NFC: add Documentation/networking/nfc.txt

Signed-off-by: NAloisio Almeida Jr <aloisio.almeida@openbossa.org>
Signed-off-by: NLauro Ramos Venancio <lauro.venancio@openbossa.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

Documentation/networking/nfc.txt

+Linux NFC subsystem
+===================
+
+The Near Field Communication (NFC) subsystem is required to standardize the
+NFC device drivers development and to create an unified userspace interface.
+
+This document covers the architecture overview, the device driver interface
+description and the userspace interface description.
+
+Architecture overview
+---------------------
+
+The NFC subsystem is responsible for:
+      - NFC adapters management;
+      - Polling for targets;
+      - Low-level data exchange;
+
+The subsystem is divided in some parts. The 'core' is responsible for
+providing the device driver interface. On the other side, it is also
+responsible for providing an interface to control operations and low-level
+data exchange.
+
+The control operations are available to userspace via generic netlink.
+
+The low-level data exchange interface is provided by the new socket family
+PF_NFC. The NFC_SOCKPROTO_RAW performs raw communication with NFC targets.
+
+
+             +--------------------------------------+
+             |              USER SPACE              |
+             +--------------------------------------+
+                 ^                       ^
+                 | low-level             | control
+                 | data exchange         | operations
+                 |                       |
+                 |                       v
+                 |                  +-----------+
+                 | AF_NFC           |  netlink  |
+                 | socket           +-----------+
+                 | raw                   ^
+                 |                       |
+                 v                       v
+             +---------+            +-----------+
+             | rawsock | <--------> |   core    |
+             +---------+            +-----------+
+                                         ^
+                                         |
+                                         v
+                                    +-----------+
+                                    |  driver   |
+                                    +-----------+
+
+Device Driver Interface
+-----------------------
+
+When registering on the NFC subsystem, the device driver must inform the core
+of the set of supported NFC protocols and the set of ops callbacks. The ops
+callbacks that must be implemented are the following:
+
+* start_poll - setup the device to poll for targets
+* stop_poll - stop on progress polling operation
+* activate_target - select and initialize one of the targets found
+* deactivate_target - deselect and deinitialize the selected target
+* data_exchange - send data and receive the response (transceive operation)
+
+Userspace interface
+--------------------
+
+The userspace interface is divided in control operations and low-level data
+exchange operation.
+
+CONTROL OPERATIONS:
+
+Generic netlink is used to implement the interface to the control operations.
+The operations are composed by commands and events, all listed below:
+
+* NFC_CMD_GET_DEVICE - get specific device info or dump the device list
+* NFC_CMD_START_POLL - setup a specific device to polling for targets
+* NFC_CMD_STOP_POLL - stop the polling operation in a specific device
+* NFC_CMD_GET_TARGET - dump the list of targets found by a specific device
+
+* NFC_EVENT_DEVICE_ADDED - reports an NFC device addition
+* NFC_EVENT_DEVICE_REMOVED - reports an NFC device removal
+* NFC_EVENT_TARGETS_FOUND - reports START_POLL results when 1 or more targets
+are found
+
+The user must call START_POLL to poll for NFC targets, passing the desired NFC
+protocols through NFC_ATTR_PROTOCOLS attribute. The device remains in polling
+state until it finds any target. However, the user can stop the polling
+operation by calling STOP_POLL command. In this case, it will be checked if
+the requester of STOP_POLL is the same of START_POLL.
+
+If the polling operation finds one or more targets, the event TARGETS_FOUND is
+sent (including the device id). The user must call GET_TARGET to get the list of
+all targets found by such device. Each reply message has target attributes with
+relevant information such as the supported NFC protocols.
+
+All polling operations requested through one netlink socket are stopped when
+it's closed.
+
+LOW-LEVEL DATA EXCHANGE:
+
+The userspace must use PF_NFC sockets to perform any data communication with
+targets. All NFC sockets use AF_NFC:
+
+struct sockaddr_nfc {
+       sa_family_t sa_family;
+       __u32 dev_idx;
+       __u32 target_idx;
+       __u32 nfc_protocol;
+};
+
+To establish a connection with one target, the user must create an
+NFC_SOCKPROTO_RAW socket and call the 'connect' syscall with the sockaddr_nfc
+struct correctly filled. All information comes from NFC_EVENT_TARGETS_FOUND
+netlink event. As a target can support more than one NFC protocol, the user
+must inform which protocol it wants to use.
+
+Internally, 'connect' will result in an activate_target call to the driver.
+When the socket is closed, the target is deactivated.
+
+The data format exchanged through the sockets is NFC protocol dependent. For
+instance, when communicating with MIFARE tags, the data exchanged are MIFARE
+commands and their responses.
+
+The first received package is the response to the first sent package and so
+on. In order to allow valid "empty" responses, every data received has a NULL
+header of 1 byte.