Documentation: HSI: Add some general description for the HSI subsystem

HSI - High-speed Synchronous Serial Interface

1. Introduction

~~~~~~~~~~~~~~~

High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol,

that is optimized for die-level interconnect between an Application Processor

and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and

implemented by multiple vendors since then.

The HSI interface supports full duplex communication over multiple channels

(typically 8) and is capable of reaching speeds up to 200 Mbit/s.

The serial protocol uses two signals, DATA and FLAG as combined data and clock

signals and an additional READY signal for flow control. An additional WAKE

signal can be used to wakeup the chips from standby modes. The signals are

commonly prefixed by AC for signals going from the application die to the

cellular die and CA for signals going the other way around.

+------------+                                 +---------------+

|  Cellular  |                                 |  Application  |

|    Die     |                                 |      Die      |

|            | - - - - - - CAWAKE - - - - - - >|               |

|           T|------------ CADATA ------------>|R              |

|           X|------------ CAFLAG ------------>|X              |

|            |<----------- ACREADY ------------|               |

|            |                                 |               |

|            |                                 |               |

|            |< - - - - -  ACWAKE - - - - - - -|               |

|           R|<----------- ACDATA -------------|T              |

|           X|<----------- ACFLAG -------------|X              |

|            |------------ CAREADY ----------->|               |

|            |                                 |               |

|            |                                 |               |

+------------+                                 +---------------+

2. HSI Subsystem in Linux

~~~~~~~~~~~~~~~~~~~~~~~~~

In the Linux kernel the hsi subsystem is supposed to be used for HSI devices.

The hsi subsystem contains drivers for hsi controllers including support for

multi-port controllers and provides a generic API for using the HSI ports.

It also contains HSI client drivers, which make use of the generic API to

implement a protocol used on the HSI interface. These client drivers can

use an arbitrary number of channels.

3. hsi-char Device

~~~~~~~~~~~~~~~~~~

Each port automatically registers a generic client driver called hsi_char,

which provides a charecter device for userspace representing the HSI port.

It can be used to communicate via HSI from userspace. Userspace may

configure the hsi_char device using the following ioctl commands:

* HSC_RESET:

 - flush the HSI port

* HSC_SET_PM

 - enable or disable the client.

* HSC_SEND_BREAK

 - send break

* HSC_SET_RX

 - set RX configuration

* HSC_GET_RX

 - get RX configuration

* HSC_SET_TX

 - set TX configuration

* HSC_GET_TX

 - get TX configuration