Documentation/DocBook/kernel-api.tmpl

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   4
   5 <book id="LinuxKernelAPI">
   6  <bookinfo>
   7   <title>The Linux Kernel API</title>
   8
   9   <legalnotice>
  10    <para>
  11      This documentation is free software; you can redistribute
  12      it and/or modify it under the terms of the GNU General Public
  13      License as published by the Free Software Foundation; either
  14      version 2 of the License, or (at your option) any later
  15      version.
  16    </para>
  17
  18    <para>
  19      This program is distributed in the hope that it will be
  20      useful, but WITHOUT ANY WARRANTY; without even the implied
  21      warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  22      See the GNU General Public License for more details.
  23    </para>
  24
  25    <para>
  26      You should have received a copy of the GNU General Public
  27      License along with this program; if not, write to the Free
  28      Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  29      MA 02111-1307 USA
  30    </para>
  31
  32    <para>
  33      For more details see the file COPYING in the source
  34      distribution of Linux.
  35    </para>
  36   </legalnotice>
  37  </bookinfo>
  38
  39 <toc></toc>
  40
  41   <chapter id="Basics">
  42      <title>Driver Basics</title>
  43      <sect1><title>Driver Entry and Exit points</title>
  44 !Iinclude/linux/init.h
  45      </sect1>
  46
  47      <sect1><title>Atomic and pointer manipulation</title>
  48 !Iarch/x86/include/asm/atomic_32.h
  49 !Iarch/x86/include/asm/unaligned.h
  50      </sect1>
  51
  52      <sect1><title>Delaying, scheduling, and timer routines</title>
  53 !Iinclude/linux/sched.h
  54 !Ekernel/sched.c
  55 !Ekernel/timer.c
  56      </sect1>
  57      <sect1><title>High-resolution timers</title>
  58 !Iinclude/linux/ktime.h
  59 !Iinclude/linux/hrtimer.h
  60 !Ekernel/hrtimer.c
  61      </sect1>
  62      <sect1><title>Workqueues and Kevents</title>
  63 !Ekernel/workqueue.c
  64      </sect1>
  65      <sect1><title>Internal Functions</title>
  66 !Ikernel/exit.c
  67 !Ikernel/signal.c
  68 !Iinclude/linux/kthread.h
  69 !Ekernel/kthread.c
  70      </sect1>
  71
  72      <sect1><title>Kernel objects manipulation</title>
  73 <!--
  74 X!Iinclude/linux/kobject.h
  75 -->
  76 !Elib/kobject.c
  77      </sect1>
  78
  79      <sect1><title>Kernel utility functions</title>
  80 !Iinclude/linux/kernel.h
  81 !Ekernel/printk.c
  82 !Ekernel/panic.c
  83 !Ekernel/sys.c
  84 !Ekernel/rcupdate.c
  85      </sect1>
  86
  87      <sect1><title>Device Resource Management</title>
  88 !Edrivers/base/devres.c
  89      </sect1>
  90
  91   </chapter>
  92
  93   <chapter id="adt">
  94      <title>Data Types</title>
  95      <sect1><title>Doubly Linked Lists</title>
  96 !Iinclude/linux/list.h
  97      </sect1>
  98   </chapter>
  99
 100   <chapter id="libc">
 101      <title>Basic C Library Functions</title>
 102
 103      <para>
 104        When writing drivers, you cannot in general use routines which are
 105        from the C Library.  Some of the functions have been found generally
 106        useful and they are listed below.  The behaviour of these functions
 107        may vary slightly from those defined by ANSI, and these deviations
 108        are noted in the text.
 109      </para>
 110
 111      <sect1><title>String Conversions</title>
 112 !Ilib/vsprintf.c
 113 !Elib/vsprintf.c
 114      </sect1>
 115      <sect1><title>String Manipulation</title>
 116 <!-- All functions are exported at now
 117 X!Ilib/string.c
 118  -->
 119 !Elib/string.c
 120      </sect1>
 121      <sect1><title>Bit Operations</title>
 122 !Iarch/x86/include/asm/bitops.h
 123      </sect1>
 124   </chapter>
 125
 126   <chapter id="kernel-lib">
 127      <title>Basic Kernel Library Functions</title>
 128
 129      <para>
 130        The Linux kernel provides more basic utility functions.
 131      </para>
 132
 133      <sect1><title>Bitmap Operations</title>
 134 !Elib/bitmap.c
 135 !Ilib/bitmap.c
 136      </sect1>
 137
 138      <sect1><title>Command-line Parsing</title>
 139 !Elib/cmdline.c
 140      </sect1>
 141
 142      <sect1 id="crc"><title>CRC Functions</title>
 143 !Elib/crc7.c
 144 !Elib/crc16.c
 145 !Elib/crc-itu-t.c
 146 !Elib/crc32.c
 147 !Elib/crc-ccitt.c
 148      </sect1>
 149   </chapter>
 150
 151   <chapter id="mm">
 152      <title>Memory Management in Linux</title>
 153      <sect1><title>The Slab Cache</title>
 154 !Iinclude/linux/slab.h
 155 !Emm/slab.c
 156      </sect1>
 157      <sect1><title>User Space Memory Access</title>
 158 !Iarch/x86/include/asm/uaccess_32.h
 159 !Earch/x86/lib/usercopy_32.c
 160      </sect1>
 161      <sect1><title>More Memory Management Functions</title>
 162 !Emm/readahead.c
 163 !Emm/filemap.c
 164 !Emm/memory.c
 165 !Emm/vmalloc.c
 166 !Imm/page_alloc.c
 167 !Emm/mempool.c
 168 !Emm/dmapool.c
 169 !Emm/page-writeback.c
 170 !Emm/truncate.c
 171      </sect1>
 172   </chapter>
 173
 174
 175   <chapter id="ipc">
 176      <title>Kernel IPC facilities</title>
 177
 178      <sect1><title>IPC utilities</title>
 179 !Iipc/util.c
 180      </sect1>
 181   </chapter>
 182
 183   <chapter id="kfifo">
 184      <title>FIFO Buffer</title>
 185      <sect1><title>kfifo interface</title>
 186 !Iinclude/linux/kfifo.h
 187 !Ekernel/kfifo.c
 188      </sect1>
 189   </chapter>
 190
 191   <chapter id="relayfs">
 192      <title>relay interface support</title>
 193
 194      <para>
 195         Relay interface support
 196         is designed to provide an efficient mechanism for tools and
 197         facilities to relay large amounts of data from kernel space to
 198         user space.
 199      </para>
 200
 201      <sect1><title>relay interface</title>
 202 !Ekernel/relay.c
 203 !Ikernel/relay.c
 204      </sect1>
 205   </chapter>
 206
 207   <chapter id="modload">
 208      <title>Module Support</title>
 209      <sect1><title>Module Loading</title>
 210 !Ekernel/kmod.c
 211      </sect1>
 212      <sect1><title>Inter Module support</title>
 213         <para>
 214            Refer to the file kernel/module.c for more information.
 215         </para>
 216 <!-- FIXME: Removed for now since no structured comments in source
 217 X!Ekernel/module.c
 218 -->
 219      </sect1>
 220   </chapter>
 221
 222   <chapter id="hardware">
 223      <title>Hardware Interfaces</title>
 224      <sect1><title>Interrupt Handling</title>
 225 !Ekernel/irq/manage.c
 226      </sect1>
 227
 228      <sect1><title>DMA Channels</title>
 229 !Ekernel/dma.c
 230      </sect1>
 231
 232      <sect1><title>Resources Management</title>
 233 !Ikernel/resource.c
 234 !Ekernel/resource.c
 235      </sect1>
 236
 237      <sect1><title>MTRR Handling</title>
 238 !Earch/x86/kernel/cpu/mtrr/main.c
 239      </sect1>
 240
 241      <sect1><title>PCI Support Library</title>
 242 !Edrivers/pci/pci.c
 243 !Edrivers/pci/pci-driver.c
 244 !Edrivers/pci/remove.c
 245 !Edrivers/pci/pci-acpi.c
 246 !Edrivers/pci/search.c
 247 !Edrivers/pci/msi.c
 248 !Edrivers/pci/bus.c
 249 <!-- FIXME: Removed for now since no structured comments in source
 250 X!Edrivers/pci/hotplug.c
 251 -->
 252 !Edrivers/pci/probe.c
 253 !Edrivers/pci/rom.c
 254      </sect1>
 255      <sect1><title>PCI Hotplug Support Library</title>
 256 !Edrivers/pci/hotplug/pci_hotplug_core.c
 257      </sect1>
 258      <sect1><title>MCA Architecture</title>
 259         <sect2><title>MCA Device Functions</title>
 260            <para>
 261               Refer to the file arch/x86/kernel/mca_32.c for more information.
 262            </para>
 263 <!-- FIXME: Removed for now since no structured comments in source
 264 X!Earch/x86/kernel/mca_32.c
 265 -->
 266         </sect2>
 267         <sect2><title>MCA Bus DMA</title>
 268 !Iarch/x86/include/asm/mca_dma.h
 269         </sect2>
 270      </sect1>
 271   </chapter>
 272
 273   <chapter id="firmware">
 274      <title>Firmware Interfaces</title>
 275      <sect1><title>DMI Interfaces</title>
 276 !Edrivers/firmware/dmi_scan.c
 277      </sect1>
 278      <sect1><title>EDD Interfaces</title>
 279 !Idrivers/firmware/edd.c
 280      </sect1>
 281   </chapter>
 282
 283   <chapter id="security">
 284      <title>Security Framework</title>
 285 !Isecurity/security.c
 286 !Esecurity/inode.c
 287   </chapter>
 288
 289   <chapter id="audit">
 290      <title>Audit Interfaces</title>
 291 !Ekernel/audit.c
 292 !Ikernel/auditsc.c
 293 !Ikernel/auditfilter.c
 294   </chapter>
 295
 296   <chapter id="accounting">
 297      <title>Accounting Framework</title>
 298 !Ikernel/acct.c
 299   </chapter>
 300
 301   <chapter id="devdrivers">
 302      <title>Device drivers infrastructure</title>
 303      <sect1><title>Device Drivers Base</title>
 304 <!--
 305 X!Iinclude/linux/device.h
 306 -->
 307 !Edrivers/base/driver.c
 308 !Edrivers/base/core.c
 309 !Edrivers/base/class.c
 310 !Edrivers/base/firmware_class.c
 311 !Edrivers/base/transport_class.c
 312 <!-- Cannot be included, because
 313      attribute_container_add_class_device_adapter
 314  and attribute_container_classdev_to_container
 315      exceed allowed 44 characters maximum
 316 X!Edrivers/base/attribute_container.c
 317 -->
 318 !Edrivers/base/sys.c
 319 <!--
 320 X!Edrivers/base/interface.c
 321 -->
 322 !Edrivers/base/platform.c
 323 !Edrivers/base/bus.c
 324      </sect1>
 325      <sect1><title>Device Drivers Power Management</title>
 326 !Edrivers/base/power/main.c
 327      </sect1>
 328      <sect1><title>Device Drivers ACPI Support</title>
 329 <!-- Internal functions only
 330 X!Edrivers/acpi/sleep/main.c
 331 X!Edrivers/acpi/sleep/wakeup.c
 332 X!Edrivers/acpi/motherboard.c
 333 X!Edrivers/acpi/bus.c
 334 -->
 335 !Edrivers/acpi/scan.c
 336 !Idrivers/acpi/scan.c
 337 <!-- No correct structured comments
 338 X!Edrivers/acpi/pci_bind.c
 339 -->
 340      </sect1>
 341      <sect1><title>Device drivers PnP support</title>
 342 !Idrivers/pnp/core.c
 343 <!-- No correct structured comments
 344 X!Edrivers/pnp/system.c
 345  -->
 346 !Edrivers/pnp/card.c
 347 !Idrivers/pnp/driver.c
 348 !Edrivers/pnp/manager.c
 349 !Edrivers/pnp/support.c
 350      </sect1>
 351      <sect1><title>Userspace IO devices</title>
 352 !Edrivers/uio/uio.c
 353 !Iinclude/linux/uio_driver.h
 354      </sect1>
 355   </chapter>
 356
 357   <chapter id="blkdev">
 358      <title>Block Devices</title>
 359 !Eblock/blk-core.c
 360 !Iblock/blk-core.c
 361 !Eblock/blk-map.c
 362 !Iblock/blk-sysfs.c
 363 !Eblock/blk-settings.c
 364 !Eblock/blk-exec.c
 365 !Eblock/blk-barrier.c
 366 !Eblock/blk-tag.c
 367 !Iblock/blk-tag.c
 368 !Eblock/blk-integrity.c
 369 !Iblock/blktrace.c
 370 !Iblock/genhd.c
 371 !Eblock/genhd.c
 372   </chapter>
 373
 374   <chapter id="chrdev">
 375         <title>Char devices</title>
 376 !Efs/char_dev.c
 377   </chapter>
 378
 379   <chapter id="miscdev">
 380      <title>Miscellaneous Devices</title>
 381 !Edrivers/char/misc.c
 382   </chapter>
 383
 384   <chapter id="parportdev">
 385      <title>Parallel Port Devices</title>
 386 !Iinclude/linux/parport.h
 387 !Edrivers/parport/ieee1284.c
 388 !Edrivers/parport/share.c
 389 !Idrivers/parport/daisy.c
 390   </chapter>
 391
 392   <chapter id="message_devices">
 393         <title>Message-based devices</title>
 394      <sect1><title>Fusion message devices</title>
 395 !Edrivers/message/fusion/mptbase.c
 396 !Idrivers/message/fusion/mptbase.c
 397 !Edrivers/message/fusion/mptscsih.c
 398 !Idrivers/message/fusion/mptscsih.c
 399 !Idrivers/message/fusion/mptctl.c
 400 !Idrivers/message/fusion/mptspi.c
 401 !Idrivers/message/fusion/mptfc.c
 402 !Idrivers/message/fusion/mptlan.c
 403      </sect1>
 404      <sect1><title>I2O message devices</title>
 405 !Iinclude/linux/i2o.h
 406 !Idrivers/message/i2o/core.h
 407 !Edrivers/message/i2o/iop.c
 408 !Idrivers/message/i2o/iop.c
 409 !Idrivers/message/i2o/config-osm.c
 410 !Edrivers/message/i2o/exec-osm.c
 411 !Idrivers/message/i2o/exec-osm.c
 412 !Idrivers/message/i2o/bus-osm.c
 413 !Edrivers/message/i2o/device.c
 414 !Idrivers/message/i2o/device.c
 415 !Idrivers/message/i2o/driver.c
 416 !Idrivers/message/i2o/pci.c
 417 !Idrivers/message/i2o/i2o_block.c
 418 !Idrivers/message/i2o/i2o_scsi.c
 419 !Idrivers/message/i2o/i2o_proc.c
 420      </sect1>
 421   </chapter>
 422
 423   <chapter id="snddev">
 424      <title>Sound Devices</title>
 425 !Iinclude/sound/core.h
 426 !Esound/sound_core.c
 427 !Iinclude/sound/pcm.h
 428 !Esound/core/pcm.c
 429 !Esound/core/device.c
 430 !Esound/core/info.c
 431 !Esound/core/rawmidi.c
 432 !Esound/core/sound.c
 433 !Esound/core/memory.c
 434 !Esound/core/pcm_memory.c
 435 !Esound/core/init.c
 436 !Esound/core/isadma.c
 437 !Esound/core/control.c
 438 !Esound/core/pcm_lib.c
 439 !Esound/core/hwdep.c
 440 !Esound/core/pcm_native.c
 441 !Esound/core/memalloc.c
 442 <!-- FIXME: Removed for now since no structured comments in source
 443 X!Isound/sound_firmware.c
 444 -->
 445   </chapter>
 446
 447   <chapter id="uart16x50">
 448      <title>16x50 UART Driver</title>
 449 !Iinclude/linux/serial_core.h
 450 !Edrivers/serial/serial_core.c
 451 !Edrivers/serial/8250.c
 452   </chapter>
 453
 454   <chapter id="fbdev">
 455      <title>Frame Buffer Library</title>
 456
 457      <para>
 458        The frame buffer drivers depend heavily on four data structures.
 459        These structures are declared in include/linux/fb.h.  They are
 460        fb_info, fb_var_screeninfo, fb_fix_screeninfo and fb_monospecs.
 461        The last three can be made available to and from userland.
 462      </para>
 463
 464      <para>
 465        fb_info defines the current state of a particular video card.
 466        Inside fb_info, there exists a fb_ops structure which is a
 467        collection of needed functions to make fbdev and fbcon work.
 468        fb_info is only visible to the kernel.
 469      </para>
 470
 471      <para>
 472        fb_var_screeninfo is used to describe the features of a video card
 473        that are user defined.  With fb_var_screeninfo, things such as
 474        depth and the resolution may be defined.
 475      </para>
 476
 477      <para>
 478        The next structure is fb_fix_screeninfo. This defines the
 479        properties of a card that are created when a mode is set and can't
 480        be changed otherwise.  A good example of this is the start of the
 481        frame buffer memory.  This "locks" the address of the frame buffer
 482        memory, so that it cannot be changed or moved.
 483      </para>
 484
 485      <para>
 486        The last structure is fb_monospecs. In the old API, there was
 487        little importance for fb_monospecs. This allowed for forbidden things
 488        such as setting a mode of 800x600 on a fix frequency monitor. With
 489        the new API, fb_monospecs prevents such things, and if used
 490        correctly, can prevent a monitor from being cooked.  fb_monospecs
 491        will not be useful until kernels 2.5.x.
 492      </para>
 493
 494      <sect1><title>Frame Buffer Memory</title>
 495 !Edrivers/video/fbmem.c
 496      </sect1>
 497 <!--
 498      <sect1><title>Frame Buffer Console</title>
 499 X!Edrivers/video/console/fbcon.c
 500      </sect1>
 501 -->
 502      <sect1><title>Frame Buffer Colormap</title>
 503 !Edrivers/video/fbcmap.c
 504      </sect1>
 505 <!-- FIXME:
 506   drivers/video/fbgen.c has no docs, which stuffs up the sgml.  Comment
 507   out until somebody adds docs.  KAO
 508      <sect1><title>Frame Buffer Generic Functions</title>
 509 X!Idrivers/video/fbgen.c
 510      </sect1>
 511 KAO -->
 512      <sect1><title>Frame Buffer Video Mode Database</title>
 513 !Idrivers/video/modedb.c
 514 !Edrivers/video/modedb.c
 515      </sect1>
 516      <sect1><title>Frame Buffer Macintosh Video Mode Database</title>
 517 !Edrivers/video/macmodes.c
 518      </sect1>
 519      <sect1><title>Frame Buffer Fonts</title>
 520         <para>
 521            Refer to the file drivers/video/console/fonts.c for more information.
 522         </para>
 523 <!-- FIXME: Removed for now since no structured comments in source
 524 X!Idrivers/video/console/fonts.c
 525 -->
 526      </sect1>
 527   </chapter>
 528
 529   <chapter id="input_subsystem">
 530      <title>Input Subsystem</title>
 531 !Iinclude/linux/input.h
 532 !Edrivers/input/input.c
 533 !Edrivers/input/ff-core.c
 534 !Edrivers/input/ff-memless.c
 535   </chapter>
 536
 537   <chapter id="spi">
 538       <title>Serial Peripheral Interface (SPI)</title>
 539   <para>
 540         SPI is the "Serial Peripheral Interface", widely used with
 541         embedded systems because it is a simple and efficient
 542         interface:  basically a multiplexed shift register.
 543         Its three signal wires hold a clock (SCK, often in the range
 544         of 1-20 MHz), a "Master Out, Slave In" (MOSI) data line, and
 545         a "Master In, Slave Out" (MISO) data line.
 546         SPI is a full duplex protocol; for each bit shifted out the
 547         MOSI line (one per clock) another is shifted in on the MISO line.
 548         Those bits are assembled into words of various sizes on the
 549         way to and from system memory.
 550         An additional chipselect line is usually active-low (nCS);
 551         four signals are normally used for each peripheral, plus
 552         sometimes an interrupt.
 553   </para>
 554   <para>
 555         The SPI bus facilities listed here provide a generalized
 556         interface to declare SPI busses and devices, manage them
 557         according to the standard Linux driver model, and perform
 558         input/output operations.
 559         At this time, only "master" side interfaces are supported,
 560         where Linux talks to SPI peripherals and does not implement
 561         such a peripheral itself.
 562         (Interfaces to support implementing SPI slaves would
 563         necessarily look different.)
 564   </para>
 565   <para>
 566         The programming interface is structured around two kinds of driver,
 567         and two kinds of device.
 568         A "Controller Driver" abstracts the controller hardware, which may
 569         be as simple as a set of GPIO pins or as complex as a pair of FIFOs
 570         connected to dual DMA engines on the other side of the SPI shift
 571         register (maximizing throughput).  Such drivers bridge between
 572         whatever bus they sit on (often the platform bus) and SPI, and
 573         expose the SPI side of their device as a
 574         <structname>struct spi_master</structname>.
 575         SPI devices are children of that master, represented as a
 576         <structname>struct spi_device</structname> and manufactured from
 577         <structname>struct spi_board_info</structname> descriptors which
 578         are usually provided by board-specific initialization code.
 579         A <structname>struct spi_driver</structname> is called a
 580         "Protocol Driver", and is bound to a spi_device using normal
 581         driver model calls.
 582   </para>
 583   <para>
 584         The I/O model is a set of queued messages.  Protocol drivers
 585         submit one or more <structname>struct spi_message</structname>
 586         objects, which are processed and completed asynchronously.
 587         (There are synchronous wrappers, however.)  Messages are
 588         built from one or more <structname>struct spi_transfer</structname>
 589         objects, each of which wraps a full duplex SPI transfer.
 590         A variety of protocol tweaking options are needed, because
 591         different chips adopt very different policies for how they
 592         use the bits transferred with SPI.
 593   </para>
 594 !Iinclude/linux/spi/spi.h
 595 !Fdrivers/spi/spi.c spi_register_board_info
 596 !Edrivers/spi/spi.c
 597   </chapter>
 598
 599   <chapter id="i2c">
 600      <title>I<superscript>2</superscript>C and SMBus Subsystem</title>
 601
 602      <para>
 603         I<superscript>2</superscript>C (or without fancy typography, "I2C")
 604         is an acronym for the "Inter-IC" bus, a simple bus protocol which is
 605         widely used where low data rate communications suffice.
 606         Since it's also a licensed trademark, some vendors use another
 607         name (such as "Two-Wire Interface", TWI) for the same bus.
 608         I2C only needs two signals (SCL for clock, SDA for data), conserving
 609         board real estate and minimizing signal quality issues.
 610         Most I2C devices use seven bit addresses, and bus speeds of up
 611         to 400 kHz; there's a high speed extension (3.4 MHz) that's not yet
 612         found wide use.
 613         I2C is a multi-master bus; open drain signaling is used to
 614         arbitrate between masters, as well as to handshake and to
 615         synchronize clocks from slower clients.
 616      </para>
 617
 618      <para>
 619         The Linux I2C programming interfaces support only the master
 620         side of bus interactions, not the slave side.
 621         The programming interface is structured around two kinds of driver,
 622         and two kinds of device.
 623         An I2C "Adapter Driver" abstracts the controller hardware; it binds
 624         to a physical device (perhaps a PCI device or platform_device) and
 625         exposes a <structname>struct i2c_adapter</structname> representing
 626         each I2C bus segment it manages.
 627         On each I2C bus segment will be I2C devices represented by a
 628         <structname>struct i2c_client</structname>.  Those devices will
 629         be bound to a <structname>struct i2c_driver</structname>,
 630         which should follow the standard Linux driver model.
 631         (At this writing, a legacy model is more widely used.)
 632         There are functions to perform various I2C protocol operations; at
 633         this writing all such functions are usable only from task context.
 634      </para>
 635
 636      <para>
 637         The System Management Bus (SMBus) is a sibling protocol.  Most SMBus
 638         systems are also I2C conformant.  The electrical constraints are
 639         tighter for SMBus, and it standardizes particular protocol messages
 640         and idioms.  Controllers that support I2C can also support most
 641         SMBus operations, but SMBus controllers don't support all the protocol
 642         options that an I2C controller will.
 643         There are functions to perform various SMBus protocol operations,
 644         either using I2C primitives or by issuing SMBus commands to
 645         i2c_adapter devices which don't support those I2C operations.
 646      </para>
 647
 648 !Iinclude/linux/i2c.h
 649 !Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info
 650 !Edrivers/i2c/i2c-core.c
 651   </chapter>
 652
 653   <chapter id="clk">
 654      <title>Clock Framework</title>
 655
 656      <para>
 657         The clock framework defines programming interfaces to support
 658         software management of the system clock tree.
 659         This framework is widely used with System-On-Chip (SOC) platforms
 660         to support power management and various devices which may need
 661         custom clock rates.
 662         Note that these "clocks" don't relate to timekeeping or real
 663         time clocks (RTCs), each of which have separate frameworks.
 664         These <structname>struct clk</structname> instances may be used
 665         to manage for example a 96 MHz signal that is used to shift bits
 666         into and out of peripherals or busses, or otherwise trigger
 667         synchronous state machine transitions in system hardware.
 668      </para>
 669
 670      <para>
 671         Power management is supported by explicit software clock gating:
 672         unused clocks are disabled, so the system doesn't waste power
 673         changing the state of transistors that aren't in active use.
 674         On some systems this may be backed by hardware clock gating,
 675         where clocks are gated without being disabled in software.
 676         Sections of chips that are powered but not clocked may be able
 677         to retain their last state.
 678         This low power state is often called a <emphasis>retention
 679         mode</emphasis>.
 680         This mode still incurs leakage currents, especially with finer
 681         circuit geometries, but for CMOS circuits power is mostly used
 682         by clocked state changes.
 683      </para>
 684
 685      <para>
 686         Power-aware drivers only enable their clocks when the device
 687         they manage is in active use.  Also, system sleep states often
 688         differ according to which clock domains are active:  while a
 689         "standby" state may allow wakeup from several active domains, a
 690         "mem" (suspend-to-RAM) state may require a more wholesale shutdown
 691         of clocks derived from higher speed PLLs and oscillators, limiting
 692         the number of possible wakeup event sources.  A driver's suspend
 693         method may need to be aware of system-specific clock constraints
 694         on the target sleep state.
 695      </para>
 696
 697      <para>
 698         Some platforms support programmable clock generators.  These
 699         can be used by external chips of various kinds, such as other
 700         CPUs, multimedia codecs, and devices with strict requirements
 701         for interface clocking.
 702      </para>
 703
 704 !Iinclude/linux/clk.h
 705   </chapter>
 706
 707 </book>