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conditional: Describe the new boot_partition config.txt conditional filter #4100

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May 13, 2025
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conditional: Describe the new boot_partition config.txt conditional filter #4100

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12 changes: 12 additions & 0 deletions documentation/asciidoc/computers/config_txt/conditional.adoc
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Expand Up @@ -123,6 +123,18 @@ cmdline=cmdline-recovery.txt

The raw value of the `PM_RSTS` register at bootup is available via `/proc/device-tree/chosen/bootloader/rsts` and the final partition number used for booting is available via `/proc/device-tree/chosen/bootloader/partition`. These are big-endian binary values.

=== The `[boot_partition=N]` filter
The `boot_partition` filter can be used to select alternate OS files (e.g. `cmdline.txt`) to be loaded, depending on which partition `config.txt` was loaded from after processing `autoboot.txt`. This is intended for use with an `A/B` boot-system with `autoboot.txt` where it is desirable to be able to have identical files installed to the boot partition for both the `A` and `B` images.

Example `config.txt` - select the matching root filesystem for the `A/B` boot file-system.
[source,ini]
----
[boot_partition=1]
cmdline=cmdline_rootfs_a.txt

[boot_partition=2]
cmdline=cmdline_rootfs_b.txt
----

=== The `[tryboot]` filter

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