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1.. SPDX-License-Identifier: GPL-2.02.. include:: <isonum.txt>3 4==========5Linux CAIF6==========7 8Copyright |copy| ST-Ericsson AB 20109 10:Author: Sjur Brendeland/ sjur.brandeland@stericsson.com11:License terms: GNU General Public License (GPL) version 212 13 14Introduction15============16 17CAIF is a MUX protocol used by ST-Ericsson cellular modems for18communication between Modem and host. The host processes can open virtual AT19channels, initiate GPRS Data connections, Video channels and Utility Channels.20The Utility Channels are general purpose pipes between modem and host.21 22ST-Ericsson modems support a number of transports between modem23and host. Currently, UART and Loopback are available for Linux.24 25 26Architecture27============28 29The implementation of CAIF is divided into:30 31* CAIF Socket Layer and GPRS IP Interface.32* CAIF Core Protocol Implementation33* CAIF Link Layer, implemented as NET devices.34 35::36 37 RTNL38 !39 ! +------+ +------+40 ! +------+! +------+!41 ! ! IP !! !Socket!!42 +-------> !interf!+ ! API !+ <- CAIF Client APIs43 ! +------+ +------!44 ! ! !45 ! +-----------+46 ! !47 ! +------+ <- CAIF Core Protocol48 ! ! CAIF !49 ! ! Core !50 ! +------+51 ! +----------!---------+52 ! ! ! !53 ! +------+ +-----+ +------+54 +--> ! HSI ! ! TTY ! ! USB ! <- Link Layer (Net Devices)55 +------+ +-----+ +------+56 57 58 59Implementation60==============61 62 63CAIF Core Protocol Layer64------------------------65 66CAIF Core layer implements the CAIF protocol as defined by ST-Ericsson.67It implements the CAIF protocol stack in a layered approach, where68each layer described in the specification is implemented as a separate layer.69The architecture is inspired by the design patterns "Protocol Layer" and70"Protocol Packet".71 72CAIF structure73^^^^^^^^^^^^^^74 75The Core CAIF implementation contains:76 77 - Simple implementation of CAIF.78 - Layered architecture (a la Streams), each layer in the CAIF79 specification is implemented in a separate c-file.80 - Clients must call configuration function to add PHY layer.81 - Clients must implement CAIF layer to consume/produce82 CAIF payload with receive and transmit functions.83 - Clients must call configuration function to add and connect the84 Client layer.85 - When receiving / transmitting CAIF Packets (cfpkt), ownership is passed86 to the called function (except for framing layers' receive function)87 88Layered Architecture89====================90 91The CAIF protocol can be divided into two parts: Support functions and Protocol92Implementation. The support functions include:93 94 - CFPKT CAIF Packet. Implementation of CAIF Protocol Packet. The95 CAIF Packet has functions for creating, destroying and adding content96 and for adding/extracting header and trailers to protocol packets.97 98The CAIF Protocol implementation contains:99 100 - CFCNFG CAIF Configuration layer. Configures the CAIF Protocol101 Stack and provides a Client interface for adding Link-Layer and102 Driver interfaces on top of the CAIF Stack.103 104 - CFCTRL CAIF Control layer. Encodes and Decodes control messages105 such as enumeration and channel setup. Also matches request and106 response messages.107 108 - CFSERVL General CAIF Service Layer functionality; handles flow109 control and remote shutdown requests.110 111 - CFVEI CAIF VEI layer. Handles CAIF AT Channels on VEI (Virtual112 External Interface). This layer encodes/decodes VEI frames.113 114 - CFDGML CAIF Datagram layer. Handles CAIF Datagram layer (IP115 traffic), encodes/decodes Datagram frames.116 117 - CFMUX CAIF Mux layer. Handles multiplexing between multiple118 physical bearers and multiple channels such as VEI, Datagram, etc.119 The MUX keeps track of the existing CAIF Channels and120 Physical Instances and selects the appropriate instance based121 on Channel-Id and Physical-ID.122 123 - CFFRML CAIF Framing layer. Handles Framing i.e. Frame length124 and frame checksum.125 126 - CFSERL CAIF Serial layer. Handles concatenation/split of frames127 into CAIF Frames with correct length.128 129::130 131 +---------+132 | Config |133 | CFCNFG |134 +---------+135 !136 +---------+ +---------+ +---------+137 | AT | | Control | | Datagram|138 | CFVEIL | | CFCTRL | | CFDGML |139 +---------+ +---------+ +---------+140 \_____________!______________/141 !142 +---------+143 | MUX |144 | |145 +---------+146 _____!_____147 / \148 +---------+ +---------+149 | CFFRML | | CFFRML |150 | Framing | | Framing |151 +---------+ +---------+152 ! !153 +---------+ +---------+154 | | | Serial |155 | | | CFSERL |156 +---------+ +---------+157 158 159In this layered approach the following "rules" apply.160 161 - All layers embed the same structure "struct cflayer"162 - A layer does not depend on any other layer's private data.163 - Layers are stacked by setting the pointers::164 165 layer->up , layer->dn166 167 - In order to send data upwards, each layer should do::168 169 layer->up->receive(layer->up, packet);170 171 - In order to send data downwards, each layer should do::172 173 layer->dn->transmit(layer->dn, packet);174 175 176CAIF Socket and IP interface177============================178 179The IP interface and CAIF socket API are implemented on top of the180CAIF Core protocol. The IP Interface and CAIF socket have an instance of181'struct cflayer', just like the CAIF Core protocol stack.182Net device and Socket implement the 'receive()' function defined by183'struct cflayer', just like the rest of the CAIF stack. In this way, transmit and184receive of packets is handled as by the rest of the layers: the 'dn->transmit()'185function is called in order to transmit data.186 187Configuration of Link Layer188---------------------------189The Link Layer is implemented as Linux network devices (struct net_device).190Payload handling and registration is done using standard Linux mechanisms.191 192The CAIF Protocol relies on a loss-less link layer without implementing193retransmission. This implies that packet drops must not happen.194Therefore a flow-control mechanism is implemented where the physical195interface can initiate flow stop for all CAIF Channels.196