Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
1 SD and MMC Device Partitions 2 ============================ 3 4 Device partitions are additional logical block devices present on the 5 SD/MMC device. 6 7 As of this writing, MMC boot partitions as supported and exposed as 8 /dev/mmcblkXboot0 and /dev/mmcblkXboot1, where X is the index of the 9 parent /dev/mmcblkX. 10 11 MMC Boot Partitions 12 =================== 13 14 Read and write access is provided to the two MMC boot partitions. Due to 15 the sensitive nature of the boot partition contents, which often store 16 a bootloader or bootloader configuration tables crucial to booting the 17 platform, write access is disabled by default to reduce the chance of 18 accidental bricking. 19 20 To enable write access to /dev/mmcblkXbootY, disable the forced read-only 21 access with: 22 23 echo 0 > /sys/block/mmcblkXbootY/force_ro 24 25 To re-enable read-only access: 26 27 echo 1 > /sys/block/mmcblkXbootY/force_ro 28 29 The boot partitions can also be locked read only until the next power on, 30 with: 31 32 echo 1 > /sys/block/mmcblkXbootY/ro_lock_until_next_power_on 33 34 This is a feature of the card and not of the kernel. If the card does 35 not support boot partition locking, the file will not exist. If the 36 feature has been disabled on the card, the file will be read-only. 37 38 The boot partitions can also be locked permanently, but this feature is 39 not accessible through sysfs in order to avoid accidental or malicious 40 bricking.