smart_card_usb
==============
Probes all USB smart card reader devices.

Function Arguments
------------------
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   * - 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/usb/devices/`` to read attributes of
each usb device and find out all smart card readers there.  Each result
must contain these fields:

- ``device_path``: Pathname of the sysfs directory.
- ``idVendor``
- ``idProduct``

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

- ``manufacturer``
- ``product``
- ``bcdDevice``