From 97d6a2e3fe619b50303f97d0eebafaf50f1f075d Mon Sep 17 00:00:00 2001 From: Fiona Klute Date: Mon, 23 Jun 2025 13:38:01 +0200 Subject: doc: mention that extlinux.conf can use environment in "append" 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 Cc: Tom Rini --- doc/develop/distro.rst | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'doc/develop') 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:: -- cgit v1.2.3