The following section is only relevant for target platforms that define CYG_HAL_BOOTUP_UPDATE and CYG_HAL_BOOTUP_VALID_ALT where the BootUp world is designed to allow for robust in-field updates of the firmware on a device.
For such systems the application started by BootUp must implement some of the support needed with regard to providing the “pending update” application image. When a new firmware update has been downloaded by the main application it will normally force a system restart, which will cause BootUp to perform the necessary update prior to starting the new main application.
The main application, when loading a pending update application into the alternative memory/device (e.g. off-chip SPI), should ensure that it does not set (write) the "valid signature" until it has validated the correct storage of the rest of the image. This requirement ensures that if a system reset occurs before the signature has been written that the image will NOT be interpreted by BootUp as being valid. This allows the BootUp loader to be simpler since it can treat a valid, complete, signature as an indication that the application was written correctly.
At a minimum (and critically) the main application needs to ensure that it invalidates the alternative image signature prior to starting the process of storing a new application image into the alternative area. This ensures a partial image is not incorrectly identified as a valid, pending, update. If CRC checking is also being used by the platform HAL then this likelihood is minimised.
Note: This requirement that the alternative storage area must have a complete valid image, whenever there is a complete signature present, ensures that in the case where the implementation does not use a CRC (or similar check) for the BootUp code to perform validation of a complete image, that any previous previous valid signature is not incorrectly accessed and treated as “valid”. For example, if the writing of a subsequent pending update is interrupted (CPU reset, etc.) before it “overwrites” the signature of the previously stored image. If a pending update is downloaded to the holding area, validated and marked as such BUT the system is not rebooted (so the pending update is not applied). Subsequently another pending update starts to download a different image to the holding area but the process is interrupted before the previous image “valid” signature is over-written. The BootUp code could then incorrectly interpret the invalid (partially overwritten) image as being valid if no other validity checks are performed. This is why the rule exists that the code performing the pending update storage should first invalidate the signature.
It is the responsibility of the main application storing the pending update to ensure that the complete image is valid PRIOR to finalising the platform specific signature. To reiterate, the BootUp package, in combination with the platform HAL support, only requires a simple “signature validity check” for robustness WHEN the code storing the pending update ensures that if there is a valid signature in the alternative area then the rest of the bytes are also valid (i.e. a complete valid image is in place).
Since the BootUp will only perform an update when the valid alternative image is different, there is no strict need to erase the signature/image until the main application needs to provide a new update. However, doing so during its normal startup will avoid BootUp having to spend CPU cycles validating and checking the alternative image prior to checking the signature and deciding not to update.
Note: To reiterate, the main application (not BootUp) is responsible for invalidating (erasing/removing) the alternative image, or at least the signature. This allows BootUp to be simpler since it only needs to be able to read from the alternative image location.
Note: The signature can either be embedded in the application (assuming the caveat of it being the last data written), or it can be a totally separate distinct section maintained in conjunction with the update application image. Such design decisions are left to the specific platform HAL support. The use of other validation mechanisms, e.g. a CRC, will only be required if the medium that is used to store the alternative (pending) image can suffer from data corruption, or transient read errors. Such a situation will be extremely unlikely for the vast majority of implementations.