net-caif: add CAIF documentation

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.

Copyright (C) ST-Ericsson AB 2010

Author: Sjur Brendeland/ sjur.brandeland@stericsson.com

License terms: GNU General Public License (GPL) version 2

---------------------------------------------------------

=== Start ===

If you have compiled CAIF for modules do:

$modprobe crc_ccitt

$modprobe caif

$modprobe caif_socket

$modprobe chnl_net

=== Preparing the setup with a STE modem ===

If you are working on integration of CAIF you should make sure

that the kernel is built with module support.

There are some things that need to be tweaked to get the host TTY correctly

set up to talk to the modem.

Since the CAIF stack is running in the kernel and we want to use the existing

TTY, we are installing our physical serial driver as a line discipline above

the TTY device.

To achieve this we need to install the N_CAIF ldisc from user space.

The benefit is that we can hook up to any TTY.

The use of Start-of-frame-extension (STX) must also be set as

module parameter "ser_use_stx".

Normally Frame Checksum is always used on UART, but this is also provided as a

module parameter "ser_use_fcs".

$ modprobe caif_serial ser_ttyname=/dev/ttyS0 ser_use_stx=yes

$ ifconfig caif_ttyS0 up

PLEASE NOTE: 	There is a limitation in Android shell.

		It only accepts one argument to insmod/modprobe!

=== Trouble shooting ===

There are debugfs parameters provided for serial communication.

/sys/kernel/debug/caif_serial/<tty-name>/

* ser_state:   Prints the bit-mask status where

  - 0x02 means SENDING, this is a transient state.

  - 0x10 means FLOW_OFF_SENT, i.e. the previous frame has not been sent

	and is blocking further send operation. Flow OFF has been propagated

	to all CAIF Channels using this TTY.

* tty_status: Prints the bit-mask tty status information

  - 0x01 - tty->warned is on.

  - 0x02 - tty->low_latency is on.

  - 0x04 - tty->packed is on.

  - 0x08 - tty->flow_stopped is on.

  - 0x10 - tty->hw_stopped is on.

  - 0x20 - tty->stopped is on.

* last_tx_msg: Binary blob Prints the last transmitted frame.

	This can be printed with

	$od --format=x1 /sys/kernel/debug/caif_serial/<tty>/last_rx_msg.

	The first two tx messages sent look like this. Note: The initial

	byte 02 is start of frame extension (STX) used for re-syncing

	upon errors.

  - Enumeration:

        0000000  02 05 00 00 03 01 d2 02

                 |  |     |  |  |  |

                 STX(1)   |  |  |  |

                    Length(2)|  |  |

                          Control Channel(1)

                             Command:Enumeration(1)

                                Link-ID(1)

                                    Checksum(2)

  - Channel Setup:

        0000000  02 07 00 00 00 21 a1 00 48 df

                 |  |     |  |  |  |  |  |

                 STX(1)   |  |  |  |  |  |

                    Length(2)|  |  |  |  |

                          Control Channel(1)

                             Command:Channel Setup(1)

                                Channel Type(1)

                                    Priority and Link-ID(1)

				      Endpoint(1)

					  Checksum(2)

* last_rx_msg: Prints the last transmitted frame.

	The RX messages for LinkSetup look almost identical but they have the

	bit 0x20 set in the command bit, and Channel Setup has added one byte

	before Checksum containing Channel ID.

	NOTE: Several CAIF Messages might be concatenated. The maximum debug

	buffer size is 128 bytes.

== Error Scenarios:

- last_tx_msg contains channel setup message and last_rx_msg is empty ->

  The host seems to be able to send over the UART, at least the CAIF ldisc get

  notified that sending is completed.

- last_tx_msg contains enumeration message and last_rx_msg is empty ->

  The host is not able to send the message from UART, the tty has not been

  able to complete the transmit operation.

- if /sys/kernel/debug/caif_serial/<tty>/tty_status is non-zero there

  might be problems transmitting over UART.

  E.g. host and modem wiring is not correct you will typically see

  tty_status = 0x10 (hw_stopped) and ser_state = 0x10 (FLOW_OFF_SENT).

  You will probably see the enumeration message in last_tx_message

  and empty last_rx_message.