diff mbox series

[v4,2/5] doc: Move environment documentation to rST

Message ID 20210919185950.3813952-3-sjg@chromium.org
State Superseded
Delegated to: Tom Rini
Headers show
Series env: Allow environment in text files | expand

Commit Message

Simon Glass Sept. 19, 2021, 6:59 p.m. UTC
Move this from the README to rST format.

Drop i2cfast since it is obviously obsolete and breaks the formatting.

Signed-off-by: Simon Glass <sjg@chromium.org>
---

Changes in v4:
- Add new patch to move environment documentation to rST

 README                    | 328 --------------------------------
 doc/usage/environment.rst | 382 ++++++++++++++++++++++++++++++++++++++
 doc/usage/index.rst       |   1 +
 3 files changed, 383 insertions(+), 328 deletions(-)
 create mode 100644 doc/usage/environment.rst

Comments

Wolfgang Denk Sept. 20, 2021, 12:18 p.m. UTC | #1
Dear Simon,

In message <20210919185950.3813952-3-sjg@chromium.org> you wrote:
> Move this from the README to rST format.

Just a few nitpicks...

> diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
> new file mode 100644
> index 00000000000..be785a8f717
> --- /dev/null
> +++ b/doc/usage/environment.rst
> @@ -0,0 +1,382 @@
> +.. SPDX-License-Identifier: GPL-2.0+
> +
> +Environment Variables
> +=====================
> +
> +
> +U-Boot supports user configuration using Environment Variables which
> +can be made persistent by saving to Flash memory.

...by saving to persistent storage, for example flash memory.

> +Environment Variables are set using "setenv", printed using
> +"printenv", and saved to Flash using "saveenv". Using "setenv"

Environment Variables are set using "env set" (alias "setenv"),
printed using "env print" (alias "printenv"), and saved to
persistent storage using "env save" (alias "saveenv").
Using "env set" ...

> +Some configuration options can be set using Environment Variables.

This probably needs an explanation what "configuration options'
means here?


> +Image locations
> +---------------
> +
> +The following image location variables contain the location of images
> +used in booting. The "Image" column gives the role of the image and is
> +not an environment variable name. The other columns are environment
> +variable names. "File Name" gives the name of the file on a TFTP
> +server, "RAM Address" gives the location in RAM the image will be
> +loaded to, and "Flash Location" gives the image's address in NOR
> +flash or offset in NAND flash.
> +
> +*Note* - these variables don't have to be defined for all boards, some
> +boards currently use other variables for these purposes, and some
> +boards use these variables for other purposes.
> +
> +================= ============== ================ ==============
> +Image             File Name      RAM Address      Flash Location
> +================= ============== ================ ==============
> +u-boot            u-boot         u-boot_addr_r    u-boot_addr
> +Linux kernel      bootfile       kernel_addr_r    kernel_addr
> +device tree blob  fdtfile        fdt_addr_r       fdt_addr
> +ramdisk           ramdiskfile    ramdisk_addr_r   ramdisk_addr
> +================= ============== ================ ==============

Maybe it should be pointed out that this is just a commonly used set
of variable names, used in some other variable definitions, but is
in no way hard coded anywhere in U-Boot code.

OK, I see that "bootfile" and ""fdtfile" are now actually used in
some code - but IMO this is a bad idea and should be fixed.


> +Automatically updated variables
> +-------------------------------
> +
> +The following environment variables may be used and automatically
> +updated by the network boot commands ("bootp" and "rarpboot"),
> +depending the information provided by your boot server:
> +
> +=========  ===================================================
> +Variable   Notes
> +=========  ===================================================
> +bootfile   see above
> +dnsip      IP address of your Domain Name Server
> +dnsip2     IP address of your secondary Domain Name Server
> +gatewayip  IP address of the Gateway (Router) to use
> +hostname   Target hostname
> +ipaddr     See above
> +netmask    Subnet Mask
> +rootpath   Pathname of the root filesystem on the NFS server
> +serverip   see above
> +=========  ===================================================

This list is incomplete; for example, sysboot sets bootfile, too.

For completeness, .flags etc. should be documented, too [but yes,
this is a separate task of course].


Best regards,

Wolfgang Denk
diff mbox series

Patch

diff --git a/README b/README
index a3f81e4aed0..31eaf2f5f81 100644
--- a/README
+++ b/README
@@ -3180,334 +3180,6 @@  TODO.
 For now: just type "help <command>".
 
 
-Environment Variables:
-======================
-
-U-Boot supports user configuration using Environment Variables which
-can be made persistent by saving to Flash memory.
-
-Environment Variables are set using "setenv", printed using
-"printenv", and saved to Flash using "saveenv". Using "setenv"
-without a value can be used to delete a variable from the
-environment. As long as you don't save the environment you are
-working with an in-memory copy. In case the Flash area containing the
-environment is erased by accident, a default environment is provided.
-
-Some configuration options can be set using Environment Variables.
-
-List of environment variables (most likely not complete):
-
-  baudrate	- see CONFIG_BAUDRATE
-
-  bootdelay	- see CONFIG_BOOTDELAY
-
-  bootcmd	- see CONFIG_BOOTCOMMAND
-
-  bootargs	- Boot arguments when booting an RTOS image
-
-  bootfile	- Name of the image to load with TFTP
-
-  bootm_low	- Memory range available for image processing in the bootm
-		  command can be restricted. This variable is given as
-		  a hexadecimal number and defines lowest address allowed
-		  for use by the bootm command. See also "bootm_size"
-		  environment variable. Address defined by "bootm_low" is
-		  also the base of the initial memory mapping for the Linux
-		  kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
-		  bootm_mapsize.
-
-  bootm_mapsize - Size of the initial memory mapping for the Linux kernel.
-		  This variable is given as a hexadecimal number and it
-		  defines the size of the memory region starting at base
-		  address bootm_low that is accessible by the Linux kernel
-		  during early boot.  If unset, CONFIG_SYS_BOOTMAPSZ is used
-		  as the default value if it is defined, and bootm_size is
-		  used otherwise.
-
-  bootm_size	- Memory range available for image processing in the bootm
-		  command can be restricted. This variable is given as
-		  a hexadecimal number and defines the size of the region
-		  allowed for use by the bootm command. See also "bootm_low"
-		  environment variable.
-
-  bootstopkeysha256, bootdelaykey, bootstopkey	- See README.autoboot
-
-  updatefile	- Location of the software update file on a TFTP server, used
-		  by the automatic software update feature. Please refer to
-		  documentation in doc/README.update for more details.
-
-  autoload	- if set to "no" (any string beginning with 'n'),
-		  "bootp" will just load perform a lookup of the
-		  configuration from the BOOTP server, but not try to
-		  load any image using TFTP
-
-  autostart	- if set to "yes", an image loaded using the "bootp",
-		  "rarpboot", "tftpboot" or "diskboot" commands will
-		  be automatically started (by internally calling
-		  "bootm")
-
-		  If set to "no", a standalone image passed to the
-		  "bootm" command will be copied to the load address
-		  (and eventually uncompressed), but NOT be started.
-		  This can be used to load and uncompress arbitrary
-		  data.
-
-  fdt_high	- if set this restricts the maximum address that the
-		  flattened device tree will be copied into upon boot.
-		  For example, if you have a system with 1 GB memory
-		  at physical address 0x10000000, while Linux kernel
-		  only recognizes the first 704 MB as low memory, you
-		  may need to set fdt_high as 0x3C000000 to have the
-		  device tree blob be copied to the maximum address
-		  of the 704 MB low memory, so that Linux kernel can
-		  access it during the boot procedure.
-
-		  If this is set to the special value 0xFFFFFFFF then
-		  the fdt will not be copied at all on boot.  For this
-		  to work it must reside in writable memory, have
-		  sufficient padding on the end of it for u-boot to
-		  add the information it needs into it, and the memory
-		  must be accessible by the kernel.
-
-  fdtcontroladdr- if set this is the address of the control flattened
-		  device tree used by U-Boot when CONFIG_OF_CONTROL is
-		  defined.
-
-  i2cfast	- (PPC405GP|PPC405EP only)
-		  if set to 'y' configures Linux I2C driver for fast
-		  mode (400kHZ). This environment variable is used in
-		  initialization code. So, for changes to be effective
-		  it must be saved and board must be reset.
-
-  initrd_high	- restrict positioning of initrd images:
-		  If this variable is not set, initrd images will be
-		  copied to the highest possible address in RAM; this
-		  is usually what you want since it allows for
-		  maximum initrd size. If for some reason you want to
-		  make sure that the initrd image is loaded below the
-		  CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
-		  variable to a value of "no" or "off" or "0".
-		  Alternatively, you can set it to a maximum upper
-		  address to use (U-Boot will still check that it
-		  does not overwrite the U-Boot stack and data).
-
-		  For instance, when you have a system with 16 MB
-		  RAM, and want to reserve 4 MB from use by Linux,
-		  you can do this by adding "mem=12M" to the value of
-		  the "bootargs" variable. However, now you must make
-		  sure that the initrd image is placed in the first
-		  12 MB as well - this can be done with
-
-		  setenv initrd_high 00c00000
-
-		  If you set initrd_high to 0xFFFFFFFF, this is an
-		  indication to U-Boot that all addresses are legal
-		  for the Linux kernel, including addresses in flash
-		  memory. In this case U-Boot will NOT COPY the
-		  ramdisk at all. This may be useful to reduce the
-		  boot time on your system, but requires that this
-		  feature is supported by your Linux kernel.
-
-  ipaddr	- IP address; needed for tftpboot command
-
-  loadaddr	- Default load address for commands like "bootp",
-		  "rarpboot", "tftpboot", "loadb" or "diskboot"
-
-  loads_echo	- see CONFIG_LOADS_ECHO
-
-  serverip	- TFTP server IP address; needed for tftpboot command
-
-  bootretry	- see CONFIG_BOOT_RETRY_TIME
-
-  bootdelaykey	- see CONFIG_AUTOBOOT_DELAY_STR
-
-  bootstopkey	- see CONFIG_AUTOBOOT_STOP_STR
-
-  ethprime	- controls which interface is used first.
-
-  ethact	- controls which interface is currently active.
-		  For example you can do the following
-
-		  => setenv ethact FEC
-		  => ping 192.168.0.1 # traffic sent on FEC
-		  => setenv ethact SCC
-		  => ping 10.0.0.1 # traffic sent on SCC
-
-  ethrotate	- When set to "no" U-Boot does not go through all
-		  available network interfaces.
-		  It just stays at the currently selected interface.
-
-  netretry	- When set to "no" each network operation will
-		  either succeed or fail without retrying.
-		  When set to "once" the network operation will
-		  fail when all the available network interfaces
-		  are tried once without success.
-		  Useful on scripts which control the retry operation
-		  themselves.
-
-  npe_ucode	- set load address for the NPE microcode
-
-  silent_linux  - If set then Linux will be told to boot silently, by
-		  changing the console to be empty. If "yes" it will be
-		  made silent. If "no" it will not be made silent. If
-		  unset, then it will be made silent if the U-Boot console
-		  is silent.
-
-  tftpsrcp	- If this is set, the value is used for TFTP's
-		  UDP source port.
-
-  tftpdstp	- If this is set, the value is used for TFTP's UDP
-		  destination port instead of the Well Know Port 69.
-
-  tftpblocksize - Block size to use for TFTP transfers; if not set,
-		  we use the TFTP server's default block size
-
-  tftptimeout	- Retransmission timeout for TFTP packets (in milli-
-		  seconds, minimum value is 1000 = 1 second). Defines
-		  when a packet is considered to be lost so it has to
-		  be retransmitted. The default is 5000 = 5 seconds.
-		  Lowering this value may make downloads succeed
-		  faster in networks with high packet loss rates or
-		  with unreliable TFTP servers.
-
-  tftptimeoutcountmax	- maximum count of TFTP timeouts (no
-		  unit, minimum value = 0). Defines how many timeouts
-		  can happen during a single file transfer before that
-		  transfer is aborted. The default is 10, and 0 means
-		  'no timeouts allowed'. Increasing this value may help
-		  downloads succeed with high packet loss rates, or with
-		  unreliable TFTP servers or client hardware.
-
-  tftpwindowsize	- if this is set, the value is used for TFTP's
-		  window size as described by RFC 7440.
-		  This means the count of blocks we can receive before
-		  sending ack to server.
-
-  vlan		- When set to a value < 4095 the traffic over
-		  Ethernet is encapsulated/received over 802.1q
-		  VLAN tagged frames.
-
-  bootpretryperiod	- Period during which BOOTP/DHCP sends retries.
-		  Unsigned value, in milliseconds. If not set, the period will
-		  be either the default (28000), or a value based on
-		  CONFIG_NET_RETRY_COUNT, if defined. This value has
-		  precedence over the valu based on CONFIG_NET_RETRY_COUNT.
-
-  memmatches	- Number of matches found by the last 'ms' command, in hex
-
-  memaddr	- Address of the last match found by the 'ms' command, in hex,
-		  or 0 if none
-
-  mempos	- Index position of the last match found by the 'ms' command,
-		  in units of the size (.b, .w, .l) of the search
-
-  zbootbase	- (x86 only) Base address of the bzImage 'setup' block
-
-  zbootaddr	- (x86 only) Address of the loaded bzImage, typically
-		  BZIMAGE_LOAD_ADDR which is 0x100000
-
-The following image location variables contain the location of images
-used in booting. The "Image" column gives the role of the image and is
-not an environment variable name. The other columns are environment
-variable names. "File Name" gives the name of the file on a TFTP
-server, "RAM Address" gives the location in RAM the image will be
-loaded to, and "Flash Location" gives the image's address in NOR
-flash or offset in NAND flash.
-
-*Note* - these variables don't have to be defined for all boards, some
-boards currently use other variables for these purposes, and some
-boards use these variables for other purposes.
-
-Image		    File Name	     RAM Address       Flash Location
------		    ---------	     -----------       --------------
-u-boot		    u-boot	     u-boot_addr_r     u-boot_addr
-Linux kernel	    bootfile	     kernel_addr_r     kernel_addr
-device tree blob    fdtfile	     fdt_addr_r	       fdt_addr
-ramdisk		    ramdiskfile	     ramdisk_addr_r    ramdisk_addr
-
-The following environment variables may be used and automatically
-updated by the network boot commands ("bootp" and "rarpboot"),
-depending the information provided by your boot server:
-
-  bootfile	- see above
-  dnsip		- IP address of your Domain Name Server
-  dnsip2	- IP address of your secondary Domain Name Server
-  gatewayip	- IP address of the Gateway (Router) to use
-  hostname	- Target hostname
-  ipaddr	- see above
-  netmask	- Subnet Mask
-  rootpath	- Pathname of the root filesystem on the NFS server
-  serverip	- see above
-
-
-There are two special Environment Variables:
-
-  serial#	- contains hardware identification information such
-		  as type string and/or serial number
-  ethaddr	- Ethernet address
-
-These variables can be set only once (usually during manufacturing of
-the board). U-Boot refuses to delete or overwrite these variables
-once they have been set once.
-
-
-Further special Environment Variables:
-
-  ver		- Contains the U-Boot version string as printed
-		  with the "version" command. This variable is
-		  readonly (see CONFIG_VERSION_VARIABLE).
-
-
-Please note that changes to some configuration parameters may take
-only effect after the next boot (yes, that's just like Windoze :-).
-
-
-Callback functions for environment variables:
----------------------------------------------
-
-For some environment variables, the behavior of u-boot needs to change
-when their values are changed.  This functionality allows functions to
-be associated with arbitrary variables.  On creation, overwrite, or
-deletion, the callback will provide the opportunity for some side
-effect to happen or for the change to be rejected.
-
-The callbacks are named and associated with a function using the
-U_BOOT_ENV_CALLBACK macro in your board or driver code.
-
-These callbacks are associated with variables in one of two ways.  The
-static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
-in the board configuration to a string that defines a list of
-associations.  The list must be in the following format:
-
-	entry = variable_name[:callback_name]
-	list = entry[,list]
-
-If the callback name is not specified, then the callback is deleted.
-Spaces are also allowed anywhere in the list.
-
-Callbacks can also be associated by defining the ".callbacks" variable
-with the same list format above.  Any association in ".callbacks" will
-override any association in the static list. You can define
-CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
-".callbacks" environment variable in the default or embedded environment.
-
-If CONFIG_REGEX is defined, the variable_name above is evaluated as a
-regular expression. This allows multiple variables to be connected to
-the same callback without explicitly listing them all out.
-
-The signature of the callback functions is:
-
-    int callback(const char *name, const char *value, enum env_op op, int flags)
-
-* name - changed environment variable
-* value - new value of the environment variable
-* op - operation (create, overwrite, or delete)
-* flags - attributes of the environment variable change, see flags H_* in
-  include/search.h
-
-The return value is 0 if the variable change is accepted and 1 otherwise.
-
-
 Note for Redundant Ethernet Interfaces:
 =======================================
 
diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
new file mode 100644
index 00000000000..be785a8f717
--- /dev/null
+++ b/doc/usage/environment.rst
@@ -0,0 +1,382 @@ 
+.. SPDX-License-Identifier: GPL-2.0+
+
+Environment Variables
+=====================
+
+
+U-Boot supports user configuration using Environment Variables which
+can be made persistent by saving to Flash memory.
+
+Environment Variables are set using "setenv", printed using
+"printenv", and saved to Flash using "saveenv". Using "setenv"
+without a value can be used to delete a variable from the
+environment. As long as you don't save the environment you are
+working with an in-memory copy. In case the Flash area containing the
+environment is erased by accident, a default environment is provided.
+
+Some configuration options can be set using Environment Variables.
+
+List of environment variables (most likely not complete):
+
+baudrate
+    see CONFIG_BAUDRATE
+
+bootdelay
+    see CONFIG_BOOTDELAY
+
+bootcmd
+    see CONFIG_BOOTCOMMAND
+
+bootargs
+    Boot arguments when booting an RTOS image
+
+bootfile
+    Name of the image to load with TFTP
+
+bootm_low
+    Memory range available for image processing in the bootm
+    command can be restricted. This variable is given as
+    a hexadecimal number and defines lowest address allowed
+    for use by the bootm command. See also "bootm_size"
+    environment variable. Address defined by "bootm_low" is
+    also the base of the initial memory mapping for the Linux
+    kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
+    bootm_mapsize.
+
+bootm_mapsize
+    Size of the initial memory mapping for the Linux kernel.
+    This variable is given as a hexadecimal number and it
+    defines the size of the memory region starting at base
+    address bootm_low that is accessible by the Linux kernel
+    during early boot.  If unset, CONFIG_SYS_BOOTMAPSZ is used
+    as the default value if it is defined, and bootm_size is
+    used otherwise.
+
+bootm_size
+    Memory range available for image processing in the bootm
+    command can be restricted. This variable is given as
+    a hexadecimal number and defines the size of the region
+    allowed for use by the bootm command. See also "bootm_low"
+    environment variable.
+
+bootstopkeysha256, bootdelaykey, bootstopkey
+    See README.autoboot
+
+updatefile
+    Location of the software update file on a TFTP server, used
+    by the automatic software update feature. Please refer to
+    documentation in doc/README.update for more details.
+
+autoload
+    if set to "no" (any string beginning with 'n'),
+    "bootp" will just load perform a lookup of the
+    configuration from the BOOTP server, but not try to
+    load any image using TFTP
+
+autostart
+    if set to "yes", an image loaded using the "bootp",
+    "rarpboot", "tftpboot" or "diskboot" commands will
+    be automatically started (by internally calling
+    "bootm")
+
+    If set to "no", a standalone image passed to the
+    "bootm" command will be copied to the load address
+    (and eventually uncompressed), but NOT be started.
+    This can be used to load and uncompress arbitrary
+    data.
+
+fdt_high
+    if set this restricts the maximum address that the
+    flattened device tree will be copied into upon boot.
+    For example, if you have a system with 1 GB memory
+    at physical address 0x10000000, while Linux kernel
+    only recognizes the first 704 MB as low memory, you
+    may need to set fdt_high as 0x3C000000 to have the
+    device tree blob be copied to the maximum address
+    of the 704 MB low memory, so that Linux kernel can
+    access it during the boot procedure.
+
+    If this is set to the special value 0xFFFFFFFF then
+    the fdt will not be copied at all on boot.  For this
+    to work it must reside in writable memory, have
+    sufficient padding on the end of it for u-boot to
+    add the information it needs into it, and the memory
+    must be accessible by the kernel.
+
+fdtcontroladdr
+    if set this is the address of the control flattened
+    device tree used by U-Boot when CONFIG_OF_CONTROL is
+    defined.
+
+initrd_high
+    restrict positioning of initrd images:
+    If this variable is not set, initrd images will be
+    copied to the highest possible address in RAM; this
+    is usually what you want since it allows for
+    maximum initrd size. If for some reason you want to
+    make sure that the initrd image is loaded below the
+    CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
+    variable to a value of "no" or "off" or "0".
+    Alternatively, you can set it to a maximum upper
+    address to use (U-Boot will still check that it
+    does not overwrite the U-Boot stack and data).
+
+    For instance, when you have a system with 16 MB
+    RAM, and want to reserve 4 MB from use by Linux,
+    you can do this by adding "mem=12M" to the value of
+    the "bootargs" variable. However, now you must make
+    sure that the initrd image is placed in the first
+    12 MB as well - this can be done with::
+
+        setenv initrd_high 00c00000
+
+    If you set initrd_high to 0xFFFFFFFF, this is an
+    indication to U-Boot that all addresses are legal
+    for the Linux kernel, including addresses in flash
+    memory. In this case U-Boot will NOT COPY the
+    ramdisk at all. This may be useful to reduce the
+    boot time on your system, but requires that this
+    feature is supported by your Linux kernel.
+
+ipaddr
+    IP address; needed for tftpboot command
+
+loadaddr
+    Default load address for commands like "bootp",
+    "rarpboot", "tftpboot", "loadb" or "diskboot"
+
+loads_echo
+    see CONFIG_LOADS_ECHO
+
+serverip
+    TFTP server IP address; needed for tftpboot command
+
+bootretry
+    see CONFIG_BOOT_RETRY_TIME
+
+bootdelaykey
+    see CONFIG_AUTOBOOT_DELAY_STR
+
+bootstopkey
+    see CONFIG_AUTOBOOT_STOP_STR
+
+ethprime
+    controls which interface is used first.
+
+ethact
+    controls which interface is currently active.
+    For example you can do the following::
+
+    => setenv ethact FEC
+    => ping 192.168.0.1 # traffic sent on FEC
+    => setenv ethact SCC
+    => ping 10.0.0.1 # traffic sent on SCC
+
+ethrotate
+    When set to "no" U-Boot does not go through all
+    available network interfaces.
+    It just stays at the currently selected interface.
+
+netretry
+    When set to "no" each network operation will
+    either succeed or fail without retrying.
+    When set to "once" the network operation will
+    fail when all the available network interfaces
+    are tried once without success.
+    Useful on scripts which control the retry operation
+    themselves.
+
+npe_ucode
+    set load address for the NPE microcode
+
+silent_linux
+    If set then Linux will be told to boot silently, by
+    changing the console to be empty. If "yes" it will be
+    made silent. If "no" it will not be made silent. If
+    unset, then it will be made silent if the U-Boot console
+    is silent.
+
+tftpsrcp
+    If this is set, the value is used for TFTP's
+    UDP source port.
+
+tftpdstp
+    If this is set, the value is used for TFTP's UDP
+    destination port instead of the Well Know Port 69.
+
+tftpblocksize
+    Block size to use for TFTP transfers; if not set,
+    we use the TFTP server's default block size
+
+tftptimeout
+    Retransmission timeout for TFTP packets (in milli-
+    seconds, minimum value is 1000 = 1 second). Defines
+    when a packet is considered to be lost so it has to
+    be retransmitted. The default is 5000 = 5 seconds.
+    Lowering this value may make downloads succeed
+    faster in networks with high packet loss rates or
+    with unreliable TFTP servers.
+
+tftptimeoutcountmax
+    maximum count of TFTP timeouts (no
+    unit, minimum value = 0). Defines how many timeouts
+    can happen during a single file transfer before that
+    transfer is aborted. The default is 10, and 0 means
+    'no timeouts allowed'. Increasing this value may help
+    downloads succeed with high packet loss rates, or with
+    unreliable TFTP servers or client hardware.
+
+tftpwindowsize
+    if this is set, the value is used for TFTP's
+    window size as described by RFC 7440.
+    This means the count of blocks we can receive before
+    sending ack to server.
+
+vlan
+    When set to a value < 4095 the traffic over
+    Ethernet is encapsulated/received over 802.1q
+    VLAN tagged frames.
+
+bootpretryperiod
+    Period during which BOOTP/DHCP sends retries.
+    Unsigned value, in milliseconds. If not set, the period will
+    be either the default (28000), or a value based on
+    CONFIG_NET_RETRY_COUNT, if defined. This value has
+    precedence over the valu based on CONFIG_NET_RETRY_COUNT.
+
+memmatches
+    Number of matches found by the last 'ms' command, in hex
+
+memaddr
+    Address of the last match found by the 'ms' command, in hex,
+    or 0 if none
+
+mempos
+    Index position of the last match found by the 'ms' command,
+    in units of the size (.b, .w, .l) of the search
+
+zbootbase
+    (x86 only) Base address of the bzImage 'setup' block
+
+zbootaddr
+    (x86 only) Address of the loaded bzImage, typically
+    BZIMAGE_LOAD_ADDR which is 0x100000
+
+
+Image locations
+---------------
+
+The following image location variables contain the location of images
+used in booting. The "Image" column gives the role of the image and is
+not an environment variable name. The other columns are environment
+variable names. "File Name" gives the name of the file on a TFTP
+server, "RAM Address" gives the location in RAM the image will be
+loaded to, and "Flash Location" gives the image's address in NOR
+flash or offset in NAND flash.
+
+*Note* - these variables don't have to be defined for all boards, some
+boards currently use other variables for these purposes, and some
+boards use these variables for other purposes.
+
+================= ============== ================ ==============
+Image             File Name      RAM Address      Flash Location
+================= ============== ================ ==============
+u-boot            u-boot         u-boot_addr_r    u-boot_addr
+Linux kernel      bootfile       kernel_addr_r    kernel_addr
+device tree blob  fdtfile        fdt_addr_r       fdt_addr
+ramdisk           ramdiskfile    ramdisk_addr_r   ramdisk_addr
+================= ============== ================ ==============
+
+
+Automatically updated variables
+-------------------------------
+
+The following environment variables may be used and automatically
+updated by the network boot commands ("bootp" and "rarpboot"),
+depending the information provided by your boot server:
+
+=========  ===================================================
+Variable   Notes
+=========  ===================================================
+bootfile   see above
+dnsip      IP address of your Domain Name Server
+dnsip2     IP address of your secondary Domain Name Server
+gatewayip  IP address of the Gateway (Router) to use
+hostname   Target hostname
+ipaddr     See above
+netmask    Subnet Mask
+rootpath   Pathname of the root filesystem on the NFS server
+serverip   see above
+=========  ===================================================
+
+
+Special environment variables
+-----------------------------
+
+There are two special Environment Variables:
+
+serial#
+    contains hardware identification information such as type string and/or
+    serial number
+ethaddr
+    Ethernet address
+
+These variables can be set only once (usually during manufacturing of
+the board). U-Boot refuses to delete or overwrite these variables
+once they have been set once.
+
+Also:
+
+ver
+    Contains the U-Boot version string as printed
+    with the "version" command. This variable is
+    readonly (see CONFIG_VERSION_VARIABLE).
+
+Please note that changes to some configuration parameters may take
+only effect after the next boot (yes, that's just like Windoze :-).
+
+
+Callback functions for environment variables
+--------------------------------------------
+
+For some environment variables, the behavior of u-boot needs to change
+when their values are changed.  This functionality allows functions to
+be associated with arbitrary variables.  On creation, overwrite, or
+deletion, the callback will provide the opportunity for some side
+effect to happen or for the change to be rejected.
+
+The callbacks are named and associated with a function using the
+U_BOOT_ENV_CALLBACK macro in your board or driver code.
+
+These callbacks are associated with variables in one of two ways.  The
+static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
+in the board configuration to a string that defines a list of
+associations.  The list must be in the following format::
+
+    entry = variable_name[:callback_name]
+    list = entry[,list]
+
+If the callback name is not specified, then the callback is deleted.
+Spaces are also allowed anywhere in the list.
+
+Callbacks can also be associated by defining the ".callbacks" variable
+with the same list format above.  Any association in ".callbacks" will
+override any association in the static list. You can define
+CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
+".callbacks" environment variable in the default or embedded environment.
+
+If CONFIG_REGEX is defined, the variable_name above is evaluated as a
+regular expression. This allows multiple variables to be connected to
+the same callback without explicitly listing them all out.
+
+The signature of the callback functions is::
+
+    int callback(const char *name, const char *value, enum env_op op, int flags)
+
+* name - changed environment variable
+* value - new value of the environment variable
+* op - operation (create, overwrite, or delete)
+* flags - attributes of the environment variable change, see flags H_* in
+  include/search.h
+
+The return value is 0 if the variable change is accepted and 1 otherwise.
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 356f2a56181..1a79d1c03eb 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -5,6 +5,7 @@  Use U-Boot
    :maxdepth: 1
 
    dfu
+   environment
    fdt_overlays
    fit
    netconsole