=========================================
Kernel CAPI Interface to Hardware Drivers
=========================================

1. Overview
===========

From the CAPI 2.0 specification:
COMMON-ISDN-API (CAPI) is an application programming interface standard used
to access ISDN equipment connected to basic rate interfaces (BRI) and primary
rate interfaces (PRI).

Kernel CAPI operates as a dispatching layer between CAPI applications and CAPI
hardware drivers. Hardware drivers register ISDN devices (controllers, in CAPI
lingo) with Kernel CAPI to indicate their readiness to provide their service
to CAPI applications. CAPI applications also register with Kernel CAPI,
requesting association with a CAPI device. Kernel CAPI then dispatches the
application registration to an available device, forwarding it to the
corresponding hardware driver. Kernel CAPI then forwards CAPI messages in both
directions between the application and the hardware driver.

Format and semantics of CAPI messages are specified in the CAPI 2.0 standard.
This standard is freely available from https://www.capi.org.


2. Driver and Device Registration
=================================

CAPI drivers optionally register themselves with Kernel CAPI by calling the
Kernel CAPI function register_capi_driver() with a pointer to a struct
capi_driver. This structure must be filled with the name and revision of the
driver, and optionally a pointer to a callback function, add_card(). The
registration can be revoked by calling the function unregister_capi_driver()
with a pointer to the same struct capi_driver.

CAPI drivers must register each of the ISDN devices they control with Kernel
CAPI by calling the Kernel CAPI function attach_capi_ctr() with a pointer to a
struct capi_ctr before they can be used. This structure must be filled with
the names of the driver and controller, and a number of callback function
pointers which are subsequently used by Kernel CAPI for communicating with the
driver. The registration can be revoked by calling the function
detach_capi_ctr() with a pointer to the same struct capi_ctr.

Before the device can be actually used, the driver must fill in the device
information fields 'manu', 'version', 'profile' and 'serial' in the capi_ctr
structure of the device, and signal its readiness by calling capi_ctr_ready().
From then on, Kernel CAPI may call the registered callback functions for the
device.

If the device becomes unusable for any reason (shutdown, disconnect ...), the
driver has to call capi_ctr_down(). This will prevent further calls to the
callback functions by Kernel CAPI.


3. Application Registration and Communication
=============================================

Kernel CAPI forwards registration requests from applications (calls to CAPI
operation CAPI_REGISTER) to an appropriate hardware driver by calling its
register_appl() callback function. A unique Application ID (ApplID, u16) is
allocated by Kernel CAPI and passed to register_appl() along with the
parameter structure provided by the application. This is analogous to the
open() operation on regular files or character devices.

After a successful return from register_appl(), CAPI messages from the
application may be passed to the driver for the device via calls to the
send_message() callback function. Conversely, the driver may call Kernel
CAPI's capi_ctr_handle_message() function to pass a received CAPI message to
Kernel CAPI for forwarding to an application, specifying its ApplID.

Deregistration requests (CAPI operation CAPI_RELEASE) from applications are
forwarded as calls to the release_appl() callback function, passing the same
ApplID as with register_appl(). After return from release_appl(), no CAPI
messages for that application may be passed to or from the device anymore.


4. Data Structures
==================

4.1 struct capi_driver
----------------------

This structure describes a Kernel CAPI driver itself. It is used in the
register_capi_driver() and unregister_capi_driver() functions, and contains
the following non-private fields, all to be set by the driver before calling
register_capi_driver():

``char name[32]``
	the name of the driver, as a zero-terminated ASCII string
``char revision[32]``
	the revision number of the driver, as a zero-terminated ASCII string
``int (*add_card)(struct capi_driver *driver, capicardparams *data)``
	a callback function pointer (may be NULL)


4.2 struct capi_ctr
-------------------

This structure describes an ISDN device (controller) handled by a Kernel CAPI
driver. After registration via the attach_capi_ctr() function it is passed to
all controller specific lower layer interface and callback functions to
identify the controller to operate on.

It contains the following non-private fields:

to be set by the driver before calling attach_capi_ctr():
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``struct module *owner``
	pointer to the driver module owning the device

``void *driverdata``
	an opaque pointer to driver specific data, not touched by Kernel CAPI

``char name[32]``
	the name of the controller, as a zero-terminated ASCII string

``char *driver_name``
	the name of the driver, as a zero-terminated ASCII string

``int (*load_firmware)(struct capi_ctr *ctrlr, capiloaddata *ldata)``
	(optional) pointer to a callback function for sending firmware and
	configuration data to the device

	The function may return before the operation has completed.

	Completion must be signalled by a call to capi_ctr_ready().

	Return value: 0 on success, error code on error
	Called in process context.

``void (*reset_ctr)(struct capi_ctr *ctrlr)``
	(optional) pointer to a callback function for stopping the device,
	releasing all registered applications

	The function may return before the operation has completed.

	Completion must be signalled by a call to capi_ctr_down().

	Called in process context.

``void (*register_appl)(struct capi_ctr *ctrlr, u16 applid, capi_register_params *rparam)``
	pointers to callback function for registration of
	applications with the device

	Calls to these functions are serialized by Kernel CAPI so that only
	one call to any of them is active at any time.

``void (*release_appl)(struct capi_ctr *ctrlr, u16 applid)``
	pointers to callback functions deregistration of
	applications with the device

	Calls to these functions are serialized by Kernel CAPI so that only
	one call to any of them is active at any time.

``u16  (*send_message)(struct capi_ctr *ctrlr, struct sk_buff *skb)``
	pointer to a callback function for sending a CAPI message to the
	device

	Return value: CAPI error code

	If the method returns 0 (CAPI_NOERROR) the driver has taken ownership
	of the skb and the caller may no longer access it. If it returns a
	non-zero (error) value then ownership of the skb returns to the caller
	who may reuse or free it.

	The return value should only be used to signal problems with respect
	to accepting or queueing the message. Errors occurring during the
	actual processing of the message should be signaled with an
	appropriate reply message.

	May be called in process or interrupt context.

	Calls to this function are not serialized by Kernel CAPI, ie. it must
	be prepared to be re-entered.

``char *(*procinfo)(struct capi_ctr *ctrlr)``
	pointer to a callback function returning the entry for the device in
	the CAPI controller info table, /proc/capi/controller

``const struct file_operations *proc_fops``
	pointers to callback functions for the device's proc file
	system entry, /proc/capi/controllers/<n>; pointer to the device's
	capi_ctr structure is available from struct proc_dir_entry::data
	which is available from struct inode.

Note:
  Callback functions except send_message() are never called in interrupt
  context.

to be filled in before calling capi_ctr_ready():
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``u8 manu[CAPI_MANUFACTURER_LEN]``
	value to return for CAPI_GET_MANUFACTURER

``capi_version version``
	value to return for CAPI_GET_VERSION

``capi_profile profile``
	value to return for CAPI_GET_PROFILE

``u8 serial[CAPI_SERIAL_LEN]``
	value to return for CAPI_GET_SERIAL


4.3 SKBs
--------

CAPI messages are passed between Kernel CAPI and the driver via send_message()
and capi_ctr_handle_message(), stored in the data portion of a socket buffer
(skb).  Each skb contains a single CAPI message coded according to the CAPI 2.0
standard.

For the data transfer messages, DATA_B3_REQ and DATA_B3_IND, the actual
payload data immediately follows the CAPI message itself within the same skb.
The Data and Data64 parameters are not used for processing. The Data64
parameter may be omitted by setting the length field of the CAPI message to 22
instead of 30.


4.4 The _cmsg Structure
-----------------------

(declared in <linux/isdn/capiutil.h>)

The _cmsg structure stores the contents of a CAPI 2.0 message in an easily
accessible form. It contains members for all possible CAPI 2.0 parameters,
including subparameters of the Additional Info and B Protocol structured
parameters, with the following exceptions:

* second Calling party number (CONNECT_IND)

* Data64 (DATA_B3_REQ and DATA_B3_IND)

* Sending complete (subparameter of Additional Info, CONNECT_REQ and INFO_REQ)

* Global Configuration (subparameter of B Protocol, CONNECT_REQ, CONNECT_RESP
  and SELECT_B_PROTOCOL_REQ)

Only those parameters appearing in the message type currently being processed
are actually used. Unused members should be set to zero.

Members are named after the CAPI 2.0 standard names of the parameters they
represent. See <linux/isdn/capiutil.h> for the exact spelling. Member data
types are:

=========== =================================================================
u8          for CAPI parameters of type 'byte'

u16         for CAPI parameters of type 'word'

u32         for CAPI parameters of type 'dword'

_cstruct    for CAPI parameters of type 'struct'
	    The member is a pointer to a buffer containing the parameter in
	    CAPI encoding (length + content). It may also be NULL, which will
	    be taken to represent an empty (zero length) parameter.
	    Subparameters are stored in encoded form within the content part.

_cmstruct   alternative representation for CAPI parameters of type 'struct'
	    (used only for the 'Additional Info' and 'B Protocol' parameters)
	    The representation is a single byte containing one of the values:
	    CAPI_DEFAULT: The parameter is empty/absent.
	    CAPI_COMPOSE: The parameter is present.
	    Subparameter values are stored individually in the corresponding
	    _cmsg structure members.
=========== =================================================================

Functions capi_cmsg2message() and capi_message2cmsg() are provided to convert
messages between their transport encoding described in the CAPI 2.0 standard
and their _cmsg structure representation. Note that capi_cmsg2message() does
not know or check the size of its destination buffer. The caller must make
sure it is big enough to accommodate the resulting CAPI message.


5. Lower Layer Interface Functions
==================================

(declared in <linux/isdn/capilli.h>)

::

  void register_capi_driver(struct capi_driver *drvr)
  void unregister_capi_driver(struct capi_driver *drvr)

register/unregister a driver with Kernel CAPI

::

  int attach_capi_ctr(struct capi_ctr *ctrlr)
  int detach_capi_ctr(struct capi_ctr *ctrlr)

register/unregister a device (controller) with Kernel CAPI

::

  void capi_ctr_ready(struct capi_ctr *ctrlr)
  void capi_ctr_down(struct capi_ctr *ctrlr)

signal controller ready/not ready

::

  void capi_ctr_suspend_output(struct capi_ctr *ctrlr)
  void capi_ctr_resume_output(struct capi_ctr *ctrlr)

signal suspend/resume

::

  void capi_ctr_handle_message(struct capi_ctr * ctrlr, u16 applid,
			       struct sk_buff *skb)

pass a received CAPI message to Kernel CAPI
for forwarding to the specified application


6. Helper Functions and Macros
==============================

Library functions (from <linux/isdn/capilli.h>):

::

  void capilib_new_ncci(struct list_head *head, u16 applid,
			u32 ncci, u32 winsize)
  void capilib_free_ncci(struct list_head *head, u16 applid, u32 ncci)
  void capilib_release_appl(struct list_head *head, u16 applid)
  void capilib_release(struct list_head *head)
  void capilib_data_b3_conf(struct list_head *head, u16 applid,
			u32 ncci, u16 msgid)
  u16  capilib_data_b3_req(struct list_head *head, u16 applid,
			u32 ncci, u16 msgid)


Macros to extract/set element values from/in a CAPI message header
(from <linux/isdn/capiutil.h>):

======================  =============================   ====================
Get Macro		Set Macro			Element (Type)
======================  =============================   ====================
CAPIMSG_LEN(m)		CAPIMSG_SETLEN(m, len)		Total Length (u16)
CAPIMSG_APPID(m)	CAPIMSG_SETAPPID(m, applid)	ApplID (u16)
CAPIMSG_COMMAND(m)	CAPIMSG_SETCOMMAND(m,cmd)	Command (u8)
CAPIMSG_SUBCOMMAND(m)	CAPIMSG_SETSUBCOMMAND(m, cmd)	Subcommand (u8)
CAPIMSG_CMD(m)		-				Command*256
							+ Subcommand (u16)
CAPIMSG_MSGID(m)	CAPIMSG_SETMSGID(m, msgid)	Message Number (u16)

CAPIMSG_CONTROL(m)	CAPIMSG_SETCONTROL(m, contr)	Controller/PLCI/NCCI
							(u32)
CAPIMSG_DATALEN(m)	CAPIMSG_SETDATALEN(m, len)	Data Length (u16)
======================  =============================   ====================


Library functions for working with _cmsg structures
(from <linux/isdn/capiutil.h>):

``unsigned capi_cmsg2message(_cmsg *cmsg, u8 *msg)``
	Assembles a CAPI 2.0 message from the parameters in ``*cmsg``,
	storing the result in ``*msg``.

``unsigned capi_message2cmsg(_cmsg *cmsg, u8 *msg)``
	Disassembles the CAPI 2.0 message in ``*msg``, storing the parameters
	in ``*cmsg``.

``unsigned capi_cmsg_header(_cmsg *cmsg, u16 ApplId, u8 Command, u8 Subcommand, u16 Messagenumber, u32 Controller)``
	Fills the header part and address field of the _cmsg structure ``*cmsg``
	with the given values, zeroing the remainder of the structure so only
	parameters with non-default values need to be changed before sending
	the message.

``void capi_cmsg_answer(_cmsg *cmsg)``
	Sets the low bit of the Subcommand field in ``*cmsg``, thereby
	converting ``_REQ`` to ``_CONF`` and ``_IND`` to ``_RESP``.

``char *capi_cmd2str(u8 Command, u8 Subcommand)``
	Returns the CAPI 2.0 message name corresponding to the given command
	and subcommand values, as a static ASCII string. The return value may
	be NULL if the command/subcommand is not one of those defined in the
	CAPI 2.0 standard.


7. Debugging
============

The module kernelcapi has a module parameter showcapimsgs controlling some
debugging output produced by the module. It can only be set when the module is
loaded, via a parameter "showcapimsgs=<n>" to the modprobe command, either on
the command line or in the configuration file.

If the lowest bit of showcapimsgs is set, kernelcapi logs controller and
application up and down events.

In addition, every registered CAPI controller has an associated traceflag
parameter controlling how CAPI messages sent from and to tha controller are
logged. The traceflag parameter is initialized with the value of the
showcapimsgs parameter when the controller is registered, but can later be
changed via the MANUFACTURER_REQ command KCAPI_CMD_TRACE.

If the value of traceflag is non-zero, CAPI messages are logged.
DATA_B3 messages are only logged if the value of traceflag is > 2.

If the lowest bit of traceflag is set, only the command/subcommand and message
length are logged. Otherwise, kernelcapi logs a readable representation of
the entire message.