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Subject[PATCHv3 01/14] Documentation: HSI: Add some general description for the HSI subsystem
Date
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>
---
Documentation/hsi.txt | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 75 insertions(+)
create mode 100644 Documentation/hsi.txt

diff --git a/Documentation/hsi.txt b/Documentation/hsi.txt
new file mode 100644
index 0000000..6ac6cd5
--- /dev/null
+++ b/Documentation/hsi.txt
@@ -0,0 +1,75 @@
+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
--
1.9.0


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