doc: Add description for bootmeth rauc

Add documentation for the bootmeth "rauc", which boots an A/B system
with RAUC from MMC.

Signed-off-by: Martin Schwan <m.schwan@phytec.de>

doc/develop/bootstd/index.rst

@@ -12,5 +12,6 @@ Standard Boot
    qfw
    android
    cros
+   rauc
    script
    sandbox

doc/develop/bootstd/overview.rst

@@ -443,6 +443,7 @@ Bootmeth drivers are provided for booting from various media:
    - :doc:`extlinux / syslinux <extlinux>` boot from a storage device
    - :doc:`extlinux / syslinux <extlinux>` boot from a network (PXE)
    - :doc:`sandbox <sandbox>` used only for testing
+   - :doc:`RAUC distro <rauc>`: A/B system with RAUC from MMC
    - :doc:`U-Boot scripts <script>` from disk, network or SPI flash
    - :doc:`QFW <qfw>`: QEMU firmware interface
    - :doc:`VBE </develop/vbe>`: Verified Boot for Embedded

doc/develop/bootstd/rauc.rst

@@ -0,0 +1,56 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+RAUC Bootmeth
+=============
+
+This bootmeth provides a way to locate and run an A/B system with RAUC as its
+update client. The booted distro must supply a script on an MMC device
+containing the final boot instructions necessary.
+
+This bootmeth assumes a symmetric A/B partition layout, with a separate boot
+partition containing the kernel image and another partition for the root
+filesystem each. The partition numbers must be specified with
+``CONFIG_BOOTMETH_RAUC_PARTITIONS``. The content must be a list of pairs, with
+the following syntax: ``1,2 3,4``, where 1 and 3 are the slots' boot partition
+and 2 and 4 the slots' root partition.
+
+Each pair of boot and rootfs partition form a "slot". The default order in which
+available slots are tried is set through ``CONFIG_BOOTMETH_RAUC_BOOT_ORDER``,
+with the left one tried first.
+
+The default number of boot tries of each slot is set by
+``CONFIG_BOOTMETH_RAUC_DEFAULT_TRIES``.
+
+In case no valid slot can be found and/or all slots have zero tries left, the
+boot order and slot tries are reset to their default values, if
+``CONFIG_BOOTMETH_RAUC_RESET_ALL_ZERO_TRIES`` is enabled. This prevents a system
+from locking up in the bootloader and tries booting again after a specified
+number of tries.
+
+The boot script must be located in each boot partition. The bootmeth searches
+for "boot.scr.uimg" first, then "boot.scr" if not found.
+
+When the bootflow is booted, the bootmeth sets these environment variables:
+
+devtype
+    device type (e.g. "mmc")
+
+devnum
+    device number, corresponding to the device 'sequence' number
+    ``dev_seq(dev)``
+
+distro_bootpart
+    partition number of the boot partition on the device (numbered from 1)
+
+distro_rootpart
+    partition number of the rootfs partition on the device (numbered from 1)
+
+raucargs
+    kernel command line arguments needed for RAUC to detect the currently booted
+    slot
+
+The script file must be a FIT or a legacy uImage. It is loaded into memory and
+executed.
+
+The compatible string "u-boot,distro-rauc" is used for the driver. It is present
+if ``CONFIG_BOOTMETH_RAUC`` is enabled.