doc:bootmenu: add description for UEFI boot support

The bootmenu enumerates the UEFI boot options
for boot device selection.
This commit adds the description how the UEFI boot work
in bootmenu. This commit also adds "Synopsis", "Description"
and "Configuration" sections to follow the U-Boot command
documentation format.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

1 files changed, 74 insertions, 0 deletions

diff --git a/doc/usage/cmd/bootmenu.rst b/doc/usage/cmd/bootmenu.rst
index 9430f8c9aa1..cb3c8d2f93c 100644
--- a/doc/usage/cmd/bootmenu.rst
+++ b/doc/usage/cmd/bootmenu.rst
@@ -4,6 +4,15 @@
 bootmenu command
 ================
 
+Synopsis
+--------
+::
+
+    bootmenu [delay]
+
+Description
+-----------
+
 The "bootmenu" command uses U-Boot menu interfaces and provides
 a simple mechanism for creating menus with different boot items.
 The cursor keys "Up" and "Down" are used for navigation through
@@ -79,6 +88,55 @@ The above example will be rendered as below::
 The selected menu entry will be highlighted - it will have inverted
 background and text colors.
 
+UEFI boot variable enumeration
+''''''''''''''''''''''''''''''
+If enabled, the bootmenu command will automatically generate and add
+UEFI-related boot menu entries for the following items.
+
+ * possible bootable media with default file names
+ * user-defined UEFI boot options
+
+The bootmenu automatically enumerates the possible bootable
+media devices supporting EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.
+This auto generated entry is named as "<interface> <devnum>:<part>" format.
+(e.g. "usb 0:1")
+
+The bootmenu displays the UEFI-related menu entries in order of "BootOrder".
+When the user selects the UEFI boot menu entry, the bootmenu sets
+the selected boot variable index to "BootNext" without non-volatile attribute,
+then call the uefi boot manager with the command "bootefi bootmgr".
+
+Example bootmenu is as below::
+
+    *** U-Boot Boot Menu ***
+
+       mmc 0:1
+       mmc 0:2
+       debian
+       nvme 0:1
+       ubuntu
+       nvme 0:2
+       usb 0:2
+       U-Boot console
+
+Default behavior when user exits from the bootmenu
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+User can exit from bootmenu by selecting the last entry
+"U-Boot console"/"Quit" or ESC/CTRL+C key.
+
+When the CONFIG_BOOTMENU_DISABLE_UBOOT_CONSOLE is disabled,
+user exits from the bootmenu and returns to the U-Boot console.
+
+When the CONFIG_BOOTMENU_DISABLE_UBOOT_CONSOLE is enabled, user can not
+enter the U-Boot console. When the user exits from the bootmenu,
+the bootmenu invokes the following default behavior.
+
+ * if CONFIG_CMD_BOOTEFI_BOOTMGR is enabled, execute "bootefi bootmgr" command
+ * "bootefi bootmgr" fails or is not enabled, then execute "run bootcmd" command.
+
+Configuration
+-------------
+
 The "bootmenu" command is enabled by::
 
     CONFIG_CMD_BOOTMENU=y
@@ -88,3 +146,19 @@ To run the bootmenu at startup add these additional settings::
     CONFIG_AUTOBOOT_KEYED=y
     CONFIG_BOOTDELAY=30
     CONFIG_AUTOBOOT_MENU_SHOW=y
+
+UEFI boot variable enumeration is enabled by::
+
+    CONFIG_CMD_BOOTEFI_BOOTMGR=y
+
+To improve the product security, entering U-Boot console from bootmenu
+can be disabled by::
+
+    CONFIG_BOOTMENU_DISABLE_UBOOT_CONSOLE=y
+
+To scan the discoverable devices connected to the buses such as
+USB and PCIe prior to bootmenu showing up, CONFIG_PREBOOT can be
+used to run the command before showing the bootmenu, i.e.::
+
+    CONFIG_USE_PREBOOT=y
+    CONFIG_PREBOOT="pci enum; usb start; scsi scan; nvme scan; virtio scan"