diff options
Diffstat (limited to 'Documentation/nfc/nfc-hci.txt')
| -rw-r--r-- | Documentation/nfc/nfc-hci.txt | 213 |
1 files changed, 213 insertions, 0 deletions
diff --git a/Documentation/nfc/nfc-hci.txt b/Documentation/nfc/nfc-hci.txt new file mode 100644 index 000000000000..89a339c9b079 --- /dev/null +++ b/Documentation/nfc/nfc-hci.txt @@ -0,0 +1,213 @@ +HCI backend for NFC Core + +Author: Eric Lapuyade, Samuel Ortiz +Contact: eric.lapuyade@intel.com, samuel.ortiz@intel.com + +General +------- + +The HCI layer implements much of the ETSI TS 102 622 V10.2.0 specification. It +enables easy writing of HCI-based NFC drivers. The HCI layer runs as an NFC Core +backend, implementing an abstract nfc device and translating NFC Core API +to HCI commands and events. + +HCI +--- + +HCI registers as an nfc device with NFC Core. Requests coming from userspace are +routed through netlink sockets to NFC Core and then to HCI. From this point, +they are translated in a sequence of HCI commands sent to the HCI layer in the +host controller (the chip). The sending context blocks while waiting for the +response to arrive. +HCI events can also be received from the host controller. They will be handled +and a translation will be forwarded to NFC Core as needed. +HCI uses 2 execution contexts: +- one for executing commands : nfc_hci_msg_tx_work(). Only one command +can be executing at any given moment. +- one for dispatching received events and commands : nfc_hci_msg_rx_work(). + +HCI Session initialization: +--------------------------- + +The Session initialization is an HCI standard which must unfortunately +support proprietary gates. This is the reason why the driver will pass a list +of proprietary gates that must be part of the session. HCI will ensure all +those gates have pipes connected when the hci device is set up. + +HCI Gates and Pipes +------------------- + +A gate defines the 'port' where some service can be found. In order to access +a service, one must create a pipe to that gate and open it. In this +implementation, pipes are totally hidden. The public API only knows gates. +This is consistent with the driver need to send commands to proprietary gates +without knowing the pipe connected to it. + +Driver interface +---------------- + +A driver would normally register itself with HCI and provide the following +entry points: + +struct nfc_hci_ops { + int (*open)(struct nfc_hci_dev *hdev); + void (*close)(struct nfc_hci_dev *hdev); + int (*hci_ready) (struct nfc_hci_dev *hdev); + int (*xmit)(struct nfc_hci_dev *hdev, struct sk_buff *skb); + int (*start_poll)(struct nfc_hci_dev *hdev, u32 protocols); + int (*target_from_gate)(struct nfc_hci_dev *hdev, u8 gate, + struct nfc_target *target); + int (*complete_target_discovered) (struct nfc_hci_dev *hdev, u8 gate, + struct nfc_target *target); + int (*data_exchange) (struct nfc_hci_dev *hdev, + struct nfc_target *target, + struct sk_buff *skb, struct sk_buff **res_skb); + int (*check_presence)(struct nfc_hci_dev *hdev, + struct nfc_target *target); +}; + +- open() and close() shall turn the hardware on and off. +- hci_ready() is an optional entry point that is called right after the hci +session has been set up. The driver can use it to do additional initialization +that must be performed using HCI commands. +- xmit() shall simply write a frame to the chip. +- start_poll() is an optional entrypoint that shall set the hardware in polling +mode. This must be implemented only if the hardware uses proprietary gates or a +mechanism slightly different from the HCI standard. +- target_from_gate() is an optional entrypoint to return the nfc protocols +corresponding to a proprietary gate. +- complete_target_discovered() is an optional entry point to let the driver +perform additional proprietary processing necessary to auto activate the +discovered target. +- data_exchange() must be implemented by the driver if proprietary HCI commands +are required to send data to the tag. Some tag types will require custom +commands, others can be written to using the standard HCI commands. The driver +can check the tag type and either do proprietary processing, or return 1 to ask +for standard processing. +- check_presence() is an optional entry point that will be called regularly +by the core to check that an activated tag is still in the field. If this is +not implemented, the core will not be able to push tag_lost events to the user +space + +On the rx path, the driver is responsible to push incoming HCP frames to HCI +using nfc_hci_recv_frame(). HCI will take care of re-aggregation and handling +This must be done from a context that can sleep. + +SHDLC +----- + +Most chips use shdlc to ensure integrity and delivery ordering of the HCP +frames between the host controller (the chip) and hosts (entities connected +to the chip, like the cpu). In order to simplify writing the driver, an shdlc +layer is available for use by the driver. +When used, the driver actually registers with shdlc, and shdlc will register +with HCI. HCI sees shdlc as the driver and thus send its HCP frames +through shdlc->xmit. +SHDLC adds a new execution context (nfc_shdlc_sm_work()) to run its state +machine and handle both its rx and tx path. + +Included Drivers +---------------- + +An HCI based driver for an NXP PN544, connected through I2C bus, and using +shdlc is included. + +Execution Contexts +------------------ + +The execution contexts are the following: +- IRQ handler (IRQH): +fast, cannot sleep. stores incoming frames into an shdlc rx queue + +- SHDLC State Machine worker (SMW) +handles shdlc rx & tx queues. Dispatches HCI cmd responses. + +- HCI Tx Cmd worker (MSGTXWQ) +Serializes execution of HCI commands. Completes execution in case of response +timeout. + +- HCI Rx worker (MSGRXWQ) +Dispatches incoming HCI commands or events. + +- Syscall context from a userspace call (SYSCALL) +Any entrypoint in HCI called from NFC Core + +Workflow executing an HCI command (using shdlc) +----------------------------------------------- + +Executing an HCI command can easily be performed synchronously using the +following API: + +int nfc_hci_send_cmd (struct nfc_hci_dev *hdev, u8 gate, u8 cmd, + const u8 *param, size_t param_len, struct sk_buff **skb) + +The API must be invoked from a context that can sleep. Most of the time, this +will be the syscall context. skb will return the result that was received in +the response. + +Internally, execution is asynchronous. So all this API does is to enqueue the +HCI command, setup a local wait queue on stack, and wait_event() for completion. +The wait is not interruptible because it is guaranteed that the command will +complete after some short timeout anyway. + +MSGTXWQ context will then be scheduled and invoke nfc_hci_msg_tx_work(). +This function will dequeue the next pending command and send its HCP fragments +to the lower layer which happens to be shdlc. It will then start a timer to be +able to complete the command with a timeout error if no response arrive. + +SMW context gets scheduled and invokes nfc_shdlc_sm_work(). This function +handles shdlc framing in and out. It uses the driver xmit to send frames and +receives incoming frames in an skb queue filled from the driver IRQ handler. +SHDLC I(nformation) frames payload are HCP fragments. They are aggregated to +form complete HCI frames, which can be a response, command, or event. + +HCI Responses are dispatched immediately from this context to unblock +waiting command execution. Response processing involves invoking the completion +callback that was provided by nfc_hci_msg_tx_work() when it sent the command. +The completion callback will then wake the syscall context. + +Workflow receiving an HCI event or command +------------------------------------------ + +HCI commands or events are not dispatched from SMW context. Instead, they are +queued to HCI rx_queue and will be dispatched from HCI rx worker +context (MSGRXWQ). This is done this way to allow a cmd or event handler +to also execute other commands (for example, handling the +NFC_HCI_EVT_TARGET_DISCOVERED event from PN544 requires to issue an +ANY_GET_PARAMETER to the reader A gate to get information on the target +that was discovered). + +Typically, such an event will be propagated to NFC Core from MSGRXWQ context. + +Error management +---------------- + +Errors that occur synchronously with the execution of an NFC Core request are +simply returned as the execution result of the request. These are easy. + +Errors that occur asynchronously (e.g. in a background protocol handling thread) +must be reported such that upper layers don't stay ignorant that something +went wrong below and know that expected events will probably never happen. +Handling of these errors is done as follows: + +- driver (pn544) fails to deliver an incoming frame: it stores the error such +that any subsequent call to the driver will result in this error. Then it calls +the standard nfc_shdlc_recv_frame() with a NULL argument to report the problem +above. shdlc stores a EREMOTEIO sticky status, which will trigger SMW to +report above in turn. + +- SMW is basically a background thread to handle incoming and outgoing shdlc +frames. This thread will also check the shdlc sticky status and report to HCI +when it discovers it is not able to run anymore because of an unrecoverable +error that happened within shdlc or below. If the problem occurs during shdlc +connection, the error is reported through the connect completion. + +- HCI: if an internal HCI error happens (frame is lost), or HCI is reported an +error from a lower layer, HCI will either complete the currently executing +command with that error, or notify NFC Core directly if no command is executing. + +- NFC Core: when NFC Core is notified of an error from below and polling is +active, it will send a tag discovered event with an empty tag list to the user +space to let it know that the poll operation will never be able to detect a tag. +If polling is not active and the error was sticky, lower levels will return it +at next invocation. |