This option is very useful for A/B boot setups with read-only
filesystems: Letting U-Boot fill in the rootfs (and possibly related
parameters) allows keeping all boot parameters except the actual slot
selection in the extlinux.conf file, where they can be updated easily.
Signed-off-by: Fiona Klute <fiona.kl...@gmx.de>
Cc: Tom Rini <tr...@konsulko.com>
---
doc/README.pxe | 3 ++-
doc/develop/distro.rst | 7 +++++++
2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/doc/README.pxe b/doc/README.pxe
index 9fff2cd5ae9..ba189080e8c 100644
--- a/doc/README.pxe
+++ b/doc/README.pxe
@@ -178,7 +178,8 @@ devicetree-overlay <path> [...] - if this label is chosen,
use tftp to retrieve
kaslrseed - set this label to request random number from hwrng as
kaslr seed.
append <string> - use <string> as the kernel command line when
booting this
- label.
+ label. Environment variable references like ${var} are
+ substituted before boot.
initrd <path> - if this label is chosen, use tftp to retrieve the initrd
at <path>. it will be stored at the address indicated in
diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst
index 1d2f9c4c32b..01efce40a29 100644
--- a/doc/develop/distro.rst
+++ b/doc/develop/distro.rst
@@ -92,6 +92,13 @@ That said, we have some differences to these documents,
namely:
* If ``-`` is passed as fdt argument and ``CONFIG_SUPPORT_PASSING_ATAGS`` is
enabled, then no device tree will be used (legacy booting / pre-dtb kernel).
+* The ``append`` string may use environment variables. For example, an
+ A/B boot setup could use ``append root=PARTLABEL=root_${bootslot}``
+ to set the root filesystem to the right one for the selected slot,
+ assuming the ``bootslot`` environment variable is set before the
+ extlinux.conf file is processed, and the partition is labeled to
+ match.
+
See also doc/README.pxe under 'pxe file format'.
One example extlinux.conf generated by the Fedora installer is::
--
2.50.0
