doc/readme-zephyr: document the serial recovery

Added description on serial recovery mode features and
its configuration.

Signed-off-by: Andrzej Puzdrowski <[email protected]>
Signed-off-by: Anna Kielar <[email protected]>

Author: nvlsianpu

docs/readme-zephyr.md

@@ -209,3 +209,55 @@ swap-using-scratch is used whereby an application has loaded a firmware update
 and marked it as test/confirmed but MCUboot will not swap the images and
 erasing the secondary slot from the zephyr application returns an error
 because the slot is marked for upgrade.
+
+## Serial recovery
+
+### Interface selection
+
+A serial recovery protocol is available over either a hardware serial port or a USB CDC ACM virtual serial port.
+The SMP server implementation can be enabled by the ``CONFIG_MCUBOOT_SERIAL=y`` Kconfig option.
+To set a type of an interface, use the ``BOOT_SERIAL_DEVICE`` Kconfig choice, and select either the ``CONFIG_BOOT_SERIAL_UART`` or the ``CONFIG_BOOT_SERIAL_CDC_ACM`` value.
+Which interface belongs to the protocol shall be set by the devicetree-chosen node:
+- `zephyr,console` - If a hardware serial port is used.
+- `zephyr,cdc-acm-uart` - If a virtual serial port is used.
+
+### Entering the serial recovery mode
+
+To enter the serial recovery mode, the device has to initiate rebooting, and a triggering event has to occur (for example, pressing a button).
+
+By default, the serial recovery GPIO pin active state enters the serial recovery mode.
+Use the ``mcuboot_button0`` devicetree button alias to assign the GPIO pin to the MCUboot.
+
+Alternatively, MCUboot can wait for a limited time to check if DFU is invoked by receiving an MCUmgr command.
+Select ``CONFIG_BOOT_SERIAL_WAIT_FOR_DFU=y`` to use this mode. ``CONFIG_BOOT_SERIAL_WAIT_FOR_DFU_TIMEOUT`` option defines
+the amount of time in milliseconds the device will wait for the trigger.
+
+### Direct image upload
+
+By default, the SMP server implementation will only use the first slot.
+To change it, invoke the `image upload` MCUmgr command with a selected image number, and make sure the ``CONFIG_MCUBOOT_SERIAL_DIRECT_IMAGE_UPLOAD=y`` Kconfig option is enabled.
+Note that the ``CONFIG_UPDATEABLE_IMAGE_NUMBER`` Kconfig option adjusts the number of image-pairs supported by the MCUboot.
+
+The mapping of image number to partition is as follows:
+* 0 and 1 - image-0, the primary slot of the first image.
+* 2 - image-1, the secondary slot of the first image.
+* 3 - image-2.
+* 4 - image-3.
+
+0 is a default upload target when no explicit selection is done.
+
+### System-specific commands
+
+Use the ``CONFIG_ENABLE_MGMT_PERUSER=y`` Kconfig option to enable the following additional commands:
+* Storage erase - This command allows erasing the storage partition (enable with ``CONFIG_BOOT_MGMT_CUSTOM_STORAGE_ERASE=y``).
+* Custom image list - This command allows fetching version and installation status (custom properties) for all images (enable with ``CONFIG_BOOT_MGMT_CUSTOM_IMG_LIST=y``).
+
+### In-place image decryption
+
+Images uploaded by the serial recovery can be decrypted on the fly by using ECIES primitives described in the [ECIES encryption](encrypted_images.md#ecies-encryption) section.
+
+Enable support for this feature by using ``CONFIG_BOOT_SERIAL_ENCRYPT_EC256=y``.
+
+### More configuration
+
+For details on other available configuration options for the serial recovery protocol, check the Kconfig options  (for example by using ``menuconfig``).