doc: mention that choosing root/usr partitions by strverscmp() on the partition label...

diff --git a/docs/DISCOVERABLE_PARTITIONS.md b/docs/DISCOVERABLE_PARTITIONS.md
index 9ce4ebc9407f58326430a046bbf6de831cf20ff1..81d07f88a65535baf429d5d9201420d637263963 100644 (file)
--- a/docs/DISCOVERABLE_PARTITIONS.md
+++ b/docs/DISCOVERABLE_PARTITIONS.md
@@ -162,7 +162,14 @@ partition is listed in `/etc/fstab` or with `root=` on the kernel command line,
 it _must_ take precedence over automatically discovered partitions.  If a
 `/home/`, `/usr/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/` or `/boot/`
 directory is found to be populated already in the root partition, the automatic
-discovery _must not_ mount any discovered file system over it.
+discovery _must not_ mount any discovered file system over it. Optionally, in
+case of the root, `/usr/` and their Verity partitions instead of strictly
+mounting the first suitable partition an OS might choose to mount the partition
+whose label compares the highest according to `strverscmp()` or a similar
+logic, in order to implement a simple partition-based A/B versioning
+scheme. The precise rules are left for the implementation to decide, but when
+in doubt earlier partitions (by their index) should always win over later
+partitions if the label comparison is inconclusive.
 
 A *container* *manager* should automatically discover and mount the root,
 `/usr/`, `/home/`, `/srv/`, `/var/`, `/var/tmp/` partitions inside a container