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