Sjur Braendeland | edc7616 | 2010-03-30 13:56:29 +0000 | [diff] [blame^] | 1 | Linux CAIF |
| 2 | =========== |
| 3 | copyright (C) ST-Ericsson AB 2010 |
| 4 | Author: Sjur Brendeland/ sjur.brandeland@stericsson.com |
| 5 | License terms: GNU General Public License (GPL) version 2 |
| 6 | |
| 7 | |
| 8 | Introduction |
| 9 | ------------ |
| 10 | CAIF is a MUX protocol used by ST-Ericsson cellular modems for |
| 11 | communication between Modem and host. The host processes can open virtual AT |
| 12 | channels, initiate GPRS Data connections, Video channels and Utility Channels. |
| 13 | The Utility Channels are general purpose pipes between modem and host. |
| 14 | |
| 15 | ST-Ericsson modems support a number of transports between modem |
| 16 | and host. Currently, UART and Loopback are available for Linux. |
| 17 | |
| 18 | |
| 19 | Architecture: |
| 20 | ------------ |
| 21 | The implementation of CAIF is divided into: |
| 22 | * CAIF Socket Layer, Kernel API, and Net Device. |
| 23 | * CAIF Core Protocol Implementation |
| 24 | * CAIF Link Layer, implemented as NET devices. |
| 25 | |
| 26 | |
| 27 | RTNL |
| 28 | ! |
| 29 | ! +------+ +------+ +------+ |
| 30 | ! +------+! +------+! +------+! |
| 31 | ! ! Sock !! !Kernel!! ! Net !! |
| 32 | ! ! API !+ ! API !+ ! Dev !+ <- CAIF Client APIs |
| 33 | ! +------+ +------! +------+ |
| 34 | ! ! ! ! |
| 35 | ! +----------!----------+ |
| 36 | ! +------+ <- CAIF Protocol Implementation |
| 37 | +-------> ! CAIF ! |
| 38 | ! Core ! |
| 39 | +------+ |
| 40 | +--------!--------+ |
| 41 | ! ! |
| 42 | +------+ +-----+ |
| 43 | ! ! ! TTY ! <- Link Layer (Net Devices) |
| 44 | +------+ +-----+ |
| 45 | |
| 46 | |
| 47 | Using the Kernel API |
| 48 | ---------------------- |
| 49 | The Kernel API is used for accessing CAIF channels from the |
| 50 | kernel. |
| 51 | The user of the API has to implement two callbacks for receive |
| 52 | and control. |
| 53 | The receive callback gives a CAIF packet as a SKB. The control |
| 54 | callback will |
| 55 | notify of channel initialization complete, and flow-on/flow- |
| 56 | off. |
| 57 | |
| 58 | |
| 59 | struct caif_device caif_dev = { |
| 60 | .caif_config = { |
| 61 | .name = "MYDEV" |
| 62 | .type = CAIF_CHTY_AT |
| 63 | } |
| 64 | .receive_cb = my_receive, |
| 65 | .control_cb = my_control, |
| 66 | }; |
| 67 | caif_add_device(&caif_dev); |
| 68 | caif_transmit(&caif_dev, skb); |
| 69 | |
| 70 | See the caif_kernel.h for details about the CAIF kernel API. |
| 71 | |
| 72 | |
| 73 | I M P L E M E N T A T I O N |
| 74 | =========================== |
| 75 | =========================== |
| 76 | |
| 77 | CAIF Core Protocol Layer |
| 78 | ========================================= |
| 79 | |
| 80 | CAIF Core layer implements the CAIF protocol as defined by ST-Ericsson. |
| 81 | It implements the CAIF protocol stack in a layered approach, where |
| 82 | each layer described in the specification is implemented as a separate layer. |
| 83 | The architecture is inspired by the design patterns "Protocol Layer" and |
| 84 | "Protocol Packet". |
| 85 | |
| 86 | == CAIF structure == |
| 87 | The Core CAIF implementation contains: |
| 88 | - Simple implementation of CAIF. |
| 89 | - Layered architecture (a la Streams), each layer in the CAIF |
| 90 | specification is implemented in a separate c-file. |
| 91 | - Clients must implement PHY layer to access physical HW |
| 92 | with receive and transmit functions. |
| 93 | - Clients must call configuration function to add PHY layer. |
| 94 | - Clients must implement CAIF layer to consume/produce |
| 95 | CAIF payload with receive and transmit functions. |
| 96 | - Clients must call configuration function to add and connect the |
| 97 | Client layer. |
| 98 | - When receiving / transmitting CAIF Packets (cfpkt), ownership is passed |
| 99 | to the called function (except for framing layers' receive functions |
| 100 | or if a transmit function returns an error, in which case the caller |
| 101 | must free the packet). |
| 102 | |
| 103 | Layered Architecture |
| 104 | -------------------- |
| 105 | The CAIF protocol can be divided into two parts: Support functions and Protocol |
| 106 | Implementation. The support functions include: |
| 107 | |
| 108 | - CFPKT CAIF Packet. Implementation of CAIF Protocol Packet. The |
| 109 | CAIF Packet has functions for creating, destroying and adding content |
| 110 | and for adding/extracting header and trailers to protocol packets. |
| 111 | |
| 112 | - CFLST CAIF list implementation. |
| 113 | |
| 114 | - CFGLUE CAIF Glue. Contains OS Specifics, such as memory |
| 115 | allocation, endianness, etc. |
| 116 | |
| 117 | The CAIF Protocol implementation contains: |
| 118 | |
| 119 | - CFCNFG CAIF Configuration layer. Configures the CAIF Protocol |
| 120 | Stack and provides a Client interface for adding Link-Layer and |
| 121 | Driver interfaces on top of the CAIF Stack. |
| 122 | |
| 123 | - CFCTRL CAIF Control layer. Encodes and Decodes control messages |
| 124 | such as enumeration and channel setup. Also matches request and |
| 125 | response messages. |
| 126 | |
| 127 | - CFSERVL General CAIF Service Layer functionality; handles flow |
| 128 | control and remote shutdown requests. |
| 129 | |
| 130 | - CFVEI CAIF VEI layer. Handles CAIF AT Channels on VEI (Virtual |
| 131 | External Interface). This layer encodes/decodes VEI frames. |
| 132 | |
| 133 | - CFDGML CAIF Datagram layer. Handles CAIF Datagram layer (IP |
| 134 | traffic), encodes/decodes Datagram frames. |
| 135 | |
| 136 | - CFMUX CAIF Mux layer. Handles multiplexing between multiple |
| 137 | physical bearers and multiple channels such as VEI, Datagram, etc. |
| 138 | The MUX keeps track of the existing CAIF Channels and |
| 139 | Physical Instances and selects the apropriate instance based |
| 140 | on Channel-Id and Physical-ID. |
| 141 | |
| 142 | - CFFRML CAIF Framing layer. Handles Framing i.e. Frame length |
| 143 | and frame checksum. |
| 144 | |
| 145 | - CFSERL CAIF Serial layer. Handles concatenation/split of frames |
| 146 | into CAIF Frames with correct length. |
| 147 | |
| 148 | |
| 149 | |
| 150 | +---------+ |
| 151 | | Config | |
| 152 | | CFCNFG | |
| 153 | +---------+ |
| 154 | ! |
| 155 | +---------+ +---------+ +---------+ |
| 156 | | AT | | Control | | Datagram| |
| 157 | | CFVEIL | | CFCTRL | | CFDGML | |
| 158 | +---------+ +---------+ +---------+ |
| 159 | \_____________!______________/ |
| 160 | ! |
| 161 | +---------+ |
| 162 | | MUX | |
| 163 | | | |
| 164 | +---------+ |
| 165 | _____!_____ |
| 166 | / \ |
| 167 | +---------+ +---------+ |
| 168 | | CFFRML | | CFFRML | |
| 169 | | Framing | | Framing | |
| 170 | +---------+ +---------+ |
| 171 | ! ! |
| 172 | +---------+ +---------+ |
| 173 | | | | Serial | |
| 174 | | | | CFSERL | |
| 175 | +---------+ +---------+ |
| 176 | |
| 177 | |
| 178 | In this layered approach the following "rules" apply. |
| 179 | - All layers embed the same structure "struct cflayer" |
| 180 | - A layer does not depend on any other layer's private data. |
| 181 | - Layers are stacked by setting the pointers |
| 182 | layer->up , layer->dn |
| 183 | - In order to send data upwards, each layer should do |
| 184 | layer->up->receive(layer->up, packet); |
| 185 | - In order to send data downwards, each layer should do |
| 186 | layer->dn->transmit(layer->dn, packet); |
| 187 | |
| 188 | |
| 189 | Linux Driver Implementation |
| 190 | =========================== |
| 191 | |
| 192 | Linux GPRS Net Device and CAIF socket are implemented on top of the |
| 193 | CAIF Core protocol. The Net device and CAIF socket have an instance of |
| 194 | 'struct cflayer', just like the CAIF Core protocol stack. |
| 195 | Net device and Socket implement the 'receive()' function defined by |
| 196 | 'struct cflayer', just like the rest of the CAIF stack. In this way, transmit and |
| 197 | receive of packets is handled as by the rest of the layers: the 'dn->transmit()' |
| 198 | function is called in order to transmit data. |
| 199 | |
| 200 | The layer on top of the CAIF Core implementation is |
| 201 | sometimes referred to as the "Client layer". |
| 202 | |
| 203 | |
| 204 | Configuration of Link Layer |
| 205 | --------------------------- |
| 206 | The Link Layer is implemented as Linux net devices (struct net_device). |
| 207 | Payload handling and registration is done using standard Linux mechanisms. |
| 208 | |
| 209 | The CAIF Protocol relies on a loss-less link layer without implementing |
| 210 | retransmission. This implies that packet drops must not happen. |
| 211 | Therefore a flow-control mechanism is implemented where the physical |
| 212 | interface can initiate flow stop for all CAIF Channels. |