isdn: prevent NULL ptr Oops in capi_cmsg2str()

[deliverable/linux.git] / Documentation / isdn / INTERFACE.CAPI

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 http://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. The CAPI message to send is stored in the
data portion of an skb. 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

void (*reset_ctr)(struct capi_ctr *ctrlr)
	pointer to a callback function for performing a reset on the device,
	releasing all registered applications

void (*register_appl)(struct capi_ctr *ctrlr, u16 applid,
			capi_register_params *rparam)
void (*release_appl)(struct capi_ctr *ctrlr, u16 applid)
	pointers to callback functions for registration and deregistration of
	applications with the device

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

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

read_proc_t *ctr_read_proc
	pointer to the read_proc callback function for the device's proc file
	system entry, /proc/capi/controllers/<n>; will be called with a
	pointer to the device's capi_ctr structure as the last (data) argument

- 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


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)