net-caif: add CAIF documentation
Documentation of the CAIF Protocol.
Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/caif/Linux-CAIF.txt b/Documentation/networking/caif/Linux-CAIF.txt
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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 apropriate 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.