1 .. SPDX-License-Identifier: GPL-2.0+
6 U-Boot supports user configuration using environment variables which
7 can be made persistent by saving to persistent storage, for example flash
10 Environment variables are set using "env set" (alias "setenv"), printed using
11 "env print" (alias "printenv"), and saved to persistent storage using
12 "env save" (alias "saveenv"). Using "env set"
13 without a value can be used to delete a variable from the
14 environment. As long as you don't save the environment, you are
15 working with an in-memory copy. In case the Flash area containing the
16 environment is erased by accident, a default environment is provided.
18 See :doc:`cmd/env` for details.
20 Some configuration is controlled by Environment Variables, so that setting the
21 variable can adjust the behaviour of U-Boot (e.g. autoboot delay, autoloading
24 Text-based Environment
25 ----------------------
27 The default environment for a board is created using a `.env` environment file
28 using a simple text format. The base filename for this is defined by
29 `CONFIG_ENV_SOURCE_FILE`, or `CONFIG_SYS_BOARD` if that is empty.
31 The file must be in the board directory and have a .env extension, so
32 assuming that there is a board vendor, the resulting filename is therefore::
34 board/<vendor>/<board>/<CONFIG_ENV_SOURCE_FILE>.env
38 board/<vendor>/<board>/<CONFIG_SYS_BOARD>.env
40 This is a plain text file where you can type your environment variables in
41 the form `var=value`. Blank lines and multi-line variables are supported.
42 The conversion script looks for a line that starts in column 1 with a string
43 and has an equals sign immediately afterwards. Spaces before the = are not
44 permitted. It is a good idea to indent your scripts so that only the 'var='
45 appears at the start of a line.
47 To add additional text to a variable you can use `var+=value`. This text is
48 merged into the variable during the make process and made available as a
49 single value to U-Boot. Variables can contain `+` characters but in the unlikely
50 event that you want to have a variable name ending in plus, put a backslash
51 before the `+` so that the script knows you are not adding to an existing
52 variable but assigning to a new one::
56 This file can include C-style comments. Blank lines and multi-line
57 variables are supported, and you can use normal C preprocessor directives
58 and CONFIG defines from your board config also.
60 For example, for snapper9260 you would create a text file called
61 `board/bluewater/snapper9260.env` containing the environment text.
70 /* U-Boot script for booting */
72 if [ -z ${tftpserverip} ]; then
73 echo "Use 'setenv tftpserverip a.b.c.d' to set IP address."
76 usb start; setenv autoload n; bootp;
77 tftpboot ${tftpserverip}:
80 /* Print a message when boot fails */
81 echo CONFIG_SYS_BOARD boot failed - please check your image
82 echo Load address is CONFIG_SYS_LOAD_ADDR
84 If CONFIG_ENV_SOURCE_FILE is empty and the default filename is not present, then
85 the old-style C environment is used instead. See below.
87 Old-style C environment
88 -----------------------
90 Traditionally, the default environment is created in `include/env_default.h`,
91 and can be augmented by various `CONFIG` defines. See that file for details. In
92 particular you can define `CONFIG_EXTRA_ENV_SETTINGS` in your board file
93 to add environment variables.
95 Board maintainers are encouraged to migrate to the text-based environment as it
96 is easier to maintain. The distro-board script still requires the old-style
97 environment but work is underway to address this.
100 List of environment variables
101 -----------------------------
103 Some device configuration options can be set using environment variables. In
104 many cases the value in the default environment comes from a CONFIG option - see
105 `include/env_default.h`) for this.
107 This is most-likely not complete:
110 If set to "yes" (actually any string starting with 1, y, Y, t, or T) an
111 image loaded with one of the commands listed below will be automatically
112 started by internally invoking the bootm command.
114 * bootelf - Boot from an ELF image in memory
115 * bootp - boot image via network using BOOTP/TFTP protocol
116 * dhcp - boot image via network using DHCP/TFTP protocol
117 * diskboot - boot from ide device
118 * nboot - boot from NAND device
119 * nfs - boot image via network using NFS protocol
120 * rarpboot - boot image via network using RARP/TFTP protocol
121 * scsiboot - boot from SCSI device
122 * tftpboot - boot image via network using TFTP protocol
123 * usbboot - boot from USB device
125 If the environment variable autostart is not set to a value starting with
126 1, y, Y, t, or T, an image passed to the "bootm" command will be copied to
127 the load address (and eventually uncompressed), but NOT be started.
128 This can be used to load and uncompress arbitrary data.
131 Used to set the baudrate of the UART - it defaults to CONFIG_BAUDRATE (which
135 Delay before automatically running bootcmd. During this time the user
136 can choose to enter the shell (or the boot menu if
137 CONFIG_AUTOBOOT_MENU_SHOW=y):
139 - 0 to autoboot with no delay, but you can stop it by key input.
140 - -1 to disable autoboot.
141 - -2 to autoboot with no delay and not check for abort
143 The default value is defined by CONFIG_BOOTDELAY.
144 The value of 'bootdelay' is overridden by the /config/bootdelay value in
145 the device-tree if CONFIG_OF_CONTROL=y.
148 The command that is run if the user does not enter the shell during the
152 Command line arguments passed when booting an operating system or binary
156 Name of the image to load with TFTP
159 Memory range available for image processing in the bootm
160 command can be restricted. This variable is given as
161 a hexadecimal number and defines lowest address allowed
162 for use by the bootm command. See also "bootm_size"
163 environment variable. Address defined by "bootm_low" is
164 also the base of the initial memory mapping for the Linux
165 kernel -- see the description of CFG_SYS_BOOTMAPSZ and
169 Size of the initial memory mapping for the Linux kernel.
170 This variable is given as a hexadecimal number and it
171 defines the size of the memory region starting at base
172 address bootm_low that is accessible by the Linux kernel
173 during early boot. If unset, CFG_SYS_BOOTMAPSZ is used
174 as the default value if it is defined, and bootm_size is
178 Memory range available for image processing in the bootm
179 command can be restricted. This variable is given as
180 a hexadecimal number and defines the size of the region
181 allowed for use by the bootm command. See also "bootm_low"
182 environment variable.
184 bootstopkeysha256, bootdelaykey, bootstopkey
188 Location of the software update file on a TFTP server, used
189 by the automatic software update feature. Please refer to
190 documentation in doc/README.update for more details.
193 if set to "no" (any string beginning with 'n'),
194 "bootp" and "dhcp" will just load perform a lookup of the
195 configuration from the BOOTP server, but not try to
199 if set this restricts the maximum address that the
200 flattened device tree will be copied into upon boot.
201 For example, if you have a system with 1 GB memory
202 at physical address 0x10000000, while Linux kernel
203 only recognizes the first 704 MB as low memory, you
204 may need to set fdt_high as 0x3C000000 to have the
205 device tree blob be copied to the maximum address
206 of the 704 MB low memory, so that Linux kernel can
207 access it during the boot procedure.
209 If this is set to the special value 0xffffffff (32-bit machines) or
210 0xffffffffffffffff (64-bit machines) then
211 the fdt will not be copied at all on boot. For this
212 to work it must reside in writable memory, have
213 sufficient padding on the end of it for u-boot to
214 add the information it needs into it, and the memory
215 must be accessible by the kernel. This usage is strongly discouraged
216 however as it also stops U-Boot from ensuring the device tree starting
217 address is properly aligned and a misaligned tree will cause OS failures.
220 if set this is the address of the control flattened
221 device tree used by U-Boot when CONFIG_OF_CONTROL is
225 restrict positioning of initrd images:
226 If this variable is not set, initrd images will be
227 copied to the highest possible address in RAM; this
228 is usually what you want since it allows for
229 maximum initrd size. If for some reason you want to
230 make sure that the initrd image is loaded below the
231 CFG_SYS_BOOTMAPSZ limit, you can set this environment
232 variable to a value of "no" or "off" or "0".
233 Alternatively, you can set it to a maximum upper
234 address to use (U-Boot will still check that it
235 does not overwrite the U-Boot stack and data).
237 For instance, when you have a system with 16 MB
238 RAM, and want to reserve 4 MB from use by Linux,
239 you can do this by adding "mem=12M" to the value of
240 the "bootargs" variable. However, now you must make
241 sure that the initrd image is placed in the first
242 12 MB as well - this can be done with::
244 setenv initrd_high 00c00000
246 If you set initrd_high to 0xffffffff (32-bit machines) or
247 0xffffffffffffffff (64-bit machines), this is an
248 indication to U-Boot that all addresses are legal
249 for the Linux kernel, including addresses in flash
250 memory. In this case U-Boot will NOT COPY the
251 ramdisk at all. This may be useful to reduce the
252 boot time on your system, but requires that this
253 feature is supported by your Linux kernel. This usage however requires
254 that the user ensure that there will be no overlap with other parts of the
255 image such as the Linux kernel BSS. It should not be enabled by default
256 and only done as part of optimizing a deployment.
259 IP address; needed for tftpboot command
262 Default load address for commands like "bootp",
263 "rarpboot", "tftpboot", "loadb" or "diskboot". Note that the optimal
264 default values here will vary between architectures. On 32bit ARM for
265 example, some offset from start of memory is used as the Linux kernel
266 zImage has a self decompressor and it's best if we stay out of where that
270 see CONFIG_LOADS_ECHO
273 TFTP server IP address; needed for tftpboot command
276 see CONFIG_BOOT_RETRY_TIME
279 see CONFIG_AUTOBOOT_DELAY_STR
282 see CONFIG_AUTOBOOT_STOP_STR
285 controls which network interface is used first.
288 controls which interface is currently active.
289 For example you can do the following::
292 => ping 192.168.0.1 # traffic sent on FEC
294 => ping 10.0.0.1 # traffic sent on SCC
297 When set to "no" U-Boot does not go through all
298 available network interfaces.
299 It just stays at the currently selected interface. When unset or set to
300 anything other than "no", U-Boot does go through all
301 available network interfaces.
304 When set to "no" each network operation will
305 either succeed or fail without retrying.
306 When set to "once" the network operation will
307 fail when all the available network interfaces
308 are tried once without success.
309 Useful on scripts which control the retry operation
313 If set then Linux will be told to boot silently, by
314 adding 'console=' to its command line. If "yes" it will be
315 made silent. If "no" it will not be made silent. If
316 unset, then it will be made silent if the U-Boot console
320 If this is set, the value is used for TFTP's
324 If this is set, the value is used for TFTP's UDP
325 destination port instead of the default port 69.
328 Block size to use for TFTP transfers; if not set,
329 we use the TFTP server's default block size
332 Retransmission timeout for TFTP packets (in milli-
333 seconds, minimum value is 1000 = 1 second). Defines
334 when a packet is considered to be lost so it has to
335 be retransmitted. The default is 5000 = 5 seconds.
336 Lowering this value may make downloads succeed
337 faster in networks with high packet loss rates or
338 with unreliable TFTP servers.
341 maximum count of TFTP timeouts (no
342 unit, minimum value = 0). Defines how many timeouts
343 can happen during a single file transfer before that
344 transfer is aborted. The default is 10, and 0 means
345 'no timeouts allowed'. Increasing this value may help
346 downloads succeed with high packet loss rates, or with
347 unreliable TFTP servers or client hardware.
350 if this is set, the value is used for TFTP's
351 window size as described by RFC 7440.
352 This means the count of blocks we can receive before
353 sending ack to server.
356 When set to a value < 4095 the traffic over
357 Ethernet is encapsulated/received over 802.1q
360 Note: This appears not to be used in U-Boot. See `README.VLAN`.
363 Period during which BOOTP/DHCP sends retries.
364 Unsigned value, in milliseconds. If not set, the period will
365 be either the default (28000), or a value based on
366 CONFIG_NET_RETRY_COUNT, if defined. This value has
367 precedence over the value based on CONFIG_NET_RETRY_COUNT.
370 Number of matches found by the last 'ms' command, in hex
373 Address of the last match found by the 'ms' command, in hex,
377 Index position of the last match found by the 'ms' command,
378 in units of the size (.b, .w, .l) of the search
381 (x86 only) Base address of the bzImage 'setup' block
384 (x86 only) Address of the loaded bzImage, typically
385 BZIMAGE_LOAD_ADDR which is 0x100000
391 The following image location variables contain the location of images
392 used in booting. The "Image" column gives the role of the image and is
393 not an environment variable name. The other columns are environment
394 variable names. "File Name" gives the name of the file on a TFTP
395 server, "RAM Address" gives the location in RAM the image will be
396 loaded to, and "Flash Location" gives the image's address in NOR
397 flash or offset in NAND flash.
399 *Note* - these variables don't have to be defined for all boards, some
400 boards currently use other variables for these purposes, and some
401 boards use these variables for other purposes.
403 Also note that most of these variables are just a commonly used set of variable
404 names, used in some other variable definitions, but are not hard-coded anywhere
407 ================= ============== ================ ==============
408 Image File Name RAM Address Flash Location
409 ================= ============== ================ ==============
410 Linux kernel bootfile kernel_addr_r kernel_addr
411 device tree blob fdtfile fdt_addr_r fdt_addr
412 ramdisk ramdiskfile ramdisk_addr_r ramdisk_addr
413 ================= ============== ================ ==============
415 When setting the RAM addresses for `kernel_addr_r`, `fdt_addr_r` and
416 `ramdisk_addr_r` there are several types of constraints to keep in mind. The
417 one type of constraint is payload requirement. For example, a device tree MUST
418 be loaded at an 8-byte aligned address as that is what the specification
419 requires. In a similar manner, the operating system may define restrictions on
420 where in memory space payloads can be. This is documented for example in Linux,
421 with both the `Booting ARM Linux`_ and `Booting AArch64 Linux`_ documents.
422 Finally, there are practical constraints. We do not know the size of a given
423 payload a user will use but each payload must not overlap or it will corrupt
424 the other payload. A similar problem can happen when a payload ends up being in
425 the OS BSS area. For these reasons we need to ensure our default values here
426 are both unlikely to lead to failure to boot and sufficiently explained so that
427 they can be optimized for boot time or adjusted for smaller memory
430 On different architectures we will have different constraints. It is important
431 that we follow whatever documented requirements are available to best ensure
432 forward compatibility. What follows are examples to highlight how to provide
433 reasonable default values in different cases.
435 Texas Instruments OMAP2PLUS (ARMv7) example
436 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
438 On these families of processors we are on a 32bit ARMv7 core. As booting some
439 form of Linux is our most common payload we will also keep in mind the
440 documented requirements for booting that Linux provides. These values are also
441 known to be fine for booting a number of other operating systems (or their
442 loaders). In this example we define the following variables and values::
445 kernel_addr_r=${loadaddr}
446 fdt_addr_r=0x88000000
447 ramdisk_addr_r=0x88080000
448 bootm_size=0x10000000
450 The first thing to keep in mind is that DRAM starts at 0x80000000. We set a
451 32MiB buffer from the start of memory as our default load address and set
452 ``kernel_addr_r`` to that. This is because the Linux ``zImage`` decompressor
453 will typically then be able to avoid doing a relocation itself. It also MUST be
454 within the first 128MiB of memory. The next value is we set ``fdt_addr_r`` to
455 be at 128MiB offset from the start of memory. This location is suggested by the
456 kernel documentation and is exceedingly unlikely to be overwritten by the
457 kernel itself given other architectural constraints. We then allow for the
458 device tree to be up to 512KiB in size before placing the ramdisk in memory. We
459 then say that everything should be within the first 256MiB of memory so that
460 U-Boot can relocate things as needed to ensure proper alignment. We pick 256MiB
461 as our value here because we know there are very few platforms on in this
462 family with less memory. It could be as high as 768MiB and still ensure that
463 everything would be visible to the kernel, but again we go with what we assume
464 is the safest assumption.
466 Automatically updated variables
467 -------------------------------
469 The following environment variables may be used and automatically
470 updated by the network boot commands ("bootp" and "rarpboot"),
471 depending the information provided by your boot server:
473 ========= ===================================================
475 ========= ===================================================
477 dnsip IP address of your Domain Name Server
478 dnsip2 IP address of your secondary Domain Name Server
479 gatewayip IP address of the Gateway (Router) to use
480 hostname Target hostname
483 rootpath Pathname of the root filesystem on the NFS server
485 ========= ===================================================
488 Special environment variables
489 -----------------------------
491 There are two special Environment Variables:
494 contains hardware identification information such as type string and/or
497 Ethernet address. If CONFIG_REGEX=y, also eth*addr (where * is an integer).
499 These variables can be set only once (usually during manufacturing of
500 the board). U-Boot refuses to delete or overwrite these variables
501 once they have been set, unless CONFIG_ENV_OVERWRITE is enabled in the board
507 Contains the U-Boot version string as printed
508 with the "version" command. This variable is
509 readonly (see CONFIG_VERSION_VARIABLE).
511 Please note that changes to some configuration parameters may take
512 only effect after the next boot (yes, that's just like Windows).
515 External environment file
516 -------------------------
518 The `CONFIG_USE_DEFAULT_ENV_FILE` option provides a way to bypass the
519 environment generation in U-Boot. If enabled, then `CONFIG_DEFAULT_ENV_FILE`
520 provides the name of a file which is converted into the environment,
521 completely bypassing the standard environment variables in `env_default.h`.
523 The format is the same as accepted by the mkenvimage tool, with lines containing
524 key=value pairs. Blank lines and lines beginning with # are ignored.
526 Future work may unify this feature with the text-based environment, perhaps
527 moving the contents of `env_default.h` to a text file.
532 See :doc:`../develop/environment` for internal development details.
534 .. _`Booting ARM Linux`: https://www.kernel.org/doc/html/latest/arm/booting.html
535 .. _`Booting AArch64 Linux`: https://www.kernel.org/doc/html/latest/arm64/booting.html