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Diffstat (limited to 'Documentation/networking/caif/Linux-CAIF.txt')
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diff --git a/Documentation/networking/caif/Linux-CAIF.txt b/Documentation/networking/caif/Linux-CAIF.txt new file mode 100644 index 000000000000..7fe7a9a33a4f --- /dev/null +++ b/Documentation/networking/caif/Linux-CAIF.txt @@ -0,0 +1,212 @@ +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. |