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-Serial Peripheral Interface (SPI)
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-=================================
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-
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-SPI is the "Serial Peripheral Interface", widely used with embedded
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-systems because it is a simple and efficient interface: basically a
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-multiplexed shift register. Its three signal wires hold a clock (SCK,
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-often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
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-line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
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-duplex protocol; for each bit shifted out the MOSI line (one per clock)
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-another is shifted in on the MISO line. Those bits are assembled into
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-words of various sizes on the way to and from system memory. An
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-additional chipselect line is usually active-low (nCS); four signals are
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-normally used for each peripheral, plus sometimes an interrupt.
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-
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-The SPI bus facilities listed here provide a generalized interface to
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-declare SPI busses and devices, manage them according to the standard
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-Linux driver model, and perform input/output operations. At this time,
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-only "master" side interfaces are supported, where Linux talks to SPI
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-peripherals and does not implement such a peripheral itself. (Interfaces
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-to support implementing SPI slaves would necessarily look different.)
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-
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-The programming interface is structured around two kinds of driver, and
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-two kinds of device. A "Controller Driver" abstracts the controller
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-hardware, which may be as simple as a set of GPIO pins or as complex as
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-a pair of FIFOs connected to dual DMA engines on the other side of the
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-SPI shift register (maximizing throughput). Such drivers bridge between
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-whatever bus they sit on (often the platform bus) and SPI, and expose
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-the SPI side of their device as a :c:type:`struct spi_master
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-<spi_master>`. SPI devices are children of that master,
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-represented as a :c:type:`struct spi_device <spi_device>` and
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-manufactured from :c:type:`struct spi_board_info
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-<spi_board_info>` descriptors which are usually provided by
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-board-specific initialization code. A :c:type:`struct spi_driver
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-<spi_driver>` is called a "Protocol Driver", and is bound to a
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-spi_device using normal driver model calls.
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-
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-The I/O model is a set of queued messages. Protocol drivers submit one
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-or more :c:type:`struct spi_message <spi_message>` objects,
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-which are processed and completed asynchronously. (There are synchronous
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-wrappers, however.) Messages are built from one or more
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-:c:type:`struct spi_transfer <spi_transfer>` objects, each of
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-which wraps a full duplex SPI transfer. A variety of protocol tweaking
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-options are needed, because different chips adopt very different
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-policies for how they use the bits transferred with SPI.
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-
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-.. kernel-doc:: include/linux/spi/spi.h
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- :internal:
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-
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-.. kernel-doc:: drivers/spi/spi.c
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- :functions: spi_register_board_info
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-
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-.. kernel-doc:: drivers/spi/spi.c
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- :export:
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-
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-I\ :sup:`2`\ C and SMBus Subsystem
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-==================================
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-
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-I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
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-the "Inter-IC" bus, a simple bus protocol which is widely used where low
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-data rate communications suffice. Since it's also a licensed trademark,
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-some vendors use another name (such as "Two-Wire Interface", TWI) for
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-the same bus. I2C only needs two signals (SCL for clock, SDA for data),
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-conserving board real estate and minimizing signal quality issues. Most
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-I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
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-there's a high speed extension (3.4 MHz) that's not yet found wide use.
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-I2C is a multi-master bus; open drain signaling is used to arbitrate
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-between masters, as well as to handshake and to synchronize clocks from
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-slower clients.
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-
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-The Linux I2C programming interfaces support only the master side of bus
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-interactions, not the slave side. The programming interface is
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-structured around two kinds of driver, and two kinds of device. An I2C
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-"Adapter Driver" abstracts the controller hardware; it binds to a
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-physical device (perhaps a PCI device or platform_device) and exposes a
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-:c:type:`struct i2c_adapter <i2c_adapter>` representing each
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-I2C bus segment it manages. On each I2C bus segment will be I2C devices
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-represented by a :c:type:`struct i2c_client <i2c_client>`.
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-Those devices will be bound to a :c:type:`struct i2c_driver
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-<i2c_driver>`, which should follow the standard Linux driver
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-model. (At this writing, a legacy model is more widely used.) There are
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-functions to perform various I2C protocol operations; at this writing
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-all such functions are usable only from task context.
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-
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-The System Management Bus (SMBus) is a sibling protocol. Most SMBus
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-systems are also I2C conformant. The electrical constraints are tighter
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-for SMBus, and it standardizes particular protocol messages and idioms.
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-Controllers that support I2C can also support most SMBus operations, but
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-SMBus controllers don't support all the protocol options that an I2C
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-controller will. There are functions to perform various SMBus protocol
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-operations, either using I2C primitives or by issuing SMBus commands to
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-i2c_adapter devices which don't support those I2C operations.
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-
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-.. kernel-doc:: include/linux/i2c.h
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- :internal:
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-
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-.. kernel-doc:: drivers/i2c/i2c-boardinfo.c
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- :functions: i2c_register_board_info
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-
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-.. kernel-doc:: drivers/i2c/i2c-core.c
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- :export:
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-
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-High Speed Synchronous Serial Interface (HSI)
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-=============================================
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-
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-1. Introduction
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----------------
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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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-
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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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-
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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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-::
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-
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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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-
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-2. HSI Subsystem in Linux
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--------------------------
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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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-
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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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-
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-3. hsi-char Device
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-------------------
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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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-
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-HSC_RESET
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- flush the HSI port
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-
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-HSC_SET_PM
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- enable or disable the client.
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-
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-HSC_SEND_BREAK
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- send break
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-
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-HSC_SET_RX
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- set RX configuration
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-
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-HSC_GET_RX
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- get RX configuration
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-
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-HSC_SET_TX
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- set TX configuration
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-
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-HSC_GET_TX
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- get TX configuration
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-
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-The kernel HSI API
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-------------------
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-
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-.. kernel-doc:: include/linux/hsi/hsi.h
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- :internal:
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-
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-.. kernel-doc:: drivers/hsi/hsi_core.c
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- :export:
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-
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Jonathan Corbet