mmc-dev-parts.rst 1.4 KB

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