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 !Iinclude/asm-x86/atomic_32.h
  49 !Iinclude/asm-x86/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 !Iinclude/asm-x86/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 !Iinclude/asm-x86/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 !Iinclude/asm-x86/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   </chapter>
 287
 288   <chapter id="audit">
 289      <title>Audit Interfaces</title>
 290 !Ekernel/audit.c
 291 !Ikernel/auditsc.c
 292 !Ikernel/auditfilter.c
 293   </chapter>
 294
 295   <chapter id="accounting">
 296      <title>Accounting Framework</title>
 297 !Ikernel/acct.c
 298   </chapter>
 299
 300   <chapter id="devdrivers">
 301      <title>Device drivers infrastructure</title>
 302      <sect1><title>Device Drivers Base</title>
 303 <!--
 304 X!Iinclude/linux/device.h
 305 -->
 306 !Edrivers/base/driver.c
 307 !Edrivers/base/core.c
 308 !Edrivers/base/class.c
 309 !Edrivers/base/firmware_class.c
 310 !Edrivers/base/transport_class.c
 311 <!-- Cannot be included, because
 312      attribute_container_add_class_device_adapter
 313  and attribute_container_classdev_to_container
 314      exceed allowed 44 characters maximum
 315 X!Edrivers/base/attribute_container.c
 316 -->
 317 !Edrivers/base/sys.c
 318 <!--
 319 X!Edrivers/base/interface.c
 320 -->
 321 !Edrivers/base/platform.c
 322 !Edrivers/base/bus.c
 323      </sect1>
 324      <sect1><title>Device Drivers Power Management</title>
 325 !Edrivers/base/power/main.c
 326      </sect1>
 327      <sect1><title>Device Drivers ACPI Support</title>
 328 <!-- Internal functions only
 329 X!Edrivers/acpi/sleep/main.c
 330 X!Edrivers/acpi/sleep/wakeup.c
 331 X!Edrivers/acpi/motherboard.c
 332 X!Edrivers/acpi/bus.c
 333 -->
 334 !Edrivers/acpi/scan.c
 335 !Idrivers/acpi/scan.c
 336 <!-- No correct structured comments
 337 X!Edrivers/acpi/pci_bind.c
 338 -->
 339      </sect1>
 340      <sect1><title>Device drivers PnP support</title>
 341 !Idrivers/pnp/core.c
 342 <!-- No correct structured comments
 343 X!Edrivers/pnp/system.c
 344  -->
 345 !Edrivers/pnp/card.c
 346 !Idrivers/pnp/driver.c
 347 !Edrivers/pnp/manager.c
 348 !Edrivers/pnp/support.c
 349      </sect1>
 350      <sect1><title>Userspace IO devices</title>
 351 !Edrivers/uio/uio.c
 352 !Iinclude/linux/uio_driver.h
 353      </sect1>
 354   </chapter>
 355
 356   <chapter id="blkdev">
 357      <title>Block Devices</title>
 358 !Eblock/blk-core.c
 359 !Iblock/blk-core.c
 360 !Eblock/blk-map.c
 361 !Iblock/blk-sysfs.c
 362 !Eblock/blk-settings.c
 363 !Eblock/blk-exec.c
 364 !Eblock/blk-barrier.c
 365 !Eblock/blk-tag.c
 366 !Iblock/blk-tag.c
 367   </chapter>
 368
 369   <chapter id="chrdev">
 370         <title>Char devices</title>
 371 !Efs/char_dev.c
 372   </chapter>
 373
 374   <chapter id="miscdev">
 375      <title>Miscellaneous Devices</title>
 376 !Edrivers/char/misc.c
 377   </chapter>
 378
 379   <chapter id="parportdev">
 380      <title>Parallel Port Devices</title>
 381 !Iinclude/linux/parport.h
 382 !Edrivers/parport/ieee1284.c
 383 !Edrivers/parport/share.c
 384 !Idrivers/parport/daisy.c
 385   </chapter>
 386
 387   <chapter id="message_devices">
 388         <title>Message-based devices</title>
 389      <sect1><title>Fusion message devices</title>
 390 !Edrivers/message/fusion/mptbase.c
 391 !Idrivers/message/fusion/mptbase.c
 392 !Edrivers/message/fusion/mptscsih.c
 393 !Idrivers/message/fusion/mptscsih.c
 394 !Idrivers/message/fusion/mptctl.c
 395 !Idrivers/message/fusion/mptspi.c
 396 !Idrivers/message/fusion/mptfc.c
 397 !Idrivers/message/fusion/mptlan.c
 398      </sect1>
 399      <sect1><title>I2O message devices</title>
 400 !Iinclude/linux/i2o.h
 401 !Idrivers/message/i2o/core.h
 402 !Edrivers/message/i2o/iop.c
 403 !Idrivers/message/i2o/iop.c
 404 !Idrivers/message/i2o/config-osm.c
 405 !Edrivers/message/i2o/exec-osm.c
 406 !Idrivers/message/i2o/exec-osm.c
 407 !Idrivers/message/i2o/bus-osm.c
 408 !Edrivers/message/i2o/device.c
 409 !Idrivers/message/i2o/device.c
 410 !Idrivers/message/i2o/driver.c
 411 !Idrivers/message/i2o/pci.c
 412 !Idrivers/message/i2o/i2o_block.c
 413 !Idrivers/message/i2o/i2o_scsi.c
 414 !Idrivers/message/i2o/i2o_proc.c
 415      </sect1>
 416   </chapter>
 417
 418   <chapter id="snddev">
 419      <title>Sound Devices</title>
 420 !Iinclude/sound/core.h
 421 !Esound/sound_core.c
 422 !Iinclude/sound/pcm.h
 423 !Esound/core/pcm.c
 424 !Esound/core/device.c
 425 !Esound/core/info.c
 426 !Esound/core/rawmidi.c
 427 !Esound/core/sound.c
 428 !Esound/core/memory.c
 429 !Esound/core/pcm_memory.c
 430 !Esound/core/init.c
 431 !Esound/core/isadma.c
 432 !Esound/core/control.c
 433 !Esound/core/pcm_lib.c
 434 !Esound/core/hwdep.c
 435 !Esound/core/pcm_native.c
 436 !Esound/core/memalloc.c
 437 <!-- FIXME: Removed for now since no structured comments in source
 438 X!Isound/sound_firmware.c
 439 -->
 440   </chapter>
 441
 442   <chapter id="uart16x50">
 443      <title>16x50 UART Driver</title>
 444 !Iinclude/linux/serial_core.h
 445 !Edrivers/serial/serial_core.c
 446 !Edrivers/serial/8250.c
 447   </chapter>
 448
 449   <chapter id="fbdev">
 450      <title>Frame Buffer Library</title>
 451
 452      <para>
 453        The frame buffer drivers depend heavily on four data structures.
 454        These structures are declared in include/linux/fb.h.  They are
 455        fb_info, fb_var_screeninfo, fb_fix_screeninfo and fb_monospecs.
 456        The last three can be made available to and from userland.
 457      </para>
 458
 459      <para>
 460        fb_info defines the current state of a particular video card.
 461        Inside fb_info, there exists a fb_ops structure which is a
 462        collection of needed functions to make fbdev and fbcon work.
 463        fb_info is only visible to the kernel.
 464      </para>
 465
 466      <para>
 467        fb_var_screeninfo is used to describe the features of a video card
 468        that are user defined.  With fb_var_screeninfo, things such as
 469        depth and the resolution may be defined.
 470      </para>
 471
 472      <para>
 473        The next structure is fb_fix_screeninfo. This defines the
 474        properties of a card that are created when a mode is set and can't
 475        be changed otherwise.  A good example of this is the start of the
 476        frame buffer memory.  This "locks" the address of the frame buffer
 477        memory, so that it cannot be changed or moved.
 478      </para>
 479
 480      <para>
 481        The last structure is fb_monospecs. In the old API, there was
 482        little importance for fb_monospecs. This allowed for forbidden things
 483        such as setting a mode of 800x600 on a fix frequency monitor. With
 484        the new API, fb_monospecs prevents such things, and if used
 485        correctly, can prevent a monitor from being cooked.  fb_monospecs
 486        will not be useful until kernels 2.5.x.
 487      </para>
 488
 489      <sect1><title>Frame Buffer Memory</title>
 490 !Edrivers/video/fbmem.c
 491      </sect1>
 492 <!--
 493      <sect1><title>Frame Buffer Console</title>
 494 X!Edrivers/video/console/fbcon.c
 495      </sect1>
 496 -->
 497      <sect1><title>Frame Buffer Colormap</title>
 498 !Edrivers/video/fbcmap.c
 499      </sect1>
 500 <!-- FIXME:
 501   drivers/video/fbgen.c has no docs, which stuffs up the sgml.  Comment
 502   out until somebody adds docs.  KAO
 503      <sect1><title>Frame Buffer Generic Functions</title>
 504 X!Idrivers/video/fbgen.c
 505      </sect1>
 506 KAO -->
 507      <sect1><title>Frame Buffer Video Mode Database</title>
 508 !Idrivers/video/modedb.c
 509 !Edrivers/video/modedb.c
 510      </sect1>
 511      <sect1><title>Frame Buffer Macintosh Video Mode Database</title>
 512 !Edrivers/video/macmodes.c
 513      </sect1>
 514      <sect1><title>Frame Buffer Fonts</title>
 515         <para>
 516            Refer to the file drivers/video/console/fonts.c for more information.
 517         </para>
 518 <!-- FIXME: Removed for now since no structured comments in source
 519 X!Idrivers/video/console/fonts.c
 520 -->
 521      </sect1>
 522   </chapter>
 523
 524   <chapter id="input_subsystem">
 525      <title>Input Subsystem</title>
 526 !Iinclude/linux/input.h
 527 !Edrivers/input/input.c
 528 !Edrivers/input/ff-core.c
 529 !Edrivers/input/ff-memless.c
 530   </chapter>
 531
 532   <chapter id="spi">
 533       <title>Serial Peripheral Interface (SPI)</title>
 534   <para>
 535         SPI is the "Serial Peripheral Interface", widely used with
 536         embedded systems because it is a simple and efficient
 537         interface:  basically a multiplexed shift register.
 538         Its three signal wires hold a clock (SCK, often in the range
 539         of 1-20 MHz), a "Master Out, Slave In" (MOSI) data line, and
 540         a "Master In, Slave Out" (MISO) data line.
 541         SPI is a full duplex protocol; for each bit shifted out the
 542         MOSI line (one per clock) another is shifted in on the MISO line.
 543         Those bits are assembled into words of various sizes on the
 544         way to and from system memory.
 545         An additional chipselect line is usually active-low (nCS);
 546         four signals are normally used for each peripheral, plus
 547         sometimes an interrupt.
 548   </para>
 549   <para>
 550         The SPI bus facilities listed here provide a generalized
 551         interface to declare SPI busses and devices, manage them
 552         according to the standard Linux driver model, and perform
 553         input/output operations.
 554         At this time, only "master" side interfaces are supported,
 555         where Linux talks to SPI peripherals and does not implement
 556         such a peripheral itself.
 557         (Interfaces to support implementing SPI slaves would
 558         necessarily look different.)
 559   </para>
 560   <para>
 561         The programming interface is structured around two kinds of driver,
 562         and two kinds of device.
 563         A "Controller Driver" abstracts the controller hardware, which may
 564         be as simple as a set of GPIO pins or as complex as a pair of FIFOs
 565         connected to dual DMA engines on the other side of the SPI shift
 566         register (maximizing throughput).  Such drivers bridge between
 567         whatever bus they sit on (often the platform bus) and SPI, and
 568         expose the SPI side of their device as a
 569         <structname>struct spi_master</structname>.
 570         SPI devices are children of that master, represented as a
 571         <structname>struct spi_device</structname> and manufactured from
 572         <structname>struct spi_board_info</structname> descriptors which
 573         are usually provided by board-specific initialization code.
 574         A <structname>struct spi_driver</structname> is called a
 575         "Protocol Driver", and is bound to a spi_device using normal
 576         driver model calls.
 577   </para>
 578   <para>
 579         The I/O model is a set of queued messages.  Protocol drivers
 580         submit one or more <structname>struct spi_message</structname>
 581         objects, which are processed and completed asynchronously.
 582         (There are synchronous wrappers, however.)  Messages are
 583         built from one or more <structname>struct spi_transfer</structname>
 584         objects, each of which wraps a full duplex SPI transfer.
 585         A variety of protocol tweaking options are needed, because
 586         different chips adopt very different policies for how they
 587         use the bits transferred with SPI.
 588   </para>
 589 !Iinclude/linux/spi/spi.h
 590 !Fdrivers/spi/spi.c spi_register_board_info
 591 !Edrivers/spi/spi.c
 592   </chapter>
 593
 594   <chapter id="i2c">
 595      <title>I<superscript>2</superscript>C and SMBus Subsystem</title>
 596
 597      <para>
 598         I<superscript>2</superscript>C (or without fancy typography, "I2C")
 599         is an acronym for the "Inter-IC" bus, a simple bus protocol which is
 600         widely used where low data rate communications suffice.
 601         Since it's also a licensed trademark, some vendors use another
 602         name (such as "Two-Wire Interface", TWI) for the same bus.
 603         I2C only needs two signals (SCL for clock, SDA for data), conserving
 604         board real estate and minimizing signal quality issues.
 605         Most I2C devices use seven bit addresses, and bus speeds of up
 606         to 400 kHz; there's a high speed extension (3.4 MHz) that's not yet
 607         found wide use.
 608         I2C is a multi-master bus; open drain signaling is used to
 609         arbitrate between masters, as well as to handshake and to
 610         synchronize clocks from slower clients.
 611      </para>
 612
 613      <para>
 614         The Linux I2C programming interfaces support only the master
 615         side of bus interactions, not the slave side.
 616         The programming interface is structured around two kinds of driver,
 617         and two kinds of device.
 618         An I2C "Adapter Driver" abstracts the controller hardware; it binds
 619         to a physical device (perhaps a PCI device or platform_device) and
 620         exposes a <structname>struct i2c_adapter</structname> representing
 621         each I2C bus segment it manages.
 622         On each I2C bus segment will be I2C devices represented by a
 623         <structname>struct i2c_client</structname>.  Those devices will
 624         be bound to a <structname>struct i2c_driver</structname>,
 625         which should follow the standard Linux driver model.
 626         (At this writing, a legacy model is more widely used.)
 627         There are functions to perform various I2C protocol operations; at
 628         this writing all such functions are usable only from task context.
 629      </para>
 630
 631      <para>
 632         The System Management Bus (SMBus) is a sibling protocol.  Most SMBus
 633         systems are also I2C conformant.  The electrical constraints are
 634         tighter for SMBus, and it standardizes particular protocol messages
 635         and idioms.  Controllers that support I2C can also support most
 636         SMBus operations, but SMBus controllers don't support all the protocol
 637         options that an I2C controller will.
 638         There are functions to perform various SMBus protocol operations,
 639         either using I2C primitives or by issuing SMBus commands to
 640         i2c_adapter devices which don't support those I2C operations.
 641      </para>
 642
 643 !Iinclude/linux/i2c.h
 644 !Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info
 645 !Edrivers/i2c/i2c-core.c
 646   </chapter>
 647
 648   <chapter id="clk">
 649      <title>Clock Framework</title>
 650
 651      <para>
 652         The clock framework defines programming interfaces to support
 653         software management of the system clock tree.
 654         This framework is widely used with System-On-Chip (SOC) platforms
 655         to support power management and various devices which may need
 656         custom clock rates.
 657         Note that these "clocks" don't relate to timekeeping or real
 658         time clocks (RTCs), each of which have separate frameworks.
 659         These <structname>struct clk</structname> instances may be used
 660         to manage for example a 96 MHz signal that is used to shift bits
 661         into and out of peripherals or busses, or otherwise trigger
 662         synchronous state machine transitions in system hardware.
 663      </para>
 664
 665      <para>
 666         Power management is supported by explicit software clock gating:
 667         unused clocks are disabled, so the system doesn't waste power
 668         changing the state of transistors that aren't in active use.
 669         On some systems this may be backed by hardware clock gating,
 670         where clocks are gated without being disabled in software.
 671         Sections of chips that are powered but not clocked may be able
 672         to retain their last state.
 673         This low power state is often called a <emphasis>retention
 674         mode</emphasis>.
 675         This mode still incurs leakage currents, especially with finer
 676         circuit geometries, but for CMOS circuits power is mostly used
 677         by clocked state changes.
 678      </para>
 679
 680      <para>
 681         Power-aware drivers only enable their clocks when the device
 682         they manage is in active use.  Also, system sleep states often
 683         differ according to which clock domains are active:  while a
 684         "standby" state may allow wakeup from several active domains, a
 685         "mem" (suspend-to-RAM) state may require a more wholesale shutdown
 686         of clocks derived from higher speed PLLs and oscillators, limiting
 687         the number of possible wakeup event sources.  A driver's suspend
 688         method may need to be aware of system-specific clock constraints
 689         on the target sleep state.
 690      </para>
 691
 692      <para>
 693         Some platforms support programmable clock generators.  These
 694         can be used by external chips of various kinds, such as other
 695         CPUs, multimedia codecs, and devices with strict requirements
 696         for interface clocking.
 697      </para>
 698
 699 !Iinclude/linux/clk.h
 700   </chapter>
 701
 702 </book>