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+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). Commands can be executed synchronously (the sending
+context blocks waiting for response) or asynchronously (the response is returned
+from HCI Rx context).
+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. There are hooks to
+let the HCI driver handle proprietary events or override standard behavior.
+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.
+In case the chip supports pre-opened gates and pseudo-static pipes, the driver
+can pass that information to HCI core.
+
+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 is generally written in two parts : the physical link management and
+the HCI management. This makes it easier to maintain a driver for a chip that
+can be connected using various phy (i2c, spi, ...)
+
+HCI Management
+--------------
+
+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 im_protocols, u32 tm_protocols);
+ int (*dep_link_up)(struct nfc_hci_dev *hdev, struct nfc_target *target,
+ u8 comm_mode, u8 *gb, size_t gb_len);
+ int (*dep_link_down)(struct nfc_hci_dev *hdev);
+ 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 (*im_transceive) (struct nfc_hci_dev *hdev,
+ struct nfc_target *target, struct sk_buff *skb,
+ data_exchange_cb_t cb, void *cb_context);
+ int (*tm_send)(struct nfc_hci_dev *hdev, struct sk_buff *skb);
+ int (*check_presence)(struct nfc_hci_dev *hdev,
+ struct nfc_target *target);
+ int (*event_received)(struct nfc_hci_dev *hdev, u8 gate, u8 event,
+ struct sk_buff *skb);
+};
+
+- 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 physical link.
+- 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.
+- dep_link_up() is called after a p2p target has been detected, to finish
+the p2p connection setup with hardware parameters that need to be passed back
+to nfc core.
+- dep_link_down() is called to bring the p2p link down.
+- 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.
+- im_transceive() 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. The data exchange command itself must be sent
+asynchronously.
+- tm_send() is called to send data in the case of a p2p connection
+- 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
+- event_received() is called to handle an event coming from the chip. Driver
+can handle the event or return 1 to let HCI attempt standard processing.
+
+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.
+
+PHY Management
+--------------
+
+The physical link (i2c, ...) management is defined by the following struture:
+
+struct nfc_phy_ops {
+ int (*write)(void *dev_id, struct sk_buff *skb);
+ int (*enable)(void *dev_id);
+ void (*disable)(void *dev_id);
+};
+
+enable(): turn the phy on (power on), make it ready to transfer data
+disable(): turn the phy off
+write(): Send a data frame to the chip. Note that to enable higher
+layers such as an llc to store the frame for re-emission, this function must
+not alter the skb. It must also not return a positive result (return 0 for
+success, negative for failure).
+
+Data coming from the chip shall be sent directly to nfc_hci_recv_frame().
+
+LLC
+---
+
+Communication between the CPU and the chip often requires some link layer
+protocol. Those are isolated as modules managed by the HCI layer. There are
+currently two modules : nop (raw transfert) and shdlc.
+A new llc must implement the following functions:
+
+struct nfc_llc_ops {
+ void *(*init) (struct nfc_hci_dev *hdev, xmit_to_drv_t xmit_to_drv,
+ rcv_to_hci_t rcv_to_hci, int tx_headroom,
+ int tx_tailroom, int *rx_headroom, int *rx_tailroom,
+ llc_failure_t llc_failure);
+ void (*deinit) (struct nfc_llc *llc);
+ int (*start) (struct nfc_llc *llc);
+ int (*stop) (struct nfc_llc *llc);
+ void (*rcv_from_drv) (struct nfc_llc *llc, struct sk_buff *skb);
+ int (*xmit_from_hci) (struct nfc_llc *llc, struct sk_buff *skb);
+};
+
+- init() : allocate and init your private storage
+- deinit() : cleanup
+- start() : establish the logical connection
+- stop () : terminate the logical connection
+- rcv_from_drv() : handle data coming from the chip, going to HCI
+- xmit_from_hci() : handle data sent by HCI, going to the chip
+
+The llc must be registered with nfc before it can be used. Do that by
+calling nfc_llc_register(const char *name, struct nfc_llc_ops *ops);
+
+Again, note that the llc does not handle the physical link. It is thus very
+easy to mix any physical link with any llc for a given chip driver.
+
+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. sends incoming frames to HCI where they are passed to
+the current llc. In case of shdlc, the frame is queued in shdlc rx queue.
+
+- SHDLC State Machine worker (SMW)
+Only when llc_shdlc is used: 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.
+
+It is also possible to execute the command asynchronously using this API:
+
+static int nfc_hci_execute_cmd_async(struct nfc_hci_dev *hdev, u8 pipe, u8 cmd,
+ const u8 *param, size_t param_len,
+ data_exchange_cb_t cb, void *cb_context)
+
+The workflow is the same, except that the API call returns immediately, and
+the callback will be called with the result from the SMW 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.