129 lines
5.2 KiB
Text
129 lines
5.2 KiB
Text
|
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.
|