From: Fiona Klute Date: Mon, 23 Jun 2025 11:38:01 +0000 (+0200) Subject: doc: mention that extlinux.conf can use environment in "append" X-Git-Tag: v2025.07~3^2~1 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=97d6a2e3fe619b50303f97d0eebafaf50f1f075d;p=pandora-u-boot.git 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 --- 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 [...] - if this label is chosen, use tftp to retrieve kaslrseed - set this label to request random number from hwrng as kaslr seed. append - use as the kernel command line when booting this - label. + label. Environment variable references like ${var} are + substituted before boot. initrd - if this label is chosen, use tftp to retrieve the initrd at . it will be stored at the address indicated in diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst index 1d2f9c4c32b..01efce40a29 100644 --- a/doc/develop/distro.rst +++ b/doc/develop/distro.rst @@ -92,6 +92,13 @@ That said, we have some differences to these documents, namely: * If ``-`` is passed as fdt argument and ``CONFIG_SUPPORT_PASSING_ATAGS`` is enabled, then no device tree will be used (legacy booting / pre-dtb kernel). +* The ``append`` string may use environment variables. For example, an + A/B boot setup could use ``append root=PARTLABEL=root_${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::