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<book id="LinuxDriversAPI">
<title>Linux Device Drivers</title>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
See the GNU General Public License for more details.
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
For more details see the file COPYING in the source
distribution of Linux.
<chapter id="Basics">
<title>Driver Basics</title>
<sect1><title>Driver Entry and Exit points</title>
<sect1><title>Atomic and pointer manipulation</title>
<sect1><title>Delaying, scheduling, and timer routines</title>
<sect1><title>Wait queues and Wake events</title>
<sect1><title>High-resolution timers</title>
<sect1><title>Workqueues and Kevents</title>
<sect1><title>Internal Functions</title>
<sect1><title>Kernel objects manipulation</title>
<sect1><title>Kernel utility functions</title>
<sect1><title>Device Resource Management</title>
<chapter id="devdrivers">
<title>Device drivers infrastructure</title>
<sect1><title>The Basic Device Driver-Model Structures </title>
<sect1><title>Device Drivers Base</title>
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<sect1><title>Device Drivers DMA Management</title>
<sect1><title>Device Drivers Power Management</title>
<sect1><title>Device Drivers ACPI Support</title>
<!-- Internal functions only
<!-- No correct structured comments
<sect1><title>Device drivers PnP support</title>
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<sect1><title>Userspace IO devices</title>
<chapter id="parportdev">
<title>Parallel Port Devices</title>
<chapter id="message_devices">
<title>Message-based devices</title>
<sect1><title>Fusion message devices</title>
<chapter id="snddev">
<title>Sound Devices</title>
<!-- FIXME: Removed for now since no structured comments in source
<chapter id="mediadev">
<title>Media Devices</title>
<sect1><title>Video2Linux devices</title>
<sect1><title>Digital TV (DVB) devices</title>
<sect1><title>Digital TV Demux API</title>
<para>The kernel demux API defines a driver-internal interface for
registering low-level, hardware specific driver to a hardware
independent demux layer. It is only of interest for Digital TV
device driver writers. The header file for this API is named
<constant>demux.h</constant> and located in
<para>The demux API should be implemented for each demux in the
system. It is used to select the TS source of a demux and to manage
the demux resources. When the demux client allocates a resource via
the demux API, it receives a pointer to the API of that
<para>Each demux receives its TS input from a DVB front-end or from
memory, as set via this demux API. In a system with more than one
front-end, the API can be used to select one of the DVB front-ends
as a TS source for a demux, unless this is fixed in the HW platform.
The demux API only controls front-ends regarding to their connections
with demuxes; the APIs used to set the other front-end parameters,
such as tuning, are not defined in this document.</para>
<para>The functions that implement the abstract interface demux should
be defined static or module private and registered to the Demux
core for external access. It is not necessary to implement every
function in the struct <constant>dmx_demux</constant>. For example,
a demux interface might support Section filtering, but not PES
filtering. The API client is expected to check the value of any
function pointer before calling the function: the value of NULL means
that the &#8220;function is not available&#8221;.</para>
<para>Whenever the functions of the demux API modify shared data,
the possibilities of lost update and race condition problems should
be addressed, e.g. by protecting parts of code with mutexes.</para>
<para>Note that functions called from a bottom half context must not
sleep. Even a simple memory allocation without using GFP_ATOMIC can
result in a kernel thread being put to sleep if swapping is needed.
For example, the Linux kernel calls the functions of a network device
interface from a bottom half context. Thus, if a demux API function
is called from network device code, the function must not sleep.
<section id="demux_callback_api">
<title>Demux Callback API</title>
<para>This kernel-space API comprises the callback functions that
deliver filtered data to the demux client. Unlike the other DVB
kABIs, these functions are provided by the client and called from
the demux code.</para>
<para>The function pointers of this abstract interface are not
packed into a structure as in the other demux APIs, because the
callback functions are registered and used independent of each
other. As an example, it is possible for the API client to provide
several callback functions for receiving TS packets and no
callbacks for PES packets or sections.</para>
<para>The functions that implement the callback API need not be
re-entrant: when a demux driver calls one of these functions,
the driver is not allowed to call the function again before
the original call returns. If a callback is triggered by a
hardware interrupt, it is recommended to use the Linux
&#8220;bottom half&#8221; mechanism or start a tasklet instead of
making the callback function call directly from a hardware
<para>This mechanism is implemented by
<link linkend='API-dmx-ts-cb'>dmx_ts_cb()</link> and
<link linkend='API-dmx-section-cb'>dmx_section_cb()</link>.</para>
<sect1><title>Remote Controller devices</title>
<sect1><title>Media Controller devices</title>
<chapter id="uart16x50">
<title>16x50 UART Driver</title>
<chapter id="fbdev">
<title>Frame Buffer Library</title>
The frame buffer drivers depend heavily on four data structures.
These structures are declared in include/linux/fb.h. They are
fb_info, fb_var_screeninfo, fb_fix_screeninfo and fb_monospecs.
The last three can be made available to and from userland.
fb_info defines the current state of a particular video card.
Inside fb_info, there exists a fb_ops structure which is a
collection of needed functions to make fbdev and fbcon work.
fb_info is only visible to the kernel.
fb_var_screeninfo is used to describe the features of a video card
that are user defined. With fb_var_screeninfo, things such as
depth and the resolution may be defined.
The next structure is fb_fix_screeninfo. This defines the
properties of a card that are created when a mode is set and can't
be changed otherwise. A good example of this is the start of the
frame buffer memory. This "locks" the address of the frame buffer
memory, so that it cannot be changed or moved.
The last structure is fb_monospecs. In the old API, there was
little importance for fb_monospecs. This allowed for forbidden things
such as setting a mode of 800x600 on a fix frequency monitor. With
the new API, fb_monospecs prevents such things, and if used
correctly, can prevent a monitor from being cooked. fb_monospecs
will not be useful until kernels 2.5.x.
<sect1><title>Frame Buffer Memory</title>
<sect1><title>Frame Buffer Console</title>
<sect1><title>Frame Buffer Colormap</title>
<!-- FIXME:
drivers/video/fbgen.c has no docs, which stuffs up the sgml. Comment
out until somebody adds docs. KAO
<sect1><title>Frame Buffer Generic Functions</title>
KAO -->
<sect1><title>Frame Buffer Video Mode Database</title>
<sect1><title>Frame Buffer Macintosh Video Mode Database</title>
<sect1><title>Frame Buffer Fonts</title>
Refer to the file lib/fonts/fonts.c for more information.
<!-- FIXME: Removed for now since no structured comments in source
<chapter id="input_subsystem">
<title>Input Subsystem</title>
<sect1><title>Input core</title>
<sect1><title>Multitouch Library</title>
<sect1><title>Polled input devices</title>
<sect1><title>Matrix keyboars/keypads</title>
<sect1><title>Sparse keymap support</title>
<chapter id="spi">
<title>Serial Peripheral Interface (SPI)</title>
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
<structname>struct spi_master</structname>.
SPI devices are children of that master, represented as a
<structname>struct spi_device</structname> and manufactured from
<structname>struct spi_board_info</structname> descriptors which
are usually provided by board-specific initialization code.
A <structname>struct spi_driver</structname> 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 <structname>struct spi_message</structname>
objects, which are processed and completed asynchronously.
(There are synchronous wrappers, however.) Messages are
built from one or more <structname>struct spi_transfer</structname>
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.
!Fdrivers/spi/spi.c spi_register_board_info
<chapter id="i2c">
<title>I<superscript>2</superscript>C and SMBus Subsystem</title>
I<superscript>2</superscript>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 <structname>struct i2c_adapter</structname> representing
each I2C bus segment it manages.
On each I2C bus segment will be I2C devices represented by a
<structname>struct i2c_client</structname>. Those devices will
be bound to a <structname>struct i2c_driver</structname>,
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.
!Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info
<chapter id="hsi">
<title>High Speed Synchronous Serial Interface (HSI)</title>
High Speed Synchronous Serial Interface (HSI) is a
serial interface mainly used for connecting application
engines (APE) with cellular modem engines (CMT) in cellular
HSI provides multiplexing for up to 16 logical channels,
low-latency and full duplex communication.
<chapter id="pwm">
<title>Pulse-Width Modulation (PWM)</title>
Pulse-width modulation is a modulation technique primarily used to
control power supplied to electrical devices.
The PWM framework provides an abstraction for providers and consumers
of PWM signals. A controller that provides one or more PWM signals is
registered as <structname>struct pwm_chip</structname>. Providers are
expected to embed this structure in a driver-specific structure. This
structure contains fields that describe a particular chip.
A chip exposes one or more PWM signal sources, each of which exposed
as a <structname>struct pwm_device</structname>. Operations can be
performed on PWM devices to control the period, duty cycle, polarity
and active state of the signal.
Note that PWM devices are exclusive resources: they can always only be
used by one consumer at a time.