Serial Peripheral Interface (SPI) ================================= SPI is the "Serial Peripheral Interface", widely used with embedded systems because it is a simple and efficient interface: basically a multiplexed shift register. Its three signal wires hold a clock (SCK, often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data line, and a "Master In, Slave Out" (MISO) data line. SPI is a full duplex protocol; for each bit shifted out the MOSI line (one per clock) another is shifted in on the MISO line. Those bits are assembled into words of various sizes on the way to and from system memory. An additional chipselect line is usually active-low (nCS); four signals are normally used for each peripheral, plus sometimes an interrupt. The SPI bus facilities listed here provide a generalized interface to declare SPI busses and devices, manage them according to the standard Linux driver model, and perform input/output operations. At this time, only "master" side interfaces are supported, where Linux talks to SPI peripherals and does not implement such a peripheral itself. (Interfaces to support implementing SPI slaves would necessarily look different.) The programming interface is structured around two kinds of driver, and two kinds of device. A "Controller Driver" abstracts the controller hardware, which may be as simple as a set of GPIO pins or as complex as a pair of FIFOs connected to dual DMA engines on the other side of the SPI shift register (maximizing throughput). Such drivers bridge between whatever bus they sit on (often the platform bus) and SPI, and expose the SPI side of their device as a :c:type:`struct spi_master `. SPI devices are children of that master, represented as a :c:type:`struct spi_device ` and manufactured from :c:type:`struct spi_board_info ` descriptors which are usually provided by board-specific initialization code. A :c:type:`struct spi_driver ` is called a "Protocol Driver", and is bound to a spi_device using normal driver model calls. The I/O model is a set of queued messages. Protocol drivers submit one or more :c:type:`struct spi_message ` objects, which are processed and completed asynchronously. (There are synchronous wrappers, however.) Messages are built from one or more :c:type:`struct spi_transfer ` objects, each of which wraps a full duplex SPI transfer. A variety of protocol tweaking options are needed, because different chips adopt very different policies for how they use the bits transferred with SPI. .. kernel-doc:: include/linux/spi/spi.h :internal: .. kernel-doc:: drivers/spi/spi.c :functions: spi_register_board_info .. kernel-doc:: drivers/spi/spi.c :export: I\ :sup:`2`\ C and SMBus Subsystem ================================== I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for the "Inter-IC" bus, a simple bus protocol which is widely used where low data rate communications suffice. Since it's also a licensed trademark, some vendors use another name (such as "Two-Wire Interface", TWI) for the same bus. I2C only needs two signals (SCL for clock, SDA for data), conserving board real estate and minimizing signal quality issues. Most I2C devices use seven bit addresses, and bus speeds of up to 400 kHz; there's a high speed extension (3.4 MHz) that's not yet found wide use. I2C is a multi-master bus; open drain signaling is used to arbitrate between masters, as well as to handshake and to synchronize clocks from slower clients. The Linux I2C programming interfaces support only the master side of bus interactions, not the slave side. The programming interface is structured around two kinds of driver, and two kinds of device. An I2C "Adapter Driver" abstracts the controller hardware; it binds to a physical device (perhaps a PCI device or platform_device) and exposes a :c:type:`struct i2c_adapter ` representing each I2C bus segment it manages. On each I2C bus segment will be I2C devices represented by a :c:type:`struct i2c_client `. Those devices will be bound to a :c:type:`struct i2c_driver `, which should follow the standard Linux driver model. (At this writing, a legacy model is more widely used.) There are functions to perform various I2C protocol operations; at this writing all such functions are usable only from task context. The System Management Bus (SMBus) is a sibling protocol. Most SMBus systems are also I2C conformant. The electrical constraints are tighter for SMBus, and it standardizes particular protocol messages and idioms. Controllers that support I2C can also support most SMBus operations, but SMBus controllers don't support all the protocol options that an I2C controller will. There are functions to perform various SMBus protocol operations, either using I2C primitives or by issuing SMBus commands to i2c_adapter devices which don't support those I2C operations. .. kernel-doc:: include/linux/i2c.h :internal: .. kernel-doc:: drivers/i2c/i2c-boardinfo.c :functions: i2c_register_board_info .. kernel-doc:: drivers/i2c/i2c-core.c :export: High Speed Synchronous Serial Interface (HSI) ============================================= 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 The kernel HSI API ------------------ .. kernel-doc:: include/linux/hsi/hsi.h :internal: .. kernel-doc:: drivers/hsi/hsi_core.c :export: