312 lines
13 KiB
ReStructuredText
312 lines
13 KiB
ReStructuredText
========================
|
|
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 structure::
|
|
|
|
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.
|