mmc-dev-parts.rst (1395B)
1============================ 2SD and MMC Device Partitions 3============================ 4 5Device partitions are additional logical block devices present on the 6SD/MMC device. 7 8As of this writing, MMC boot partitions as supported and exposed as 9/dev/mmcblkXboot0 and /dev/mmcblkXboot1, where X is the index of the 10parent /dev/mmcblkX. 11 12MMC Boot Partitions 13=================== 14 15Read and write access is provided to the two MMC boot partitions. Due to 16the sensitive nature of the boot partition contents, which often store 17a bootloader or bootloader configuration tables crucial to booting the 18platform, write access is disabled by default to reduce the chance of 19accidental bricking. 20 21To enable write access to /dev/mmcblkXbootY, disable the forced read-only 22access with:: 23 24 echo 0 > /sys/block/mmcblkXbootY/force_ro 25 26To re-enable read-only access:: 27 28 echo 1 > /sys/block/mmcblkXbootY/force_ro 29 30The boot partitions can also be locked read only until the next power on, 31with:: 32 33 echo 1 > /sys/block/mmcblkXbootY/ro_lock_until_next_power_on 34 35This is a feature of the card and not of the kernel. If the card does 36not support boot partition locking, the file will not exist. If the 37feature has been disabled on the card, the file will be read-only. 38 39The boot partitions can also be locked permanently, but this feature is 40not accessible through sysfs in order to avoid accidental or malicious 41bricking.