can vary in size
231 and use any available RAM.
232
233kernel_addr_r:
234
235 Mandatory. The location in RAM where the kernel will be loaded to when
236 processing the kernel option in the extlinux.conf.
237
238 The kernel should be located within the first 128M of RAM in order for the
239 kernel CONFIG_AUTO_ZRELADDR option to work, which is likely enabled on any
240 distro kernel. Since the kernel will decompress itself to 0x8000 after the
Masahiro Yamada28fd00b2015-07-07 18:47:17 +0900241 start of RAM, kernel_addr_r should not overlap that area, or the kernel will
Dennis Gilmoreffb4f6f2015-01-22 11:34:20 -0700242 have to copy itself somewhere else first before decompression.
243
244 A size of 16MB for the kernel is likely adequate.
245
246pxe_addr_r:
247
248 Mandatory. The location in RAM where extlinux.conf will be loaded to prior
249 to processing.
250
251 A size of 1MB for extlinux.conf is more than adequate.
252
253scriptaddr:
254
255 Mandatory, if the boot script is boot.scr rather than extlinux.conf. The
256 location in RAM where boot.scr will be loaded to prior to execution.
257
258 A size of 1MB for extlinux.conf is more than adequate.
259
260For suggestions on memory locations for ARM systems, you must follow the
261guidelines specified in Documentation/arm/Booting in the Linux kernel tree.
262
263For a commented example of setting these values, please see the definition of
264MEM_LAYOUT_ENV_SETTINGS in include/configs/tegra124-common.h.
265
266Boot Target Configuration
267-------------------------
268
269<config_distro_bootcmd.h> defines $bootcmd and many helper command variables
270that automatically search attached disks for boot configuration files and
271execute them. Boards must provide configure <config_distro_bootcmd.h> so that
272it supports the correct set of possible boot device types. To provide this
273configuration, simply define macro BOOT_TARGET_DEVICES prior to including
274<config_distro_bootcmd.h>. For example:
275
276------------------------------------------------------------
277#ifndef CONFIG_SPL_BUILD
278#define BOOT_TARGET_DEVICES(func) \
279 func(MMC, mmc, 1) \
280 func(MMC, mmc, 0) \
281 func(USB, usb, 0) \
282 func(PXE, pxe, na) \
283 func(DHCP, dhcp, na)
284#include <config_distro_bootcmd.h>
285#endif
286------------------------------------------------------------
287
288Each entry in the macro defines a single boot device (e.g. a specific eMMC
289device or SD card) or type of boot device (e.g. USB disk). The parameters to
290the func macro (passed in by the internal implementation of the header) are:
291
292- Upper-case disk type (MMC, SATA, SCSI, IDE, USB, DHCP, PXE).
293- Lower-case disk type (same options as above).
294- ID of the specific disk (MMC only) or ignored for other types.
295
296User Configuration
297==================
298
299Once the user has installed U-Boot, it is expected that the environment will
300be reset to the default values in order to enable $bootcmd and friends, as set
301up by <config_distro_bootcmd.h>. After this, various environment variables may
302be altered to influence the boot process:
303
304boot_targets:
305
306 The list of boot locations searched.
307
308 Example: mmc0, mmc1, usb, pxe
309
310 Entries may be removed or re-ordered in this list to affect the boot order.
311
312boot_prefixes:
313
314 For disk-based booting, the list of directories within a partition that are
315 searched for boot configuration files (extlinux.conf, boot.scr).
316
317 Example: / /boot/
318
319 Entries may be removed or re-ordered in this list to affect the set of
320 directories which are searched.
321
322boot_scripts:
323
324 The name of U-Boot style boot.scr files that $bootcmd searches for.
325
326 Example: boot.scr.uimg boot.scr
327
328 (Typically we expect extlinux.conf to be used, but execution of boot.scr is
329 maintained for backwards-compatibility.)
330
331 Entries may be removed or re-ordered in this list to affect the set of
332 filenames which are supported.
333
334scan_dev_for_extlinux:
335
336 If you want to disable extlinux.conf on all disks, set the value to something
337 innocuous, e.g. setenv scan_dev_for_extlinux true.
338
339scan_dev_for_scripts:
340
341 If you want to disable boot.scr on all disks, set the value to something
342 innocuous, e.g. setenv scan_dev_for_scripts true.
Karsten Merker8c249292015-03-21 14:15:38 +0100343
344
345Interactively booting from a specific device at the u-boot prompt
346=================================================================
347
348For interactively booting from a user-selected device at the u-boot command
349prompt, the environment provides predefined bootcmd_<target> variables for
350every target defined in boot_targets, which can be run be the user.
351
352If the target is a storage device, the format of the target is always
353<device type><device number>, e.g. mmc0. Specifying the device number is
354mandatory for storage devices, even if only support for a single instance
355of the storage device is actually implemented.
356
357For network targets (dhcp, pxe), only the device type gets specified;
358they do not have a device number.
359
360Examples:
361
362 - run bootcmd_usb0
363 boots from the first USB mass storage device
364
365 - run bootcmd_mmc1
366 boots from the second MMC device
367
368 - run bootcmd_pxe
369 boots by tftp using a pxelinux.cfg
370
371The list of possible targets consists of:
372
373- network targets
374 * dhcp
375 * pxe
376
377- storage targets (to which a device number must be appended)
378 * mmc
379 * sata
380 * scsi
381 * ide
382 * usb
383
384Other *boot* variables than the ones defined above are only for internal use
385of the boot environment and are not guaranteed to exist or work in the same
386way in future u-boot versions. In particular the <device type>_boot
387variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
388detail and must not be used as a public interface.