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Linux NFC subsystem
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===================
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The Near Field Communication (NFC) subsystem is required to standardize the
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NFC device drivers development and to create an unified userspace interface.
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This document covers the architecture overview, the device driver interface
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description and the userspace interface description.
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Architecture overview
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=====================
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The NFC subsystem is responsible for:
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- NFC adapters management;
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- Polling for targets;
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- Low-level data exchange;
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The subsystem is divided in some parts. The 'core' is responsible for
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providing the device driver interface. On the other side, it is also
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responsible for providing an interface to control operations and low-level
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data exchange.
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The control operations are available to userspace via generic netlink.
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The low-level data exchange interface is provided by the new socket family
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PF_NFC. The NFC_SOCKPROTO_RAW performs raw communication with NFC targets.
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.. code-block:: none
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+--------------------------------------+
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| USER SPACE |
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+--------------------------------------+
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^ ^
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| low-level | control
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| data exchange | operations
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| |
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| v
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| +-----------+
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| AF_NFC | netlink |
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| socket +-----------+
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| raw ^
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| |
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v v
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+---------+ +-----------+
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| rawsock | <--------> | core |
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+---------+ +-----------+
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^
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|
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v
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+-----------+
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| driver |
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+-----------+
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Device Driver Interface
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=======================
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When registering on the NFC subsystem, the device driver must inform the core
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of the set of supported NFC protocols and the set of ops callbacks. The ops
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callbacks that must be implemented are the following:
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* start_poll - setup the device to poll for targets
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* stop_poll - stop on progress polling operation
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* activate_target - select and initialize one of the targets found
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* deactivate_target - deselect and deinitialize the selected target
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* data_exchange - send data and receive the response (transceive operation)
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Userspace interface
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===================
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The userspace interface is divided in control operations and low-level data
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exchange operation.
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CONTROL OPERATIONS:
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Generic netlink is used to implement the interface to the control operations.
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The operations are composed by commands and events, all listed below:
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* NFC_CMD_GET_DEVICE - get specific device info or dump the device list
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* NFC_CMD_START_POLL - setup a specific device to polling for targets
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* NFC_CMD_STOP_POLL - stop the polling operation in a specific device
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* NFC_CMD_GET_TARGET - dump the list of targets found by a specific device
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* NFC_EVENT_DEVICE_ADDED - reports an NFC device addition
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* NFC_EVENT_DEVICE_REMOVED - reports an NFC device removal
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* NFC_EVENT_TARGETS_FOUND - reports START_POLL results when 1 or more targets
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are found
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The user must call START_POLL to poll for NFC targets, passing the desired NFC
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protocols through NFC_ATTR_PROTOCOLS attribute. The device remains in polling
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state until it finds any target. However, the user can stop the polling
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operation by calling STOP_POLL command. In this case, it will be checked if
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the requester of STOP_POLL is the same of START_POLL.
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If the polling operation finds one or more targets, the event TARGETS_FOUND is
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sent (including the device id). The user must call GET_TARGET to get the list of
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all targets found by such device. Each reply message has target attributes with
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relevant information such as the supported NFC protocols.
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All polling operations requested through one netlink socket are stopped when
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it's closed.
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LOW-LEVEL DATA EXCHANGE:
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The userspace must use PF_NFC sockets to perform any data communication with
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targets. All NFC sockets use AF_NFC::
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struct sockaddr_nfc {
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sa_family_t sa_family;
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__u32 dev_idx;
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__u32 target_idx;
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__u32 nfc_protocol;
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};
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To establish a connection with one target, the user must create an
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NFC_SOCKPROTO_RAW socket and call the 'connect' syscall with the sockaddr_nfc
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struct correctly filled. All information comes from NFC_EVENT_TARGETS_FOUND
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netlink event. As a target can support more than one NFC protocol, the user
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must inform which protocol it wants to use.
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Internally, 'connect' will result in an activate_target call to the driver.
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When the socket is closed, the target is deactivated.
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The data format exchanged through the sockets is NFC protocol dependent. For
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instance, when communicating with MIFARE tags, the data exchanged are MIFARE
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commands and their responses.
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The first received package is the response to the first sent package and so
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on. In order to allow valid "empty" responses, every data received has a NULL
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header of 1 byte.
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