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Documentation: HSI: Add some general description for the HSI subsystem
Add a document, which gives a rough introduction about what HSI
is and how its handled by the Linux kernel.
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
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Documentation/hsi.txt
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Documentation/hsi.txt
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HSI - High-speed Synchronous Serial Interface
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1. Introduction
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~~~~~~~~~~~~~~~
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High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol,
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that is optimized for die-level interconnect between an Application Processor
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and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and
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implemented by multiple vendors since then.
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The HSI interface supports full duplex communication over multiple channels
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(typically 8) and is capable of reaching speeds up to 200 Mbit/s.
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The serial protocol uses two signals, DATA and FLAG as combined data and clock
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signals and an additional READY signal for flow control. An additional WAKE
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signal can be used to wakeup the chips from standby modes. The signals are
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commonly prefixed by AC for signals going from the application die to the
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cellular die and CA for signals going the other way around.
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+------------+ +---------------+
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| Cellular | | Application |
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| Die | | Die |
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| | - - - - - - CAWAKE - - - - - - >| |
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| T|------------ CADATA ------------>|R |
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| X|------------ CAFLAG ------------>|X |
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| |<----------- ACREADY ------------| |
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| | | |
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| | | |
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| |< - - - - - ACWAKE - - - - - - -| |
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| R|<----------- ACDATA -------------|T |
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| X|<----------- ACFLAG -------------|X |
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| |------------ CAREADY ----------->| |
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| | | |
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| | | |
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+------------+ +---------------+
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2. HSI Subsystem in Linux
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~~~~~~~~~~~~~~~~~~~~~~~~~
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In the Linux kernel the hsi subsystem is supposed to be used for HSI devices.
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The hsi subsystem contains drivers for hsi controllers including support for
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multi-port controllers and provides a generic API for using the HSI ports.
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It also contains HSI client drivers, which make use of the generic API to
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implement a protocol used on the HSI interface. These client drivers can
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use an arbitrary number of channels.
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3. hsi-char Device
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~~~~~~~~~~~~~~~~~~
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Each port automatically registers a generic client driver called hsi_char,
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which provides a charecter device for userspace representing the HSI port.
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It can be used to communicate via HSI from userspace. Userspace may
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configure the hsi_char device using the following ioctl commands:
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* HSC_RESET:
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- flush the HSI port
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* HSC_SET_PM
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- enable or disable the client.
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* HSC_SEND_BREAK
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- send break
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* HSC_SET_RX
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- set RX configuration
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* HSC_GET_RX
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- get RX configuration
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* HSC_SET_TX
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- set TX configuration
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* HSC_GET_TX
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- get TX configuration