RAM Load and Direct XIP firmware downgrade capability #2225
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…/direct XIP rather than relying on version number comparison This commit adds new configuration option MCUBOOT_RAM_LOAD_USE_UPDATE_COUNTER (for use with MCUBOOT_RAM_LOAD) and MCUBOOT_DIRECT_XIP_USE_UPDATE_COUNTER (for use with MCUBOOT_DIRECT_XIP). By default, in RAM load/direct XIP modes, the slot that will boot is the one having the higher version number. This means it is not possible to downgrade to earlier image versions. By configuring one of the new *_USE_UPDATE_COUNTER options, a counter value in the image trailer will be consulted first to determine which image to boot, falling back to comparing the image version if the trailer is missing or there is a tie. During firmware update, the application code should read the counter value for the running image, increment it, and store that value in the image trailer of the updated slot. The counter is stored in the 'swap size' field which is not used by the RAM load/direct XIP modes.
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This PR adds new configuration options MCUBOOT_RAM_LOAD_USE_UPDATE_COUNTER (for use with MCUBOOT_RAM_LOAD) and MCUBOOT_DIRECT_XIP_USE_UPDATE_COUNTER (for use with MCUBOOT_DIRECT_XIP).
By default, in RAM load/direct XIP modes, the slot that will boot is the one having the higher version number. This means it is not possible to downgrade to earlier image versions. By configuring one of the new *_USE_UPDATE_COUNTER options, a counter value in the image trailer will be consulted first to determine which image to boot, falling back to comparing the image version if the trailer is missing or there is a tie.
During firmware update, the application code should read the counter value for the running image, increment it, and store that value in the image trailer of the updated slot. The counter is stored in the 'swap size' field which is not used by the RAM load/direct XIP modes.