Documentation/block/cmdline-partition.txt

Embedded device command line partition parsing
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Support for reading the block device partition table from the command line.
It is typically used for fixed block (eMMC) embedded devices.
It has no MBR, so saves storage space. Bootloader can be easily accessed
by absolute address of data on the block device.
Users can easily change the partition.

The format for the command line is just like mtdparts:

blkdevparts=<blkdev-def>[;<blkdev-def>]
  <blkdev-def> := <blkdev-id>:<partdef>[,<partdef>]
    <partdef> := <size>[@<offset>](part-name)

<blkdev-id>
    block device disk name, embedded device used fixed block device,
    it's disk name also fixed. such as: mmcblk0, mmcblk1, mmcblk0boot0.

<size>
    partition size, in bytes, such as: 512, 1m, 1G.

<offset>
    partition start address, in bytes.

(part-name)
    partition name, kernel send uevent with "PARTNAME". application can create
    a link to block device partition with the name "PARTNAME".
    user space application can access partition by partition name.

Example:
    eMMC disk name is "mmcblk0" and "mmcblk0boot0"

  bootargs:
    'blkdevparts=mmcblk0:1G(data0),1G(data1),-;mmcblk0boot0:1m(boot),-(kernel)'

  dmesg:
    mmcblk0: p1(data0) p2(data1) p3()
    mmcblk0boot0: p1(boot) p2(kernel)