1 High Speed Synchronous Serial Interface (HSI)
2 =============================================
3 
4 Introduction
5 ---------------
6 
7 High Speed Synchronous Interface (HSI) is a full duplex, low latency protocol,
8 that is optimized for die-level interconnect between an Application Processor
9 and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and
10 implemented by multiple vendors since then.
11 
12 The HSI interface supports full duplex communication over multiple channels
13 (typically 8) and is capable of reaching speeds up to 200 Mbit/s.
14 
15 The serial protocol uses two signals, DATA and FLAG as combined data and clock
16 signals and an additional READY signal for flow control. An additional WAKE
17 signal can be used to wakeup the chips from standby modes. The signals are
18 commonly prefixed by AC for signals going from the application die to the
19 cellular die and CA for signals going the other way around.
20 
21 ::
22 
23     +------------+                                 +---------------+
24     |  Cellular  |                                 |  Application  |
25     |    Die     |                                 |      Die      |
26     |            | - - - - - - CAWAKE - - - - - - >|               |
27     |           T|------------ CADATA ------------>|R              |
28     |           X|------------ CAFLAG ------------>|X              |
29     |            |<----------- ACREADY ------------|               |
30     |            |                                 |               |
31     |            |                                 |               |
32     |            |< - - - - -  ACWAKE - - - - - - -|               |
33     |           R|<----------- ACDATA -------------|T              |
34     |           X|<----------- ACFLAG -------------|X              |
35     |            |------------ CAREADY ----------->|               |
36     |            |                                 |               |
37     |            |                                 |               |
38     +------------+                                 +---------------+
39 
40 HSI Subsystem in Linux
41 -------------------------
42 
43 In the Linux kernel the hsi subsystem is supposed to be used for HSI devices.
44 The hsi subsystem contains drivers for hsi controllers including support for
45 multi-port controllers and provides a generic API for using the HSI ports.
46 
47 It also contains HSI client drivers, which make use of the generic API to
48 implement a protocol used on the HSI interface. These client drivers can
49 use an arbitrary number of channels.
50 
51 hsi-char Device
52 ------------------
53 
54 Each port automatically registers a generic client driver called hsi_char,
55 which provides a character device for userspace representing the HSI port.
56 It can be used to communicate via HSI from userspace. Userspace may
57 configure the hsi_char device using the following ioctl commands:
58 
59 HSC_RESET
60  flush the HSI port
61 
62 HSC_SET_PM
63  enable or disable the client.
64 
65 HSC_SEND_BREAK
66  send break
67 
68 HSC_SET_RX
69  set RX configuration
70 
71 HSC_GET_RX
72  get RX configuration
73 
74 HSC_SET_TX
75  set TX configuration
76 
77 HSC_GET_TX
78  get TX configuration
79 
80 The kernel HSI API
81 ------------------
82 
83 .. kernel-doc:: include/linux/hsi/hsi.h
84    :internal:
85 
86 .. kernel-doc:: drivers/hsi/hsi_core.c
87    :export:
88 
89