1========================
2HCI backend for NFC Core
3========================
4
5- Author: Eric Lapuyade, Samuel Ortiz
6- Contact: eric.lapuyade@intel.com, samuel.ortiz@intel.com
7
8General
9-------
10
11The HCI layer implements much of the ETSI TS 102 622 V10.2.0 specification. It
12enables easy writing of HCI-based NFC drivers. The HCI layer runs as an NFC Core
13backend, implementing an abstract nfc device and translating NFC Core API
14to HCI commands and events.
15
16HCI
17---
18
19HCI registers as an nfc device with NFC Core. Requests coming from userspace are
20routed through netlink sockets to NFC Core and then to HCI. From this point,
21they are translated in a sequence of HCI commands sent to the HCI layer in the
22host controller (the chip). Commands can be executed synchronously (the sending
23context blocks waiting for response) or asynchronously (the response is returned
24from HCI Rx context).
25HCI events can also be received from the host controller. They will be handled
26and a translation will be forwarded to NFC Core as needed. There are hooks to
27let the HCI driver handle proprietary events or override standard behavior.
28HCI uses 2 execution contexts:
29
30- one for executing commands : nfc_hci_msg_tx_work(). Only one command
31  can be executing at any given moment.
32- one for dispatching received events and commands : nfc_hci_msg_rx_work().
33
34HCI Session initialization
35--------------------------
36
37The Session initialization is an HCI standard which must unfortunately
38support proprietary gates. This is the reason why the driver will pass a list
39of proprietary gates that must be part of the session. HCI will ensure all
40those gates have pipes connected when the hci device is set up.
41In case the chip supports pre-opened gates and pseudo-static pipes, the driver
42can pass that information to HCI core.
43
44HCI Gates and Pipes
45-------------------
46
47A gate defines the 'port' where some service can be found. In order to access
48a service, one must create a pipe to that gate and open it. In this
49implementation, pipes are totally hidden. The public API only knows gates.
50This is consistent with the driver need to send commands to proprietary gates
51without knowing the pipe connected to it.
52
53Driver interface
54----------------
55
56A driver is generally written in two parts : the physical link management and
57the HCI management. This makes it easier to maintain a driver for a chip that
58can be connected using various phy (i2c, spi, ...)
59
60HCI Management
61--------------
62
63A driver would normally register itself with HCI and provide the following
64entry points::
65
66  struct nfc_hci_ops {
67	int (*open)(struct nfc_hci_dev *hdev);
68	void (*close)(struct nfc_hci_dev *hdev);
69	int (*hci_ready) (struct nfc_hci_dev *hdev);
70	int (*xmit) (struct nfc_hci_dev *hdev, struct sk_buff *skb);
71	int (*start_poll) (struct nfc_hci_dev *hdev,
72			   u32 im_protocols, u32 tm_protocols);
73	int (*dep_link_up)(struct nfc_hci_dev *hdev, struct nfc_target *target,
74			   u8 comm_mode, u8 *gb, size_t gb_len);
75	int (*dep_link_down)(struct nfc_hci_dev *hdev);
76	int (*target_from_gate) (struct nfc_hci_dev *hdev, u8 gate,
77				 struct nfc_target *target);
78	int (*complete_target_discovered) (struct nfc_hci_dev *hdev, u8 gate,
79					   struct nfc_target *target);
80	int (*im_transceive) (struct nfc_hci_dev *hdev,
81			      struct nfc_target *target, struct sk_buff *skb,
82			      data_exchange_cb_t cb, void *cb_context);
83	int (*tm_send)(struct nfc_hci_dev *hdev, struct sk_buff *skb);
84	int (*check_presence)(struct nfc_hci_dev *hdev,
85			      struct nfc_target *target);
86	int (*event_received)(struct nfc_hci_dev *hdev, u8 gate, u8 event,
87			      struct sk_buff *skb);
88  };
89
90- open() and close() shall turn the hardware on and off.
91- hci_ready() is an optional entry point that is called right after the hci
92  session has been set up. The driver can use it to do additional initialization
93  that must be performed using HCI commands.
94- xmit() shall simply write a frame to the physical link.
95- start_poll() is an optional entrypoint that shall set the hardware in polling
96  mode. This must be implemented only if the hardware uses proprietary gates or a
97  mechanism slightly different from the HCI standard.
98- dep_link_up() is called after a p2p target has been detected, to finish
99  the p2p connection setup with hardware parameters that need to be passed back
100  to nfc core.
101- dep_link_down() is called to bring the p2p link down.
102- target_from_gate() is an optional entrypoint to return the nfc protocols
103  corresponding to a proprietary gate.
104- complete_target_discovered() is an optional entry point to let the driver
105  perform additional proprietary processing necessary to auto activate the
106  discovered target.
107- im_transceive() must be implemented by the driver if proprietary HCI commands
108  are required to send data to the tag. Some tag types will require custom
109  commands, others can be written to using the standard HCI commands. The driver
110  can check the tag type and either do proprietary processing, or return 1 to ask
111  for standard processing. The data exchange command itself must be sent
112  asynchronously.
113- tm_send() is called to send data in the case of a p2p connection
114- check_presence() is an optional entry point that will be called regularly
115  by the core to check that an activated tag is still in the field. If this is
116  not implemented, the core will not be able to push tag_lost events to the user
117  space
118- event_received() is called to handle an event coming from the chip. Driver
119  can handle the event or return 1 to let HCI attempt standard processing.
120
121On the rx path, the driver is responsible to push incoming HCP frames to HCI
122using nfc_hci_recv_frame(). HCI will take care of re-aggregation and handling
123This must be done from a context that can sleep.
124
125PHY Management
126--------------
127
128The physical link (i2c, ...) management is defined by the following structure::
129
130  struct nfc_phy_ops {
131	int (*write)(void *dev_id, struct sk_buff *skb);
132	int (*enable)(void *dev_id);
133	void (*disable)(void *dev_id);
134  };
135
136enable():
137	turn the phy on (power on), make it ready to transfer data
138disable():
139	turn the phy off
140write():
141	Send a data frame to the chip. Note that to enable higher
142	layers such as an llc to store the frame for re-emission, this
143	function must not alter the skb. It must also not return a positive
144	result (return 0 for success, negative for failure).
145
146Data coming from the chip shall be sent directly to nfc_hci_recv_frame().
147
148LLC
149---
150
151Communication between the CPU and the chip often requires some link layer
152protocol. Those are isolated as modules managed by the HCI layer. There are
153currently two modules : nop (raw transfer) and shdlc.
154A new llc must implement the following functions::
155
156  struct nfc_llc_ops {
157	void *(*init) (struct nfc_hci_dev *hdev, xmit_to_drv_t xmit_to_drv,
158		       rcv_to_hci_t rcv_to_hci, int tx_headroom,
159		       int tx_tailroom, int *rx_headroom, int *rx_tailroom,
160		       llc_failure_t llc_failure);
161	void (*deinit) (struct nfc_llc *llc);
162	int (*start) (struct nfc_llc *llc);
163	int (*stop) (struct nfc_llc *llc);
164	void (*rcv_from_drv) (struct nfc_llc *llc, struct sk_buff *skb);
165	int (*xmit_from_hci) (struct nfc_llc *llc, struct sk_buff *skb);
166  };
167
168init():
169	allocate and init your private storage
170deinit():
171	cleanup
172start():
173	establish the logical connection
174stop ():
175	terminate the logical connection
176rcv_from_drv():
177	handle data coming from the chip, going to HCI
178xmit_from_hci():
179	handle data sent by HCI, going to the chip
180
181The llc must be registered with nfc before it can be used. Do that by
182calling::
183
184	nfc_llc_register(const char *name, const struct nfc_llc_ops *ops);
185
186Again, note that the llc does not handle the physical link. It is thus very
187easy to mix any physical link with any llc for a given chip driver.
188
189Included Drivers
190----------------
191
192An HCI based driver for an NXP PN544, connected through I2C bus, and using
193shdlc is included.
194
195Execution Contexts
196------------------
197
198The execution contexts are the following:
199- IRQ handler (IRQH):
200fast, cannot sleep. sends incoming frames to HCI where they are passed to
201the current llc. In case of shdlc, the frame is queued in shdlc rx queue.
202
203- SHDLC State Machine worker (SMW)
204
205  Only when llc_shdlc is used: handles shdlc rx & tx queues.
206
207  Dispatches HCI cmd responses.
208
209- HCI Tx Cmd worker (MSGTXWQ)
210
211  Serializes execution of HCI commands.
212
213  Completes execution in case of response timeout.
214
215- HCI Rx worker (MSGRXWQ)
216
217  Dispatches incoming HCI commands or events.
218
219- Syscall context from a userspace call (SYSCALL)
220
221  Any entrypoint in HCI called from NFC Core
222
223Workflow executing an HCI command (using shdlc)
224-----------------------------------------------
225
226Executing an HCI command can easily be performed synchronously using the
227following API::
228
229  int nfc_hci_send_cmd (struct nfc_hci_dev *hdev, u8 gate, u8 cmd,
230			const u8 *param, size_t param_len, struct sk_buff **skb)
231
232The API must be invoked from a context that can sleep. Most of the time, this
233will be the syscall context. skb will return the result that was received in
234the response.
235
236Internally, execution is asynchronous. So all this API does is to enqueue the
237HCI command, setup a local wait queue on stack, and wait_event() for completion.
238The wait is not interruptible because it is guaranteed that the command will
239complete after some short timeout anyway.
240
241MSGTXWQ context will then be scheduled and invoke nfc_hci_msg_tx_work().
242This function will dequeue the next pending command and send its HCP fragments
243to the lower layer which happens to be shdlc. It will then start a timer to be
244able to complete the command with a timeout error if no response arrive.
245
246SMW context gets scheduled and invokes nfc_shdlc_sm_work(). This function
247handles shdlc framing in and out. It uses the driver xmit to send frames and
248receives incoming frames in an skb queue filled from the driver IRQ handler.
249SHDLC I(nformation) frames payload are HCP fragments. They are aggregated to
250form complete HCI frames, which can be a response, command, or event.
251
252HCI Responses are dispatched immediately from this context to unblock
253waiting command execution. Response processing involves invoking the completion
254callback that was provided by nfc_hci_msg_tx_work() when it sent the command.
255The completion callback will then wake the syscall context.
256
257It is also possible to execute the command asynchronously using this API::
258
259  static int nfc_hci_execute_cmd_async(struct nfc_hci_dev *hdev, u8 pipe, u8 cmd,
260				       const u8 *param, size_t param_len,
261				       data_exchange_cb_t cb, void *cb_context)
262
263The workflow is the same, except that the API call returns immediately, and
264the callback will be called with the result from the SMW context.
265
266Workflow receiving an HCI event or command
267------------------------------------------
268
269HCI commands or events are not dispatched from SMW context. Instead, they are
270queued to HCI rx_queue and will be dispatched from HCI rx worker
271context (MSGRXWQ). This is done this way to allow a cmd or event handler
272to also execute other commands (for example, handling the
273NFC_HCI_EVT_TARGET_DISCOVERED event from PN544 requires to issue an
274ANY_GET_PARAMETER to the reader A gate to get information on the target
275that was discovered).
276
277Typically, such an event will be propagated to NFC Core from MSGRXWQ context.
278
279Error management
280----------------
281
282Errors that occur synchronously with the execution of an NFC Core request are
283simply returned as the execution result of the request. These are easy.
284
285Errors that occur asynchronously (e.g. in a background protocol handling thread)
286must be reported such that upper layers don't stay ignorant that something
287went wrong below and know that expected events will probably never happen.
288Handling of these errors is done as follows:
289
290- driver (pn544) fails to deliver an incoming frame: it stores the error such
291  that any subsequent call to the driver will result in this error. Then it
292  calls the standard nfc_shdlc_recv_frame() with a NULL argument to report the
293  problem above. shdlc stores a EREMOTEIO sticky status, which will trigger
294  SMW to report above in turn.
295
296- SMW is basically a background thread to handle incoming and outgoing shdlc
297  frames. This thread will also check the shdlc sticky status and report to HCI
298  when it discovers it is not able to run anymore because of an unrecoverable
299  error that happened within shdlc or below. If the problem occurs during shdlc
300  connection, the error is reported through the connect completion.
301
302- HCI: if an internal HCI error happens (frame is lost), or HCI is reported an
303  error from a lower layer, HCI will either complete the currently executing
304  command with that error, or notify NFC Core directly if no command is
305  executing.
306
307- NFC Core: when NFC Core is notified of an error from below and polling is
308  active, it will send a tag discovered event with an empty tag list to the user
309  space to let it know that the poll operation will never be able to detect a
310  tag. If polling is not active and the error was sticky, lower levels will
311  return it at next invocation.
312