| Kyle Swenson | 8d8f654 | 2021-03-15 11:02:55 -0600 | [diff] [blame] | 1 | HSI - High-speed Synchronous Serial Interface |
| 2 | |
| 3 | 1. Introduction |
| 4 | ~~~~~~~~~~~~~~~ |
| 5 | |
| 6 | High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol, |
| 7 | that is optimized for die-level interconnect between an Application Processor |
| 8 | and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and |
| 9 | implemented by multiple vendors since then. |
| 10 | |
| 11 | The HSI interface supports full duplex communication over multiple channels |
| 12 | (typically 8) and is capable of reaching speeds up to 200 Mbit/s. |
| 13 | |
| 14 | The serial protocol uses two signals, DATA and FLAG as combined data and clock |
| 15 | signals and an additional READY signal for flow control. An additional WAKE |
| 16 | signal can be used to wakeup the chips from standby modes. The signals are |
| 17 | commonly prefixed by AC for signals going from the application die to the |
| 18 | cellular die and CA for signals going the other way around. |
| 19 | |
| 20 | +------------+ +---------------+ |
| 21 | | Cellular | | Application | |
| 22 | | Die | | Die | |
| 23 | | | - - - - - - CAWAKE - - - - - - >| | |
| 24 | | T|------------ CADATA ------------>|R | |
| 25 | | X|------------ CAFLAG ------------>|X | |
| 26 | | |<----------- ACREADY ------------| | |
| 27 | | | | | |
| 28 | | | | | |
| 29 | | |< - - - - - ACWAKE - - - - - - -| | |
| 30 | | R|<----------- ACDATA -------------|T | |
| 31 | | X|<----------- ACFLAG -------------|X | |
| 32 | | |------------ CAREADY ----------->| | |
| 33 | | | | | |
| 34 | | | | | |
| 35 | +------------+ +---------------+ |
| 36 | |
| 37 | 2. HSI Subsystem in Linux |
| 38 | ~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 39 | |
| 40 | In the Linux kernel the hsi subsystem is supposed to be used for HSI devices. |
| 41 | The hsi subsystem contains drivers for hsi controllers including support for |
| 42 | multi-port controllers and provides a generic API for using the HSI ports. |
| 43 | |
| 44 | It also contains HSI client drivers, which make use of the generic API to |
| 45 | implement a protocol used on the HSI interface. These client drivers can |
| 46 | use an arbitrary number of channels. |
| 47 | |
| 48 | 3. hsi-char Device |
| 49 | ~~~~~~~~~~~~~~~~~~ |
| 50 | |
| 51 | Each port automatically registers a generic client driver called hsi_char, |
| 52 | which provides a charecter device for userspace representing the HSI port. |
| 53 | It can be used to communicate via HSI from userspace. Userspace may |
| 54 | configure the hsi_char device using the following ioctl commands: |
| 55 | |
| 56 | * HSC_RESET: |
| 57 | - flush the HSI port |
| 58 | |
| 59 | * HSC_SET_PM |
| 60 | - enable or disable the client. |
| 61 | |
| 62 | * HSC_SEND_BREAK |
| 63 | - send break |
| 64 | |
| 65 | * HSC_SET_RX |
| 66 | - set RX configuration |
| 67 | |
| 68 | * HSC_GET_RX |
| 69 | - get RX configuration |
| 70 | |
| 71 | * HSC_SET_TX |
| 72 | - set TX configuration |
| 73 | |
| 74 | * HSC_GET_TX |
| 75 | - get TX configuration |