From 119e9db233ad0f4cbd4cfb9f9385a7bca378b37d Mon Sep 17 00:00:00 2001 From: Zeng Chi Date: Mon, 21 Sep 2026 18:24:42 +0800 Subject: KVM: Don't pre-reserve xarray entries when storing empty/NULL attributes Skip the xarray reservation loop when clearing all memory attributes, as storing NULL only erases the entry and never needs to allocate, so no reservation (and no cleanup of a failed one) is required in that case. Suggested-by: Sean Christopherson Cc: David Ballesteros Signed-off-by: Zeng Chi Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com [sean: split to separate patch] Signed-off-by: Sean Christopherson --- Documentation/networking/nfc.rst | 132 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 132 insertions(+) create mode 100644 Documentation/networking/nfc.rst (limited to 'Documentation/networking/nfc.rst') diff --git a/Documentation/networking/nfc.rst b/Documentation/networking/nfc.rst new file mode 100644 index 000000000..401735006 --- /dev/null +++ b/Documentation/networking/nfc.rst @@ -0,0 +1,132 @@ +=================== +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. + +.. code-block:: none + + +--------------------------------------+ + | 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. -- cgit v1.3.1