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 Used to set the baudrate of the UART - it defaults to CONFIG_BAUDRATE (which
114 Delay before automatically running bootcmd. During this time the user
115 can choose to enter the shell (or the boot menu if
116 CONFIG_AUTOBOOT_MENU_SHOW=y):
118 - 0 to autoboot with no delay, but you can stop it by key input.
119 - -1 to disable autoboot.
120 - -2 to autoboot with no delay and not check for abort
122 The default value is defined by CONFIG_BOOTDELAY.
123 The value of 'bootdelay' is overridden by the /config/bootdelay value in
124 the device-tree if CONFIG_OF_CONTROL=y.
127 The command that is run if the user does not enter the shell during the
131 Command line arguments passed when booting an operating system or binary
135 Name of the image to load with TFTP
138 Memory range available for image processing in the bootm
139 command can be restricted. This variable is given as
140 a hexadecimal number and defines lowest address allowed
141 for use by the bootm command. See also "bootm_size"
142 environment variable. Address defined by "bootm_low" is
143 also the base of the initial memory mapping for the Linux
144 kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
148 Size of the initial memory mapping for the Linux kernel.
149 This variable is given as a hexadecimal number and it
150 defines the size of the memory region starting at base
151 address bootm_low that is accessible by the Linux kernel
152 during early boot. If unset, CONFIG_SYS_BOOTMAPSZ is used
153 as the default value if it is defined, and bootm_size is
157 Memory range available for image processing in the bootm
158 command can be restricted. This variable is given as
159 a hexadecimal number and defines the size of the region
160 allowed for use by the bootm command. See also "bootm_low"
161 environment variable.
163 bootstopkeysha256, bootdelaykey, bootstopkey
167 Location of the software update file on a TFTP server, used
168 by the automatic software update feature. Please refer to
169 documentation in doc/README.update for more details.
172 if set to "no" (any string beginning with 'n'),
173 "bootp" and "dhcp" will just load perform a lookup of the
174 configuration from the BOOTP server, but not try to
178 if set to "yes", an image loaded using the "bootp", "dhcp",
179 "rarpboot", "tftpboot" or "diskboot" commands will
180 be automatically started (by internally calling
183 If unset, or set to "1"/"yes"/"true" (case insensitive, just the first
184 character is enough), a standalone image
185 passed to the "bootm" command will be copied to the load address
186 (and eventually uncompressed), but NOT be started.
187 This can be used to load and uncompress arbitrary
191 if set this restricts the maximum address that the
192 flattened device tree will be copied into upon boot.
193 For example, if you have a system with 1 GB memory
194 at physical address 0x10000000, while Linux kernel
195 only recognizes the first 704 MB as low memory, you
196 may need to set fdt_high as 0x3C000000 to have the
197 device tree blob be copied to the maximum address
198 of the 704 MB low memory, so that Linux kernel can
199 access it during the boot procedure.
201 If this is set to the special value 0xffffffff (32-bit machines) or
202 0xffffffffffffffff (64-bit machines) then
203 the fdt will not be copied at all on boot. For this
204 to work it must reside in writable memory, have
205 sufficient padding on the end of it for u-boot to
206 add the information it needs into it, and the memory
207 must be accessible by the kernel.
210 if set this is the address of the control flattened
211 device tree used by U-Boot when CONFIG_OF_CONTROL is
215 restrict positioning of initrd images:
216 If this variable is not set, initrd images will be
217 copied to the highest possible address in RAM; this
218 is usually what you want since it allows for
219 maximum initrd size. If for some reason you want to
220 make sure that the initrd image is loaded below the
221 CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
222 variable to a value of "no" or "off" or "0".
223 Alternatively, you can set it to a maximum upper
224 address to use (U-Boot will still check that it
225 does not overwrite the U-Boot stack and data).
227 For instance, when you have a system with 16 MB
228 RAM, and want to reserve 4 MB from use by Linux,
229 you can do this by adding "mem=12M" to the value of
230 the "bootargs" variable. However, now you must make
231 sure that the initrd image is placed in the first
232 12 MB as well - this can be done with::
234 setenv initrd_high 00c00000
236 If you set initrd_high to 0xffffffff (32-bit machines) or
237 0xffffffffffffffff (64-bit machines), this is an
238 indication to U-Boot that all addresses are legal
239 for the Linux kernel, including addresses in flash
240 memory. In this case U-Boot will NOT COPY the
241 ramdisk at all. This may be useful to reduce the
242 boot time on your system, but requires that this
243 feature is supported by your Linux kernel.
246 IP address; needed for tftpboot command
249 Default load address for commands like "bootp",
250 "rarpboot", "tftpboot", "loadb" or "diskboot"
253 see CONFIG_LOADS_ECHO
256 TFTP server IP address; needed for tftpboot command
259 see CONFIG_BOOT_RETRY_TIME
262 see CONFIG_AUTOBOOT_DELAY_STR
265 see CONFIG_AUTOBOOT_STOP_STR
268 controls which network interface is used first.
271 controls which interface is currently active.
272 For example you can do the following::
275 => ping 192.168.0.1 # traffic sent on FEC
277 => ping 10.0.0.1 # traffic sent on SCC
280 When set to "no" U-Boot does not go through all
281 available network interfaces.
282 It just stays at the currently selected interface. When unset or set to
283 anything other than "no", U-Boot does go through all
284 available network interfaces.
287 When set to "no" each network operation will
288 either succeed or fail without retrying.
289 When set to "once" the network operation will
290 fail when all the available network interfaces
291 are tried once without success.
292 Useful on scripts which control the retry operation
296 If set then Linux will be told to boot silently, by
297 adding 'console=' to its command line. If "yes" it will be
298 made silent. If "no" it will not be made silent. If
299 unset, then it will be made silent if the U-Boot console
303 If this is set, the value is used for TFTP's
307 If this is set, the value is used for TFTP's UDP
308 destination port instead of the default port 69.
311 Block size to use for TFTP transfers; if not set,
312 we use the TFTP server's default block size
315 Retransmission timeout for TFTP packets (in milli-
316 seconds, minimum value is 1000 = 1 second). Defines
317 when a packet is considered to be lost so it has to
318 be retransmitted. The default is 5000 = 5 seconds.
319 Lowering this value may make downloads succeed
320 faster in networks with high packet loss rates or
321 with unreliable TFTP servers.
324 maximum count of TFTP timeouts (no
325 unit, minimum value = 0). Defines how many timeouts
326 can happen during a single file transfer before that
327 transfer is aborted. The default is 10, and 0 means
328 'no timeouts allowed'. Increasing this value may help
329 downloads succeed with high packet loss rates, or with
330 unreliable TFTP servers or client hardware.
333 if this is set, the value is used for TFTP's
334 window size as described by RFC 7440.
335 This means the count of blocks we can receive before
336 sending ack to server.
339 When set to a value < 4095 the traffic over
340 Ethernet is encapsulated/received over 802.1q
343 Note: This appears not to be used in U-Boot. See `README.VLAN`.
346 Period during which BOOTP/DHCP sends retries.
347 Unsigned value, in milliseconds. If not set, the period will
348 be either the default (28000), or a value based on
349 CONFIG_NET_RETRY_COUNT, if defined. This value has
350 precedence over the value based on CONFIG_NET_RETRY_COUNT.
353 Number of matches found by the last 'ms' command, in hex
356 Address of the last match found by the 'ms' command, in hex,
360 Index position of the last match found by the 'ms' command,
361 in units of the size (.b, .w, .l) of the search
364 (x86 only) Base address of the bzImage 'setup' block
367 (x86 only) Address of the loaded bzImage, typically
368 BZIMAGE_LOAD_ADDR which is 0x100000
374 The following image location variables contain the location of images
375 used in booting. The "Image" column gives the role of the image and is
376 not an environment variable name. The other columns are environment
377 variable names. "File Name" gives the name of the file on a TFTP
378 server, "RAM Address" gives the location in RAM the image will be
379 loaded to, and "Flash Location" gives the image's address in NOR
380 flash or offset in NAND flash.
382 *Note* - these variables don't have to be defined for all boards, some
383 boards currently use other variables for these purposes, and some
384 boards use these variables for other purposes.
386 Also note that most of these variables are just a commonly used set of variable
387 names, used in some other variable definitions, but are not hard-coded anywhere
390 ================= ============== ================ ==============
391 Image File Name RAM Address Flash Location
392 ================= ============== ================ ==============
393 Linux kernel bootfile kernel_addr_r kernel_addr
394 device tree blob fdtfile fdt_addr_r fdt_addr
395 ramdisk ramdiskfile ramdisk_addr_r ramdisk_addr
396 ================= ============== ================ ==============
399 Automatically updated variables
400 -------------------------------
402 The following environment variables may be used and automatically
403 updated by the network boot commands ("bootp" and "rarpboot"),
404 depending the information provided by your boot server:
406 ========= ===================================================
408 ========= ===================================================
410 dnsip IP address of your Domain Name Server
411 dnsip2 IP address of your secondary Domain Name Server
412 gatewayip IP address of the Gateway (Router) to use
413 hostname Target hostname
416 rootpath Pathname of the root filesystem on the NFS server
418 ========= ===================================================
421 Special environment variables
422 -----------------------------
424 There are two special Environment Variables:
427 contains hardware identification information such as type string and/or
430 Ethernet address. If CONFIG_REGEX=y, also eth*addr (where * is an integer).
432 These variables can be set only once (usually during manufacturing of
433 the board). U-Boot refuses to delete or overwrite these variables
434 once they have been set, unless CONFIG_ENV_OVERWRITE is enabled in the board
440 Contains the U-Boot version string as printed
441 with the "version" command. This variable is
442 readonly (see CONFIG_VERSION_VARIABLE).
444 Please note that changes to some configuration parameters may take
445 only effect after the next boot (yes, that's just like Windows).
448 External environment file
449 -------------------------
451 The `CONFIG_USE_DEFAULT_ENV_FILE` option provides a way to bypass the
452 environment generation in U-Boot. If enabled, then `CONFIG_DEFAULT_ENV_FILE`
453 provides the name of a file which is converted into the environment,
454 completely bypassing the standard environment variables in `env_default.h`.
456 The format is the same as accepted by the mkenvimage tool, with lines containing
457 key=value pairs. Blank lines and lines beginning with # are ignored.
459 Future work may unify this feature with the text-based environment, perhaps
460 moving the contents of `env_default.h` to a text file.
465 See :doc:`../develop/environment` for internal development details.