FWU Multi Bank Updates in U-Boot

The FWU Multi Bank Update feature implements the firmware update mechanism described in the PSA Firmware Update for A-profile Arm Architecture specification [1]. Certain aspects of the Dependable Boot specification [2] are also implemented. The feature provides a mechanism to have multiple banks of updatable firmware images and for updating the firmware images on the non-booted bank. On a successful update, the platform boots from the updated bank on subsequent boot. The UEFI capsule-on-disk update feature is used for performing the actual updates of the updatable firmware images.

The bookkeeping of the updatable images is done through a structure called metadata. Currently, the FWU metadata supports identification of images based on image GUIDs stored on a GPT partitioned storage media. There are plans to extend the metadata structure for non GPT partitioned devices as well.

Accessing the FWU metadata is done through generic API’s which are defined in a driver which complies with the U-Boot’s driver model. A new uclass UCLASS_FWU_MDATA has been added for accessing the FWU metadata. Individual drivers can be added based on the type of storage media, and its partitioning method. Details of the storage device containing the FWU metadata partitions are specified through a U-Boot specific device tree property fwu-mdata-store. Please refer to U-Boot fwu-mdata-gpt.yaml for the device tree bindings.

Enabling the FWU Multi Bank Update feature

The feature can be enabled by specifying the following configs:



in the .config file

By enabling the CONFIG_CMD_FWU_METADATA config option, the fwu_mdata_read command can be used to check the current state of the FWU metadata structure.

The first group of configuration settings enable the UEFI capsule-on-disk update functionality. The second group of configs enable the FWU Multi Bank Update functionality. Please refer to the section Enabling UEFI Capsule Update feature for more details on generation of the UEFI capsule.

Setting up the device for GPT partitioned storage

Before enabling the functionality in U-Boot, a GPT partitioned storage device is required. Assuming a GPT partitioned storage device, the storage media needs to be partitioned with the correct number of partitions, given the number of banks and number of images per bank that the platform is going to support. Each updatable firmware image will be stored on a separate partition. In addition, the two copies of the FWU metadata will be stored on two separate partitions. These partitions need to be created at the time of the platform’s provisioning.

As an example, a platform supporting two banks with each bank containing three images would need to have 2 * 3 = 6 partitions plus the two metadata partitions, or 8 partitions. In addition the storage media can have additional partitions of non-updatable images, like the EFI System Partition(ESP), a partition for the root file system etc. An example list of images on the storage medium would be

  • FWU metadata 1

  • U-Boot 1

  • OP-TEE 1

  • FWU metadata 2

  • OP-TEE 2

  • U-Boot 2

  • ESP

  • rootfs

When generating the partitions, a few aspects need to be taken care of. Each GPT partition entry in the GPT header has two GUIDs:

* PartitionTypeGUID
* UniquePartitionGUID

The PartitionTypeGUID value should correspond to the image_type_uuid field of the FWU metadata. This field is used to identify a given type of updatable firmware image, e.g. U-Boot, OP-TEE, FIP etc. This GUID should also be used for specifying the –guid parameter when generating the capsule.

The UniquePartitionGUID value should correspond to the image_uuid field in the FWU metadata. This GUID is used to identify images of a given image type in different banks.

The FWU specification defines the GUID value to be used for the metadata partitions. This would be the PartitionTypeGUID for the metadata partitions. Similarly, the UEFI specification defines the ESP GUID to be be used.

When generating the metadata, the image_type_uuid and the image_uuid values should match the PartitionTypeGUID and the UniquePartitionGUID values respectively.

Performing the Update

Once the storage media has been partitioned and populated with the metadata partitions, the UEFI capsule-on-disk update functionality can be used for performing the update. Refer to the section Enabling UEFI Capsule Update feature for details on how the update can be invoked.

On a successful update, the FWU metadata gets updated to reflect the bank from which the platform would be booting on subsequent boot.

Based on the value of bit15 of the Flags member of the capsule header, the updated images would either be accepted by the U-Boot’s UEFI implementation, or by the Operating System. If the Operating System is accepting the firmware images, it does so by generating an empty accept capsule. The Operating System can also reject the updated firmware by generating a revert capsule. The empty capsule can be applied by using the exact same procedure used for performing the capsule-on-disk update.

The task of accepting the different firmware images, post an update may be done by multiple, separate components in the Operating System. To help identify the firmware image that is being accepted, the accept capsule passes the image GUID of the firmware image being accepted. The relevant code in U-Boot then sets the Accept bit of the corresponding firmware image for which the accept capsule was found. Only when all the firmware components in a bank have been accepted does the platform transition from trial state to regular state.

The revert capsule on the other hand does not pass any image GUID, since reverting any image of the bank has the same result of the platform booting from the other bank on subsequent boot.

In the scenario that bit15 of the Flags member of the capsule header has not been set, the images being updated are accepted by the U-Boot’s UEFI firmware implementation by default, on successful update of the image.

Generating an empty capsule

The empty capsule can be generated using the mkeficapsule utility. To build the tool, enable:


Run the following commands to generate the accept/revert capsules:

.. code-block:: bash
$ ./tools/mkeficapsule

[–fw-accept –guid <image guid>] | [–fw-revert] <capsule_file_name>

Some examples of using the mkeficapsule tool for generation of the empty capsule would be:

.. code-block:: bash

$ ./tools/mkeficapsule –fw-accept –guid <image guid> <accept_capsule_name> $ ./tools/mkeficapsule –fw-revert <revert_capsule_name>