.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

==========
Linux CAIF
==========

Copyright |copy| 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 and GPRS IP Interface.
* CAIF Core Protocol Implementation
* CAIF Link Layer, implemented as NET devices.

::

  RTNL
   !
   !	      +------+	 +------+
   !	     +------+!	+------+!
   !	     !	IP  !!	!Socket!!
   +-------> !interf!+	! API  !+	<- CAIF Client APIs
   !	     +------+	+------!
   !		!	    !
   !		+-----------+
   !		      !
   !		   +------+		<- CAIF Core Protocol
   !		   ! CAIF !
   !		   ! Core !
   !		   +------+
   !	   +----------!---------+
   !	   !	      !		!
   !	+------+   +-----+   +------+
   +--> ! HSI  !   ! TTY !   ! USB  !	<- Link Layer (Net Devices)
	+------+   +-----+   +------+


Implementation
==============


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 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 function)

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.

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);


CAIF Socket and IP interface
============================

The IP interface and CAIF socket API are implemented on top of the
CAIF Core protocol. The IP Interface 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.

Configuration of Link Layer
---------------------------
The Link Layer is implemented as Linux network 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.