[PATCH 6/6] NFC: add Documentation/networking/nfc.txt

[Lauro Ramos Venancio <lauro.venancio@openbossa.org>]

From: Aloisio Almeida Jr <aloisio.almeida@openbossa.org>

Signed-off-by: Aloisio Almeida Jr <aloisio.almeida@openbossa.org>
Signed-off-by: Lauro Ramos Venancio <lauro.venancio@openbossa.org>
---
 Documentation/networking/nfc.txt |  127 ++++++++++++++++++++++++++++++++++++++
 1 files changed, 127 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/networking/nfc.txt

diff --git a/Documentation/networking/nfc.txt b/Documentation/networking/nfc.txt
new file mode 100644
index 0000000..e17ac19
--- /dev/null
+++ b/Documentation/networking/nfc.txt
@@ -0,0 +1,127 @@
+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.p
+
+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 exchage 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 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 was 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 an 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 specifc device
+
+* NFC_EVENT_DEVICE_ADDED - reports a NFC device addition
+* NFC_EVENT_DEVICE_REMOVED - reports a 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. The event attributes have relevant information about the target, such as
+the supported NFC protocols. The TARGETS_FOUND event has the list of all
+targets found by a specific device.
+
+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 a
+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 a 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.
-- 
1.7.1