bootstd: Add command to enable setting of bootmeth specific properties

We have previously added logic to allow a "fallback" option to be
specified in the extlinux configuration. Provide a command that allows
us to set this as the preferred default option when booting.

Combined with the bootcount functionality, this allows the "altbootcmd"
to provide a means of falling back to a previously known good state
after a failed update. For example, if "bootcmd" is set to:

    bootflow scan -lb

We would set "altbootcmd" to:

    bootmeth set extlinux fallback 1; bootflow scan -lb

Causing the boot process to boot from the fallback option.

Reviewed-by: Simon Glass <[email protected]>
Signed-off-by: Martyn Welch <[email protected]>

2 files changed, 45 insertions, 3 deletions

diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index c6f003851b2..a2913cd47be 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -103,6 +103,12 @@ provide a `read_bootflow()` method which checks whatever bootdevs it likes, then
 returns the bootflow, if found. Some of these bootmeths may be very slow, if
 they scan a lot of devices.
 
+The extlinux bootmeth also allows for bootmeth specific configuration to be
+set. A bootmeth that wishes to support this provides the `set_property()`
+method. This allows string properties and values to be passed to the bootmeth.
+It is up to the bootmeth to determine what action to take when this method is
+called.
+
 
 Boot process
 ------------
@@ -459,8 +465,8 @@ Three commands are available:
     See :doc:`/usage/cmd/bootflow`
 
 `bootmeth`
-    Allow listing of available bootmethds and setting the order in which they
-    are tried. See :doc:`/usage/cmd/bootmeth`
+    Allow listing of available bootmethds, setting the order in which they are
+    tried and bootmeth specific configuration. See :doc:`/usage/cmd/bootmeth`
 
 .. _BootflowStates:
 
diff --git a/doc/usage/cmd/bootmeth.rst b/doc/usage/cmd/bootmeth.rst
index c3d2ec1574b..4f899d92b2e 100644
--- a/doc/usage/cmd/bootmeth.rst
+++ b/doc/usage/cmd/bootmeth.rst
@@ -12,7 +12,8 @@ Synopsis
 ::
 
     bootmeth list [-a]          - list selected bootmeths (-a for all)
-    bootmeth order "[<bm> ...]" - select the order of bootmeths\n"
+    bootmeth order "[<bm> ...]" - select the order of bootmeths
+    bootmeth set <bootmeth> <property> <value> - set optional property
 
 
 Description
@@ -112,3 +113,38 @@ which are not::
         -    4  efi_mgr             EFI bootmgr flow
     -----  ---  ------------------  ------------------
     (5 bootmeths)
+
+
+bootmeth set
+~~~~~~~~~~~~
+
+Allows setting of bootmeth specific configuration. This allows finer grain
+control over the boot process in specific instances.
+
+
+Supported Configuration Options
+-------------------------------
+
+The following configuration options are currently supported:
+
+========  ===================  ======  ===============================
+Property  Supported Bootmeths  Values  Description
+========  ===================  ======  ===============================
+fallback  extlinux             0 | 1     Enable or disable fallback path
+========  ===================  ======  ===============================
+
+
+Bootmeth set Example
+--------------------
+
+With the bootcount functionality enabled, when the bootlimit is reached, the
+`altbootcmd` environment variable lists the command used for booting rather
+than `bootcmd`. We can set the fallback configuration to cause the fallback
+boot option to be preferred, to revert to a previous known working boot option
+after a failed update for example. So if `bootcmd` is set to::
+
+    bootflow scan -lb
+
+We would set "altbootcmd" to::
+
+    bootmeth set extlinux fallback 1; bootflow scan -lb