docs: split up serial-interfaces.rst

It never made sense to keep these documents together; move each into its
own file.

Drop the section numbering on hsi.txt on its way to its own file.
Suggested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/serial-interfaces.rst → Documentation/driver-api/hsi.rst

-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_master>`. SPI devices are children of that master,
-represented as a :c:type:`struct spi_device <spi_device>` and
-manufactured from :c:type:`struct spi_board_info
-<spi_board_info>` descriptors which are usually provided by
-board-specific initialization code. A :c:type:`struct spi_driver
-<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 <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 <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 <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 <i2c_client>`.
-Those devices will be bound to a :c:type:`struct i2c_driver
-<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
+Introduction
 ---------------
 
 High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol,
@@ -138,7 +37,7 @@ cellular die and CA for signals going the other way around.
     |            |                                 |               |
     +------------+                                 +---------------+
 
-2. HSI Subsystem in Linux
+HSI Subsystem in Linux
 -------------------------
 
 In the Linux kernel the hsi subsystem is supposed to be used for HSI devices.
@@ -149,7 +48,7 @@ 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
+hsi-char Device
 ------------------
 
 Each port automatically registers a generic client driver called hsi_char,

Documentation/driver-api/i2c.rst

+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 <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 <i2c_client>`.
+Those devices will be bound to a :c:type:`struct i2c_driver
+<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:

Documentation/driver-api/index.rst

@@ -20,5 +20,7 @@ available subsections can be seen below.
    sound
    frame-buffer
    input
-   serial-interfaces
+   spi
+   i2c
+   hsi
    miscellaneous

Documentation/driver-api/spi.rst

+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_master>`. SPI devices are children of that master,
+represented as a :c:type:`struct spi_device <spi_device>` and
+manufactured from :c:type:`struct spi_board_info
+<spi_board_info>` descriptors which are usually provided by
+board-specific initialization code. A :c:type:`struct spi_driver
+<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 <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 <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: