| David Brazdil | 0f672f6 | 2019-12-10 10:32:29 +0000 | [diff] [blame^] | 1 | ============================ |
| 2 | SD and MMC Device Partitions |
| 3 | ============================ |
| 4 | |
| 5 | Device partitions are additional logical block devices present on the |
| 6 | SD/MMC device. |
| 7 | |
| 8 | As of this writing, MMC boot partitions as supported and exposed as |
| 9 | /dev/mmcblkXboot0 and /dev/mmcblkXboot1, where X is the index of the |
| 10 | parent /dev/mmcblkX. |
| 11 | |
| 12 | MMC Boot Partitions |
| 13 | =================== |
| 14 | |
| 15 | Read and write access is provided to the two MMC boot partitions. Due to |
| 16 | the sensitive nature of the boot partition contents, which often store |
| 17 | a bootloader or bootloader configuration tables crucial to booting the |
| 18 | platform, write access is disabled by default to reduce the chance of |
| 19 | accidental bricking. |
| 20 | |
| 21 | To enable write access to /dev/mmcblkXbootY, disable the forced read-only |
| 22 | access with:: |
| 23 | |
| 24 | echo 0 > /sys/block/mmcblkXbootY/force_ro |
| 25 | |
| 26 | To re-enable read-only access:: |
| 27 | |
| 28 | echo 1 > /sys/block/mmcblkXbootY/force_ro |
| 29 | |
| 30 | The boot partitions can also be locked read only until the next power on, |
| 31 | with:: |
| 32 | |
| 33 | echo 1 > /sys/block/mmcblkXbootY/ro_lock_until_next_power_on |
| 34 | |
| 35 | This is a feature of the card and not of the kernel. If the card does |
| 36 | not support boot partition locking, the file will not exist. If the |
| 37 | feature has been disabled on the card, the file will be read-only. |
| 38 | |
| 39 | The boot partitions can also be locked permanently, but this feature is |
| 40 | not accessible through sysfs in order to avoid accidental or malicious |
| 41 | bricking. |