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, 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
47Using the Kernel API
48----------------------
49The Kernel API is used for accessing CAIF channels from the
50kernel.
51The user of the API has to implement two callbacks for receive
52and control.
53The receive callback gives a CAIF packet as a SKB. The control
54callback will
55notify of channel initialization complete, and flow-on/flow-
56off.
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
70See the caif_kernel.h for details about the CAIF kernel API.
71
72
73I M P L E M E N T A T I O N
74===========================
75===========================
76
77CAIF Core Protocol Layer
78=========================================
79
80CAIF Core layer implements the CAIF protocol as defined by ST-Ericsson.
81It implements the CAIF protocol stack in a layered approach, where
82each layer described in the specification is implemented as a separate layer.
83The architecture is inspired by the design patterns "Protocol Layer" and
84"Protocol Packet".
85
86== CAIF structure ==
87The 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
103Layered Architecture
104--------------------
105The CAIF protocol can be divided into two parts: Support functions and Protocol
106Implementation. 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
117The 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 appropriate 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
178In 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
189Linux Driver Implementation
190===========================
191
192Linux GPRS Net Device and CAIF socket are implemented on top of the
193CAIF Core protocol. The Net device and CAIF socket have an instance of
194'struct cflayer', just like the CAIF Core protocol stack.
195Net 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
197receive of packets is handled as by the rest of the layers: the 'dn->transmit()'
198function is called in order to transmit data.
199
200The layer on top of the CAIF Core implementation is
201sometimes referred to as the "Client layer".
202
203
204Configuration of Link Layer
205---------------------------
206The Link Layer is implemented as Linux net devices (struct net_device).
207Payload handling and registration is done using standard Linux mechanisms.
208
209The CAIF Protocol relies on a loss-less link layer without implementing
210retransmission. This implies that packet drops must not happen.
211Therefore a flow-control mechanism is implemented where the physical
212interface can initiate flow stop for all CAIF Channels.
213