SBL implements a robust, secure and power fail-safe firmware update mechanism. SBL image can be built to contain redundant boot components and depending on hardware assisted boot partition switch, to support fail-safe update.
If firmware update is interrupted by a power failure and one boot partition is corrupted, SBL boots from the alternate partition to avoid bricking the platform. The firmware update authenticates the update image (a.k.a. capsule) before updating the firmware.
Firmware update feature is implemented as a SBL payload.
When firmware update is signaled, Stage 2 loads firmware update payload to start update flow. During the update, firmware update payload keeps a state machine to keep track of the update progress as follows:
- Boot from partition A
- If update flow is requested, load FWU payload
- FWU loads and verifies capsule data. If successful, update partition B
- Set partition B as ‘active’
- Boot from partition B
- If update flow is in progress, load FWU payload
- FWU loads and verifies capsule data. If successful, update partition A
- Set partition A as ‘active’
- End update flow state machine and reboot
Triggering Firmware Update¶
Triggering firmware update is a platform specific mechanism. The location of the firmware update capsule image is passed to Firmware update payload through configuration data.
To generate and sign firmware update capsule, see Update Firmware.
Firmware update can be triggered from OS or from SBL shell. See Trigger Update From Shell.
Capsule update defined by UEFI specification is different from SBL capsule format.