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