Documentation/DocBook/kernel-api.tmpl - linux-mtk - Git at Google


            
            <?xml version="1.0" encoding="UTF-8"?>

            
            <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"

            
            	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>


            <book id="LinuxKernelAPI">

            
             <bookinfo>

            
              <title>The Linux Kernel API</title>


              <legalnotice>

            
               <para>

            
                 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

            
                 version.

            
               </para>


               <para>

            
                 This program is distributed in the hope that it will be

            
                 useful, but WITHOUT ANY WARRANTY; without even the implied

            
                 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

            
                 See the GNU General Public License for more details.

            
               </para>


               <para>

            
                 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

            
               </para>


               <para>

            
                 For more details see the file COPYING in the source

            
                 distribution of Linux.

            
               </para>

            
              </legalnotice>

            
             </bookinfo>


            <toc></toc>


              <chapter id="Basics">

            
                 <title>Driver Basics</title>

            
                 <sect1><title>Driver Entry and Exit points</title>

            
            !Iinclude/linux/init.h

            
                 </sect1>


                 <sect1><title>Atomic and pointer manipulation</title>

            
            !Iarch/x86/include/asm/atomic_32.h

            
            !Iarch/x86/include/asm/unaligned.h

            
                 </sect1>


                 <sect1><title>Delaying, scheduling, and timer routines</title>

            
            !Iinclude/linux/sched.h

            
            !Ekernel/sched.c

            
            !Ekernel/timer.c

            
                 </sect1>

            
                 <sect1><title>High-resolution timers</title>

            
            !Iinclude/linux/ktime.h

            
            !Iinclude/linux/hrtimer.h

            
            !Ekernel/hrtimer.c

            
                 </sect1>

            
                 <sect1><title>Workqueues and Kevents</title>

            
            !Ekernel/workqueue.c

            
                 </sect1>

            
                 <sect1><title>Internal Functions</title>

            
            !Ikernel/exit.c

            
            !Ikernel/signal.c

            
            !Iinclude/linux/kthread.h

            
            !Ekernel/kthread.c

            
                 </sect1>


                 <sect1><title>Kernel objects manipulation</title>

            
            <!--

            
            X!Iinclude/linux/kobject.h

            
            -->

            
            !Elib/kobject.c

            
                 </sect1>


                 <sect1><title>Kernel utility functions</title>

            
            !Iinclude/linux/kernel.h

            
            !Ekernel/printk.c

            
            !Ekernel/panic.c

            
            !Ekernel/sys.c

            
            !Ekernel/rcupdate.c

            
                 </sect1>


                 <sect1><title>Device Resource Management</title>

            
            !Edrivers/base/devres.c

            
                 </sect1>


              </chapter>


              <chapter id="adt">

            
                 <title>Data Types</title>

            
                 <sect1><title>Doubly Linked Lists</title>

            
            !Iinclude/linux/list.h

            
                 </sect1>

            
              </chapter>


              <chapter id="libc">

            
                 <title>Basic C Library Functions</title>


                 <para>

            
                   When writing drivers, you cannot in general use routines which are

            
                   from the C Library.  Some of the functions have been found generally

            
                   useful and they are listed below.  The behaviour of these functions

            
                   may vary slightly from those defined by ANSI, and these deviations

            
                   are noted in the text.

            
                 </para>


                 <sect1><title>String Conversions</title>

            
            !Ilib/vsprintf.c

            
            !Elib/vsprintf.c

            
                 </sect1>

            
                 <sect1><title>String Manipulation</title>

            
            <!-- All functions are exported at now

            
            X!Ilib/string.c

            
             -->

            
            !Elib/string.c

            
                 </sect1>

            
                 <sect1><title>Bit Operations</title>

            
            !Iarch/x86/include/asm/bitops.h

            
                 </sect1>

            
              </chapter>


              <chapter id="kernel-lib">

            
                 <title>Basic Kernel Library Functions</title>


                 <para>

            
                   The Linux kernel provides more basic utility functions.

            
                 </para>


                 <sect1><title>Bitmap Operations</title>

            
            !Elib/bitmap.c

            
            !Ilib/bitmap.c

            
                 </sect1>


                 <sect1><title>Command-line Parsing</title>

            
            !Elib/cmdline.c

            
                 </sect1>


                 <sect1 id="crc"><title>CRC Functions</title>

            
            !Elib/crc7.c

            
            !Elib/crc16.c

            
            !Elib/crc-itu-t.c

            
            !Elib/crc32.c

            
            !Elib/crc-ccitt.c

            
                 </sect1>

            
              </chapter>


              <chapter id="mm">

            
                 <title>Memory Management in Linux</title>

            
                 <sect1><title>The Slab Cache</title>

            
            !Iinclude/linux/slab.h

            
            !Emm/slab.c

            
                 </sect1>

            
                 <sect1><title>User Space Memory Access</title>

            
            !Iarch/x86/include/asm/uaccess_32.h

            
            !Earch/x86/lib/usercopy_32.c

            
                 </sect1>

            
                 <sect1><title>More Memory Management Functions</title>

            
            !Emm/readahead.c

            
            !Emm/filemap.c

            
            !Emm/memory.c

            
            !Emm/vmalloc.c

            
            !Imm/page_alloc.c

            
            !Emm/mempool.c

            
            !Emm/dmapool.c

            
            !Emm/page-writeback.c

            
            !Emm/truncate.c

            
                 </sect1>

            
              </chapter>


              <chapter id="ipc">

            
                 <title>Kernel IPC facilities</title>


                 <sect1><title>IPC utilities</title>

            
            !Iipc/util.c

            
                 </sect1>

            
              </chapter>


              <chapter id="kfifo">

            
                 <title>FIFO Buffer</title>

            
                 <sect1><title>kfifo interface</title>

            
            !Iinclude/linux/kfifo.h

            
            !Ekernel/kfifo.c

            
                 </sect1>

            
              </chapter>


              <chapter id="relayfs">

            
                 <title>relay interface support</title>


                 <para>

            
            	Relay interface support

            
            	is designed to provide an efficient mechanism for tools and

            
            	facilities to relay large amounts of data from kernel space to

            
            	user space.

            
                 </para>


                 <sect1><title>relay interface</title>

            
            !Ekernel/relay.c

            
            !Ikernel/relay.c

            
                 </sect1>

            
              </chapter>


              <chapter id="modload">

            
                 <title>Module Support</title>

            
                 <sect1><title>Module Loading</title>

            
            !Ekernel/kmod.c

            
                 </sect1>

            
                 <sect1><title>Inter Module support</title>

            
                    <para>

            
                       Refer to the file kernel/module.c for more information.

            
                    </para>

            
            <!-- FIXME: Removed for now since no structured comments in source

            
            X!Ekernel/module.c

            
            -->

            
                 </sect1>

            
              </chapter>


              <chapter id="hardware">

            
                 <title>Hardware Interfaces</title>

            
                 <sect1><title>Interrupt Handling</title>

            
            !Ekernel/irq/manage.c

            
                 </sect1>


                 <sect1><title>DMA Channels</title>

            
            !Ekernel/dma.c

            
                 </sect1>


                 <sect1><title>Resources Management</title>

            
            !Ikernel/resource.c

            
            !Ekernel/resource.c

            
                 </sect1>


                 <sect1><title>MTRR Handling</title>

            
            !Earch/x86/kernel/cpu/mtrr/main.c

            
                 </sect1>


                 <sect1><title>PCI Support Library</title>

            
            !Edrivers/pci/pci.c

            
            !Edrivers/pci/pci-driver.c

            
            !Edrivers/pci/remove.c

            
            !Edrivers/pci/pci-acpi.c

            
            !Edrivers/pci/search.c

            
            !Edrivers/pci/msi.c

            
            !Edrivers/pci/bus.c

            
            <!-- FIXME: Removed for now since no structured comments in source

            
            X!Edrivers/pci/hotplug.c

            
            -->

            
            !Edrivers/pci/probe.c

            
            !Edrivers/pci/rom.c

            
                 </sect1>

            
                 <sect1><title>PCI Hotplug Support Library</title>

            
            !Edrivers/pci/hotplug/pci_hotplug_core.c

            
                 </sect1>

            
                 <sect1><title>MCA Architecture</title>

            
            	<sect2><title>MCA Device Functions</title>

            
                       <para>

            
                          Refer to the file arch/x86/kernel/mca_32.c for more information.

            
                       </para>

            
            <!-- FIXME: Removed for now since no structured comments in source

            
            X!Earch/x86/kernel/mca_32.c

            
            -->

            
            	</sect2>

            
            	<sect2><title>MCA Bus DMA</title>

            
            !Iarch/x86/include/asm/mca_dma.h

            
            	</sect2>

            
                 </sect1>

            
              </chapter>


              <chapter id="firmware">

            
                 <title>Firmware Interfaces</title>

            
                 <sect1><title>DMI Interfaces</title>

            
            !Edrivers/firmware/dmi_scan.c

            
                 </sect1>

            
                 <sect1><title>EDD Interfaces</title>

            
            !Idrivers/firmware/edd.c

            
                 </sect1>

            
              </chapter>


              <chapter id="security">

            
                 <title>Security Framework</title>

            
            !Isecurity/security.c

            
            !Esecurity/inode.c

            
              </chapter>


              <chapter id="audit">

            
                 <title>Audit Interfaces</title>

            
            !Ekernel/audit.c

            
            !Ikernel/auditsc.c

            
            !Ikernel/auditfilter.c

            
              </chapter>


              <chapter id="accounting">

            
                 <title>Accounting Framework</title>

            
            !Ikernel/acct.c

            
              </chapter>


              <chapter id="devdrivers">

            
                 <title>Device drivers infrastructure</title>

            
                 <sect1><title>Device Drivers Base</title>

            
            <!--

            
            X!Iinclude/linux/device.h

            
            -->

            
            !Edrivers/base/driver.c

            
            !Edrivers/base/core.c

            
            !Edrivers/base/class.c

            
            !Edrivers/base/firmware_class.c

            
            !Edrivers/base/transport_class.c

            
            <!-- Cannot be included, because

            
                 attribute_container_add_class_device_adapter

            
             and attribute_container_classdev_to_container

            
                 exceed allowed 44 characters maximum

            
            X!Edrivers/base/attribute_container.c

            
            -->

            
            !Edrivers/base/sys.c

            
            <!--

            
            X!Edrivers/base/interface.c

            
            -->

            
            !Edrivers/base/platform.c

            
            !Edrivers/base/bus.c

            
                 </sect1>

            
                 <sect1><title>Device Drivers Power Management</title>

            
            !Edrivers/base/power/main.c

            
                 </sect1>

            
                 <sect1><title>Device Drivers ACPI Support</title>

            
            <!-- Internal functions only

            
            X!Edrivers/acpi/sleep/main.c

            
            X!Edrivers/acpi/sleep/wakeup.c

            
            X!Edrivers/acpi/motherboard.c

            
            X!Edrivers/acpi/bus.c

            
            -->

            
            !Edrivers/acpi/scan.c

            
            !Idrivers/acpi/scan.c

            
            <!-- No correct structured comments

            
            X!Edrivers/acpi/pci_bind.c

            
            -->

            
                 </sect1>

            
                 <sect1><title>Device drivers PnP support</title>

            
            !Idrivers/pnp/core.c

            
            <!-- No correct structured comments

            
            X!Edrivers/pnp/system.c

            
             -->

            
            !Edrivers/pnp/card.c

            
            !Idrivers/pnp/driver.c

            
            !Edrivers/pnp/manager.c

            
            !Edrivers/pnp/support.c

            
                 </sect1>

            
                 <sect1><title>Userspace IO devices</title>

            
            !Edrivers/uio/uio.c

            
            !Iinclude/linux/uio_driver.h

            
                 </sect1>

            
              </chapter>


              <chapter id="blkdev">

            
                 <title>Block Devices</title>

            
            !Eblock/blk-core.c

            
            !Iblock/blk-core.c

            
            !Eblock/blk-map.c

            
            !Iblock/blk-sysfs.c

            
            !Eblock/blk-settings.c

            
            !Eblock/blk-exec.c

            
            !Eblock/blk-barrier.c

            
            !Eblock/blk-tag.c

            
            !Iblock/blk-tag.c

            
            !Eblock/blk-integrity.c

            
            !Iblock/blktrace.c

            
            !Iblock/genhd.c

            
            !Eblock/genhd.c

            
              </chapter>


              <chapter id="chrdev">

            
            	<title>Char devices</title>

            
            !Efs/char_dev.c

            
              </chapter>


              <chapter id="miscdev">

            
                 <title>Miscellaneous Devices</title>

            
            !Edrivers/char/misc.c

            
              </chapter>


              <chapter id="parportdev">

            
                 <title>Parallel Port Devices</title>

            
            !Iinclude/linux/parport.h

            
            !Edrivers/parport/ieee1284.c

            
            !Edrivers/parport/share.c

            
            !Idrivers/parport/daisy.c

            
              </chapter>


              <chapter id="message_devices">

            
            	<title>Message-based devices</title>

            
                 <sect1><title>Fusion message devices</title>

            
            !Edrivers/message/fusion/mptbase.c

            
            !Idrivers/message/fusion/mptbase.c

            
            !Edrivers/message/fusion/mptscsih.c

            
            !Idrivers/message/fusion/mptscsih.c

            
            !Idrivers/message/fusion/mptctl.c

            
            !Idrivers/message/fusion/mptspi.c

            
            !Idrivers/message/fusion/mptfc.c

            
            !Idrivers/message/fusion/mptlan.c

            
                 </sect1>

            
                 <sect1><title>I2O message devices</title>

            
            !Iinclude/linux/i2o.h

            
            !Idrivers/message/i2o/core.h

            
            !Edrivers/message/i2o/iop.c

            
            !Idrivers/message/i2o/iop.c

            
            !Idrivers/message/i2o/config-osm.c

            
            !Edrivers/message/i2o/exec-osm.c

            
            !Idrivers/message/i2o/exec-osm.c

            
            !Idrivers/message/i2o/bus-osm.c

            
            !Edrivers/message/i2o/device.c

            
            !Idrivers/message/i2o/device.c

            
            !Idrivers/message/i2o/driver.c

            
            !Idrivers/message/i2o/pci.c

            
            !Idrivers/message/i2o/i2o_block.c

            
            !Idrivers/message/i2o/i2o_scsi.c

            
            !Idrivers/message/i2o/i2o_proc.c

            
                 </sect1>

            
              </chapter>


              <chapter id="snddev">

            
                 <title>Sound Devices</title>

            
            !Iinclude/sound/core.h

            
            !Esound/sound_core.c

            
            !Iinclude/sound/pcm.h

            
            !Esound/core/pcm.c

            
            !Esound/core/device.c

            
            !Esound/core/info.c

            
            !Esound/core/rawmidi.c

            
            !Esound/core/sound.c

            
            !Esound/core/memory.c

            
            !Esound/core/pcm_memory.c

            
            !Esound/core/init.c

            
            !Esound/core/isadma.c

            
            !Esound/core/control.c

            
            !Esound/core/pcm_lib.c

            
            !Esound/core/hwdep.c

            
            !Esound/core/pcm_native.c

            
            !Esound/core/memalloc.c

            
            <!-- FIXME: Removed for now since no structured comments in source

            
            X!Isound/sound_firmware.c

            
            -->

            
              </chapter>


              <chapter id="uart16x50">

            
                 <title>16x50 UART Driver</title>

            
            !Iinclude/linux/serial_core.h

            
            !Edrivers/serial/serial_core.c

            
            !Edrivers/serial/8250.c

            
              </chapter>


              <chapter id="fbdev">

            
                 <title>Frame Buffer Library</title>


                 <para>

            
                   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.

            
                 </para>


                 <para>

            
                   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.

            
                 </para>


                 <para>

            
                   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.

            
                 </para>


                 <para>

            
                   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.

            
                 </para>


                 <para>

            
                   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.

            
                 </para>


                 <sect1><title>Frame Buffer Memory</title>

            
            !Edrivers/video/fbmem.c

            
                 </sect1>

            
            <!--

            
                 <sect1><title>Frame Buffer Console</title>

            
            X!Edrivers/video/console/fbcon.c

            
                 </sect1>

            
            -->

            
                 <sect1><title>Frame Buffer Colormap</title>

            
            !Edrivers/video/fbcmap.c

            
                 </sect1>

            
            <!-- 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>

            
            X!Idrivers/video/fbgen.c

            
                 </sect1>

            
            KAO -->

            
                 <sect1><title>Frame Buffer Video Mode Database</title>

            
            !Idrivers/video/modedb.c

            
            !Edrivers/video/modedb.c

            
                 </sect1>

            
                 <sect1><title>Frame Buffer Macintosh Video Mode Database</title>

            
            !Edrivers/video/macmodes.c

            
                 </sect1>

            
                 <sect1><title>Frame Buffer Fonts</title>

            
                    <para>

            
                       Refer to the file drivers/video/console/fonts.c for more information.

            
                    </para>

            
            <!-- FIXME: Removed for now since no structured comments in source

            
            X!Idrivers/video/console/fonts.c

            
            -->

            
                 </sect1>

            
              </chapter>


              <chapter id="input_subsystem">

            
                 <title>Input Subsystem</title>

            
            !Iinclude/linux/input.h

            
            !Edrivers/input/input.c

            
            !Edrivers/input/ff-core.c

            
            !Edrivers/input/ff-memless.c

            
              </chapter>


              <chapter id="spi">

            
                  <title>Serial Peripheral Interface (SPI)</title>

            
              <para>

            
            	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.

            
              </para>

            
              <para>

            
            	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.)

            
              </para>

            
              <para>

            
            	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.

            
              </para>

            
              <para>

            
            	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.

            
              </para>

            
            !Iinclude/linux/spi/spi.h

            
            !Fdrivers/spi/spi.c spi_register_board_info

            
            !Edrivers/spi/spi.c

            
              </chapter>


              <chapter id="i2c">

            
                 <title>I<superscript>2</superscript>C and SMBus Subsystem</title>


                 <para>

            
            	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.

            
                 </para>


                 <para>

            
            	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.

            
                 </para>


                 <para>

            
            	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.

            
                 </para>


            !Iinclude/linux/i2c.h

            
            !Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info

            
            !Edrivers/i2c/i2c-core.c

            
              </chapter>


              <chapter id="clk">

            
                 <title>Clock Framework</title>


                 <para>

            
            	The clock framework defines programming interfaces to support

            
            	software management of the system clock tree.

            
            	This framework is widely used with System-On-Chip (SOC) platforms

            
            	to support power management and various devices which may need

            
            	custom clock rates.

            
            	Note that these "clocks" don't relate to timekeeping or real

            
            	time clocks (RTCs), each of which have separate frameworks.

            
            	These <structname>struct clk</structname> instances may be used

            
            	to manage for example a 96 MHz signal that is used to shift bits

            
            	into and out of peripherals or busses, or otherwise trigger

            
            	synchronous state machine transitions in system hardware.

            
                 </para>


                 <para>

            
            	Power management is supported by explicit software clock gating:

            
            	unused clocks are disabled, so the system doesn't waste power

            
            	changing the state of transistors that aren't in active use.

            
            	On some systems this may be backed by hardware clock gating,

            
            	where clocks are gated without being disabled in software.

            
            	Sections of chips that are powered but not clocked may be able

            
            	to retain their last state.

            
            	This low power state is often called a <emphasis>retention

            
            	mode</emphasis>.

            
            	This mode still incurs leakage currents, especially with finer

            
            	circuit geometries, but for CMOS circuits power is mostly used

            
            	by clocked state changes.

            
                 </para>


                 <para>

            
            	Power-aware drivers only enable their clocks when the device

            
            	they manage is in active use.  Also, system sleep states often

            
            	differ according to which clock domains are active:  while a

            
            	"standby" state may allow wakeup from several active domains, a

            
            	"mem" (suspend-to-RAM) state may require a more wholesale shutdown

            
            	of clocks derived from higher speed PLLs and oscillators, limiting

            
            	the number of possible wakeup event sources.  A driver's suspend

            
            	method may need to be aware of system-specific clock constraints

            
            	on the target sleep state.

            
                 </para>


                 <para>

            
                    Some platforms support programmable clock generators.  These

            
            	can be used by external chips of various kinds, such as other

            
            	CPUs, multimedia codecs, and devices with strict requirements

            
            	for interface clocking.

            
                 </para>


            !Iinclude/linux/clk.h

            
              </chapter>


            </book>
	<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

	<book id="LinuxKernelAPI">
	<bookinfo>
	<title>The Linux Kernel API</title>

	<legalnotice>
	<para>
	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
	version.
	</para>

	<para>
	This program is distributed in the hope that it will be
	useful, but WITHOUT ANY WARRANTY; without even the implied
	warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
	See the GNU General Public License for more details.
	</para>

	<para>
	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
	</para>

	<para>
	For more details see the file COPYING in the source
	distribution of Linux.
	</para>
	</legalnotice>
	</bookinfo>

	<toc></toc>

	<chapter id="Basics">
	<title>Driver Basics</title>
	<sect1><title>Driver Entry and Exit points</title>
	!Iinclude/linux/init.h
	</sect1>

	<sect1><title>Atomic and pointer manipulation</title>
	!Iarch/x86/include/asm/atomic_32.h
	!Iarch/x86/include/asm/unaligned.h
	</sect1>

	<sect1><title>Delaying, scheduling, and timer routines</title>
	!Iinclude/linux/sched.h
	!Ekernel/sched.c
	!Ekernel/timer.c
	</sect1>
	<sect1><title>High-resolution timers</title>
	!Iinclude/linux/ktime.h
	!Iinclude/linux/hrtimer.h
	!Ekernel/hrtimer.c
	</sect1>
	<sect1><title>Workqueues and Kevents</title>
	!Ekernel/workqueue.c
	</sect1>
	<sect1><title>Internal Functions</title>
	!Ikernel/exit.c
	!Ikernel/signal.c
	!Iinclude/linux/kthread.h
	!Ekernel/kthread.c
	</sect1>

	<sect1><title>Kernel objects manipulation</title>
	<!--
	X!Iinclude/linux/kobject.h
	-->
	!Elib/kobject.c
	</sect1>

	<sect1><title>Kernel utility functions</title>
	!Iinclude/linux/kernel.h
	!Ekernel/printk.c
	!Ekernel/panic.c
	!Ekernel/sys.c
	!Ekernel/rcupdate.c
	</sect1>

	<sect1><title>Device Resource Management</title>
	!Edrivers/base/devres.c
	</sect1>

	</chapter>

	<chapter id="adt">
	<title>Data Types</title>
	<sect1><title>Doubly Linked Lists</title>
	!Iinclude/linux/list.h
	</sect1>
	</chapter>

	<chapter id="libc">
	<title>Basic C Library Functions</title>

	<para>
	When writing drivers, you cannot in general use routines which are
	from the C Library. Some of the functions have been found generally
	useful and they are listed below. The behaviour of these functions
	may vary slightly from those defined by ANSI, and these deviations
	are noted in the text.
	</para>

	<sect1><title>String Conversions</title>
	!Ilib/vsprintf.c
	!Elib/vsprintf.c
	</sect1>
	<sect1><title>String Manipulation</title>
	<!-- All functions are exported at now
	X!Ilib/string.c
	-->
	!Elib/string.c
	</sect1>
	<sect1><title>Bit Operations</title>
	!Iarch/x86/include/asm/bitops.h
	</sect1>
	</chapter>

	<chapter id="kernel-lib">
	<title>Basic Kernel Library Functions</title>

	<para>
	The Linux kernel provides more basic utility functions.
	</para>

	<sect1><title>Bitmap Operations</title>
	!Elib/bitmap.c
	!Ilib/bitmap.c
	</sect1>

	<sect1><title>Command-line Parsing</title>
	!Elib/cmdline.c
	</sect1>

	<sect1 id="crc"><title>CRC Functions</title>
	!Elib/crc7.c
	!Elib/crc16.c
	!Elib/crc-itu-t.c
	!Elib/crc32.c
	!Elib/crc-ccitt.c
	</sect1>
	</chapter>

	<chapter id="mm">
	<title>Memory Management in Linux</title>
	<sect1><title>The Slab Cache</title>
	!Iinclude/linux/slab.h
	!Emm/slab.c
	</sect1>
	<sect1><title>User Space Memory Access</title>
	!Iarch/x86/include/asm/uaccess_32.h
	!Earch/x86/lib/usercopy_32.c
	</sect1>
	<sect1><title>More Memory Management Functions</title>
	!Emm/readahead.c
	!Emm/filemap.c
	!Emm/memory.c
	!Emm/vmalloc.c
	!Imm/page_alloc.c
	!Emm/mempool.c
	!Emm/dmapool.c
	!Emm/page-writeback.c
	!Emm/truncate.c
	</sect1>
	</chapter>


	<chapter id="ipc">
	<title>Kernel IPC facilities</title>

	<sect1><title>IPC utilities</title>
	!Iipc/util.c
	</sect1>
	</chapter>

	<chapter id="kfifo">
	<title>FIFO Buffer</title>
	<sect1><title>kfifo interface</title>
	!Iinclude/linux/kfifo.h
	!Ekernel/kfifo.c
	</sect1>
	</chapter>

	<chapter id="relayfs">
	<title>relay interface support</title>

	<para>
	Relay interface support
	is designed to provide an efficient mechanism for tools and
	facilities to relay large amounts of data from kernel space to
	user space.
	</para>

	<sect1><title>relay interface</title>
	!Ekernel/relay.c
	!Ikernel/relay.c
	</sect1>
	</chapter>

	<chapter id="modload">
	<title>Module Support</title>
	<sect1><title>Module Loading</title>
	!Ekernel/kmod.c
	</sect1>
	<sect1><title>Inter Module support</title>
	<para>
	Refer to the file kernel/module.c for more information.
	</para>
	<!-- FIXME: Removed for now since no structured comments in source
	X!Ekernel/module.c
	-->
	</sect1>
	</chapter>

	<chapter id="hardware">
	<title>Hardware Interfaces</title>
	<sect1><title>Interrupt Handling</title>
	!Ekernel/irq/manage.c
	</sect1>

	<sect1><title>DMA Channels</title>
	!Ekernel/dma.c
	</sect1>

	<sect1><title>Resources Management</title>
	!Ikernel/resource.c
	!Ekernel/resource.c
	</sect1>

	<sect1><title>MTRR Handling</title>
	!Earch/x86/kernel/cpu/mtrr/main.c
	</sect1>

	<sect1><title>PCI Support Library</title>
	!Edrivers/pci/pci.c
	!Edrivers/pci/pci-driver.c
	!Edrivers/pci/remove.c
	!Edrivers/pci/pci-acpi.c
	!Edrivers/pci/search.c
	!Edrivers/pci/msi.c
	!Edrivers/pci/bus.c
	<!-- FIXME: Removed for now since no structured comments in source
	X!Edrivers/pci/hotplug.c
	-->
	!Edrivers/pci/probe.c
	!Edrivers/pci/rom.c
	</sect1>
	<sect1><title>PCI Hotplug Support Library</title>
	!Edrivers/pci/hotplug/pci_hotplug_core.c
	</sect1>
	<sect1><title>MCA Architecture</title>
	<sect2><title>MCA Device Functions</title>
	<para>
	Refer to the file arch/x86/kernel/mca_32.c for more information.
	</para>
	<!-- FIXME: Removed for now since no structured comments in source
	X!Earch/x86/kernel/mca_32.c
	-->
	</sect2>
	<sect2><title>MCA Bus DMA</title>
	!Iarch/x86/include/asm/mca_dma.h
	</sect2>
	</sect1>
	</chapter>

	<chapter id="firmware">
	<title>Firmware Interfaces</title>
	<sect1><title>DMI Interfaces</title>
	!Edrivers/firmware/dmi_scan.c
	</sect1>
	<sect1><title>EDD Interfaces</title>
	!Idrivers/firmware/edd.c
	</sect1>
	</chapter>

	<chapter id="security">
	<title>Security Framework</title>
	!Isecurity/security.c
	!Esecurity/inode.c
	</chapter>

	<chapter id="audit">
	<title>Audit Interfaces</title>
	!Ekernel/audit.c
	!Ikernel/auditsc.c
	!Ikernel/auditfilter.c
	</chapter>

	<chapter id="accounting">
	<title>Accounting Framework</title>
	!Ikernel/acct.c
	</chapter>

	<chapter id="devdrivers">
	<title>Device drivers infrastructure</title>
	<sect1><title>Device Drivers Base</title>
	<!--
	X!Iinclude/linux/device.h
	-->
	!Edrivers/base/driver.c
	!Edrivers/base/core.c
	!Edrivers/base/class.c
	!Edrivers/base/firmware_class.c
	!Edrivers/base/transport_class.c
	<!-- Cannot be included, because
	attribute_container_add_class_device_adapter
	and attribute_container_classdev_to_container
	exceed allowed 44 characters maximum
	X!Edrivers/base/attribute_container.c
	-->
	!Edrivers/base/sys.c
	<!--
	X!Edrivers/base/interface.c
	-->
	!Edrivers/base/platform.c
	!Edrivers/base/bus.c
	</sect1>
	<sect1><title>Device Drivers Power Management</title>
	!Edrivers/base/power/main.c
	</sect1>
	<sect1><title>Device Drivers ACPI Support</title>
	<!-- Internal functions only
	X!Edrivers/acpi/sleep/main.c
	X!Edrivers/acpi/sleep/wakeup.c
	X!Edrivers/acpi/motherboard.c
	X!Edrivers/acpi/bus.c
	-->
	!Edrivers/acpi/scan.c
	!Idrivers/acpi/scan.c
	<!-- No correct structured comments
	X!Edrivers/acpi/pci_bind.c
	-->
	</sect1>
	<sect1><title>Device drivers PnP support</title>
	!Idrivers/pnp/core.c
	<!-- No correct structured comments
	X!Edrivers/pnp/system.c
	-->
	!Edrivers/pnp/card.c
	!Idrivers/pnp/driver.c
	!Edrivers/pnp/manager.c
	!Edrivers/pnp/support.c
	</sect1>
	<sect1><title>Userspace IO devices</title>
	!Edrivers/uio/uio.c
	!Iinclude/linux/uio_driver.h
	</sect1>
	</chapter>

	<chapter id="blkdev">
	<title>Block Devices</title>
	!Eblock/blk-core.c
	!Iblock/blk-core.c
	!Eblock/blk-map.c
	!Iblock/blk-sysfs.c
	!Eblock/blk-settings.c
	!Eblock/blk-exec.c
	!Eblock/blk-barrier.c
	!Eblock/blk-tag.c
	!Iblock/blk-tag.c
	!Eblock/blk-integrity.c
	!Iblock/blktrace.c
	!Iblock/genhd.c
	!Eblock/genhd.c
	</chapter>

	<chapter id="chrdev">
	<title>Char devices</title>
	!Efs/char_dev.c
	</chapter>

	<chapter id="miscdev">
	<title>Miscellaneous Devices</title>
	!Edrivers/char/misc.c
	</chapter>

	<chapter id="parportdev">
	<title>Parallel Port Devices</title>
	!Iinclude/linux/parport.h
	!Edrivers/parport/ieee1284.c
	!Edrivers/parport/share.c
	!Idrivers/parport/daisy.c
	</chapter>

	<chapter id="message_devices">
	<title>Message-based devices</title>
	<sect1><title>Fusion message devices</title>
	!Edrivers/message/fusion/mptbase.c
	!Idrivers/message/fusion/mptbase.c
	!Edrivers/message/fusion/mptscsih.c
	!Idrivers/message/fusion/mptscsih.c
	!Idrivers/message/fusion/mptctl.c
	!Idrivers/message/fusion/mptspi.c
	!Idrivers/message/fusion/mptfc.c
	!Idrivers/message/fusion/mptlan.c
	</sect1>
	<sect1><title>I2O message devices</title>
	!Iinclude/linux/i2o.h
	!Idrivers/message/i2o/core.h
	!Edrivers/message/i2o/iop.c
	!Idrivers/message/i2o/iop.c
	!Idrivers/message/i2o/config-osm.c
	!Edrivers/message/i2o/exec-osm.c
	!Idrivers/message/i2o/exec-osm.c
	!Idrivers/message/i2o/bus-osm.c
	!Edrivers/message/i2o/device.c
	!Idrivers/message/i2o/device.c
	!Idrivers/message/i2o/driver.c
	!Idrivers/message/i2o/pci.c
	!Idrivers/message/i2o/i2o_block.c
	!Idrivers/message/i2o/i2o_scsi.c
	!Idrivers/message/i2o/i2o_proc.c
	</sect1>
	</chapter>

	<chapter id="snddev">
	<title>Sound Devices</title>
	!Iinclude/sound/core.h
	!Esound/sound_core.c
	!Iinclude/sound/pcm.h
	!Esound/core/pcm.c
	!Esound/core/device.c
	!Esound/core/info.c
	!Esound/core/rawmidi.c
	!Esound/core/sound.c
	!Esound/core/memory.c
	!Esound/core/pcm_memory.c
	!Esound/core/init.c
	!Esound/core/isadma.c
	!Esound/core/control.c
	!Esound/core/pcm_lib.c
	!Esound/core/hwdep.c
	!Esound/core/pcm_native.c
	!Esound/core/memalloc.c
	<!-- FIXME: Removed for now since no structured comments in source
	X!Isound/sound_firmware.c
	-->
	</chapter>

	<chapter id="uart16x50">
	<title>16x50 UART Driver</title>
	!Iinclude/linux/serial_core.h
	!Edrivers/serial/serial_core.c
	!Edrivers/serial/8250.c
	</chapter>

	<chapter id="fbdev">
	<title>Frame Buffer Library</title>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<sect1><title>Frame Buffer Memory</title>
	!Edrivers/video/fbmem.c
	</sect1>
	<!--
	<sect1><title>Frame Buffer Console</title>
	X!Edrivers/video/console/fbcon.c
	</sect1>
	-->
	<sect1><title>Frame Buffer Colormap</title>
	!Edrivers/video/fbcmap.c
	</sect1>
	<!-- 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>
	X!Idrivers/video/fbgen.c
	</sect1>
	KAO -->
	<sect1><title>Frame Buffer Video Mode Database</title>
	!Idrivers/video/modedb.c
	!Edrivers/video/modedb.c
	</sect1>
	<sect1><title>Frame Buffer Macintosh Video Mode Database</title>
	!Edrivers/video/macmodes.c
	</sect1>
	<sect1><title>Frame Buffer Fonts</title>
	<para>
	Refer to the file drivers/video/console/fonts.c for more information.
	</para>
	<!-- FIXME: Removed for now since no structured comments in source
	X!Idrivers/video/console/fonts.c
	-->
	</sect1>
	</chapter>

	<chapter id="input_subsystem">
	<title>Input Subsystem</title>
	!Iinclude/linux/input.h
	!Edrivers/input/input.c
	!Edrivers/input/ff-core.c
	!Edrivers/input/ff-memless.c
	</chapter>

	<chapter id="spi">
	<title>Serial Peripheral Interface (SPI)</title>
	<para>
	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.
	</para>
	<para>
	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.)
	</para>
	<para>
	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.
	</para>
	<para>
	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.
	</para>
	!Iinclude/linux/spi/spi.h
	!Fdrivers/spi/spi.c spi_register_board_info
	!Edrivers/spi/spi.c
	</chapter>

	<chapter id="i2c">
	<title>I<superscript>2</superscript>C and SMBus Subsystem</title>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	!Iinclude/linux/i2c.h
	!Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info
	!Edrivers/i2c/i2c-core.c
	</chapter>

	<chapter id="clk">
	<title>Clock Framework</title>

	<para>
	The clock framework defines programming interfaces to support
	software management of the system clock tree.
	This framework is widely used with System-On-Chip (SOC) platforms
	to support power management and various devices which may need
	custom clock rates.
	Note that these "clocks" don't relate to timekeeping or real
	time clocks (RTCs), each of which have separate frameworks.
	These <structname>struct clk</structname> instances may be used
	to manage for example a 96 MHz signal that is used to shift bits
	into and out of peripherals or busses, or otherwise trigger
	synchronous state machine transitions in system hardware.
	</para>

	<para>
	Power management is supported by explicit software clock gating:
	unused clocks are disabled, so the system doesn't waste power
	changing the state of transistors that aren't in active use.
	On some systems this may be backed by hardware clock gating,
	where clocks are gated without being disabled in software.
	Sections of chips that are powered but not clocked may be able
	to retain their last state.
	This low power state is often called a <emphasis>retention
	mode</emphasis>.
	This mode still incurs leakage currents, especially with finer
	circuit geometries, but for CMOS circuits power is mostly used
	by clocked state changes.
	</para>

	<para>
	Power-aware drivers only enable their clocks when the device
	they manage is in active use. Also, system sleep states often
	differ according to which clock domains are active: while a
	"standby" state may allow wakeup from several active domains, a
	"mem" (suspend-to-RAM) state may require a more wholesale shutdown
	of clocks derived from higher speed PLLs and oscillators, limiting
	the number of possible wakeup event sources. A driver's suspend
	method may need to be aware of system-specific clock constraints
	on the target sleep state.
	</para>

	<para>
	Some platforms support programmable clock generators. These
	can be used by external chips of various kinds, such as other
	CPUs, multimedia codecs, and devices with strict requirements
	for interface clocking.
	</para>

	!Iinclude/linux/clk.h
	</chapter>

	</book>