mmc
===
Probes all eMMC devices listed in the sysfs ``/sys/bus/mmc/devices/``.

Function Arguments
------------------
.. list-table::
   :widths: 20 10 60
   :header-rows: 1
   :align: left

   * - Name
     - Type
     - Description

   * - dir_path
     - str, None
     - (optional; default: ``None``) The path used to search for device sysfs data. First all symlinks are resolved, to the the "real" path. Then iteratively search toward parent folder until the remaining path contains the relevent data fields.

Description
-----------
This function goes through ``/sys/bus/mmc/devices/`` to read attributes of
each eMMC device listed there.  Each result should contain these fields:

- ``device_path``: Pathname of the sysfs directory.
- ``cid``: Card Identification Register.
- ``csd``: Card Specific Data Register.
- ``manfid``: Manufacturer ID (from CID Register).
- ``name``: Product Name (from CID Register).
- ``oemid``: OEM/Application ID (from CID Register).
- ``serial``: Product Serial Number (from CID Register).

The result might also contain these optional fields if they are exported in
the sysfs entry:

- ``fwrev``: Firmware/Product Revision (from CID Register, SD and MMCv1 only).
- ``hwrev``: Hardware/Product Revision (from CID Register, SD and MMCv1 only).

Please reference the kernel
`document <https://www.kernel.org/doc/Documentation/mmc/mmc-dev-attrs.txt>`_
for more information.

Examples
--------
Let's say the Chromebook has two eMMC devices.  One of which
(at ``/sys/bus/mmc/devices/mmc0:0001``) has the attributes:

- ``cid=123412341234``
- \.\.\.

And the other one (at ``/sys/bus/mmc/devices/mmc1:0001``) has the
attributes:

- ``cid=246824682468``
- \.\.\.

Then the probe statement::

  {
    "eval": "mmc"
  }

will have the corresponding probed result::

  [
    {
      "bus_type": "mmc",
      "cid": "123412341234",
      ...
    },
    {
      "bus_type": "mmc",
      "cid": "246824682468",
      ...
    }
  ]

To verify if the Chromebook has the eMMC device which ``cid`` is
``123412341234``, you can write a probe statement like::

  {
    "eval": "mmc",
    "expect": {
      "cid": "123412341234"
    }
  }

The corresponding probed result will be empty if and only if there's no
eMMC device which ``cid`` is ``123412341234`` found.

Another use case is that you can ask this function to parse a specific
eMMC device sysfs directly like ::

  {
    "eval" "mmc:/sys/bus/mmc/devices/mmc1:0001"
  }