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+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_reseted(). 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_reseted(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)
+