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-Linux CAIF
-===========
-copyright (C) ST-Ericsson AB 2010
-Author: Sjur Brendeland/ sjur.brandeland@stericsson.com
-License terms: GNU General Public License (GPL) version 2
-
-
-Introduction
-------------
-CAIF is a MUX protocol used by ST-Ericsson cellular modems for
-communication between Modem and host. The host processes can open virtual AT
-channels, initiate GPRS Data connections, Video channels and Utility Channels.
-The Utility Channels are general purpose pipes between modem and host.
-
-ST-Ericsson modems support a number of transports between modem
-and host. Currently, UART and Loopback are available for Linux.
-
-
-Architecture:
-------------
-The implementation of CAIF is divided into:
-* CAIF Socket Layer, Kernel API, and Net Device.
-* CAIF Core Protocol Implementation
-* CAIF Link Layer, implemented as NET devices.
-
-
- RTNL
- !
- ! +------+ +------+ +------+
- ! +------+! +------+! +------+!
- ! ! Sock !! !Kernel!! ! Net !!
- ! ! API !+ ! API !+ ! Dev !+ <- CAIF Client APIs
- ! +------+ +------! +------+
- ! ! ! !
- ! +----------!----------+
- ! +------+ <- CAIF Protocol Implementation
- +-------> ! CAIF !
- ! Core !
- +------+
- +--------!--------+
- ! !
- +------+ +-----+
- ! ! ! TTY ! <- Link Layer (Net Devices)
- +------+ +-----+
-
-
-Using the Kernel API
-----------------------
-The Kernel API is used for accessing CAIF channels from the
-kernel.
-The user of the API has to implement two callbacks for receive
-and control.
-The receive callback gives a CAIF packet as a SKB. The control
-callback will
-notify of channel initialization complete, and flow-on/flow-
-off.
-
-
- struct caif_device caif_dev = {
- .caif_config = {
- .name = "MYDEV"
- .type = CAIF_CHTY_AT
- }
- .receive_cb = my_receive,
- .control_cb = my_control,
- };
- caif_add_device(&caif_dev);
- caif_transmit(&caif_dev, skb);
-
-See the caif_kernel.h for details about the CAIF kernel API.
-
-
-I M P L E M E N T A T I O N
-===========================
-===========================
-
-CAIF Core Protocol Layer
-=========================================
-
-CAIF Core layer implements the CAIF protocol as defined by ST-Ericsson.
-It implements the CAIF protocol stack in a layered approach, where
-each layer described in the specification is implemented as a separate layer.
-The architecture is inspired by the design patterns "Protocol Layer" and
-"Protocol Packet".
-
-== CAIF structure ==
-The Core CAIF implementation contains:
- - Simple implementation of CAIF.
- - Layered architecture (a la Streams), each layer in the CAIF
- specification is implemented in a separate c-file.
- - Clients must implement PHY layer to access physical HW
- with receive and transmit functions.
- - Clients must call configuration function to add PHY layer.
- - Clients must implement CAIF layer to consume/produce
- CAIF payload with receive and transmit functions.
- - Clients must call configuration function to add and connect the
- Client layer.
- - When receiving / transmitting CAIF Packets (cfpkt), ownership is passed
- to the called function (except for framing layers' receive functions
- or if a transmit function returns an error, in which case the caller
- must free the packet).
-
-Layered Architecture
---------------------
-The CAIF protocol can be divided into two parts: Support functions and Protocol
-Implementation. The support functions include:
-
- - CFPKT CAIF Packet. Implementation of CAIF Protocol Packet. The
- CAIF Packet has functions for creating, destroying and adding content
- and for adding/extracting header and trailers to protocol packets.
-
- - CFLST CAIF list implementation.
-
- - CFGLUE CAIF Glue. Contains OS Specifics, such as memory
- allocation, endianness, etc.
-
-The CAIF Protocol implementation contains:
-
- - CFCNFG CAIF Configuration layer. Configures the CAIF Protocol
- Stack and provides a Client interface for adding Link-Layer and
- Driver interfaces on top of the CAIF Stack.
-
- - CFCTRL CAIF Control layer. Encodes and Decodes control messages
- such as enumeration and channel setup. Also matches request and
- response messages.
-
- - CFSERVL General CAIF Service Layer functionality; handles flow
- control and remote shutdown requests.
-
- - CFVEI CAIF VEI layer. Handles CAIF AT Channels on VEI (Virtual
- External Interface). This layer encodes/decodes VEI frames.
-
- - CFDGML CAIF Datagram layer. Handles CAIF Datagram layer (IP
- traffic), encodes/decodes Datagram frames.
-
- - CFMUX CAIF Mux layer. Handles multiplexing between multiple
- physical bearers and multiple channels such as VEI, Datagram, etc.
- The MUX keeps track of the existing CAIF Channels and
- Physical Instances and selects the appropriate instance based
- on Channel-Id and Physical-ID.
-
- - CFFRML CAIF Framing layer. Handles Framing i.e. Frame length
- and frame checksum.
-
- - CFSERL CAIF Serial layer. Handles concatenation/split of frames
- into CAIF Frames with correct length.
-
-
-
- +---------+
- | Config |
- | CFCNFG |
- +---------+
- !
- +---------+ +---------+ +---------+
- | AT | | Control | | Datagram|
- | CFVEIL | | CFCTRL | | CFDGML |
- +---------+ +---------+ +---------+
- \_____________!______________/
- !
- +---------+
- | MUX |
- | |
- +---------+
- _____!_____
- / \
- +---------+ +---------+
- | CFFRML | | CFFRML |
- | Framing | | Framing |
- +---------+ +---------+
- ! !
- +---------+ +---------+
- | | | Serial |
- | | | CFSERL |
- +---------+ +---------+
-
-
-In this layered approach the following "rules" apply.
- - All layers embed the same structure "struct cflayer"
- - A layer does not depend on any other layer's private data.
- - Layers are stacked by setting the pointers
- layer->up , layer->dn
- - In order to send data upwards, each layer should do
- layer->up->receive(layer->up, packet);
- - In order to send data downwards, each layer should do
- layer->dn->transmit(layer->dn, packet);
-
-
-Linux Driver Implementation
-===========================
-
-Linux GPRS Net Device and CAIF socket are implemented on top of the
-CAIF Core protocol. The Net device and CAIF socket have an instance of
-'struct cflayer', just like the CAIF Core protocol stack.
-Net device and Socket implement the 'receive()' function defined by
-'struct cflayer', just like the rest of the CAIF stack. In this way, transmit and
-receive of packets is handled as by the rest of the layers: the 'dn->transmit()'
-function is called in order to transmit data.
-
-The layer on top of the CAIF Core implementation is
-sometimes referred to as the "Client layer".
-
-
-Configuration of Link Layer
----------------------------
-The Link Layer is implemented as Linux net devices (struct net_device).
-Payload handling and registration is done using standard Linux mechanisms.
-
-The CAIF Protocol relies on a loss-less link layer without implementing
-retransmission. This implies that packet drops must not happen.
-Therefore a flow-control mechanism is implemented where the physical
-interface can initiate flow stop for all CAIF Channels.