The Linux Kernel API
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For more details see the file COPYING in the source
distribution of Linux.
Driver Basics
Driver Entry and Exit points
!Iinclude/linux/init.h
Atomic and pointer manipulation
!Iinclude/asm-x86/atomic_32.h
!Iinclude/asm-x86/unaligned.h
Delaying, scheduling, and timer routines
!Iinclude/linux/sched.h
!Ekernel/sched.c
!Ekernel/timer.c
High-resolution timers
!Iinclude/linux/ktime.h
!Iinclude/linux/hrtimer.h
!Ekernel/hrtimer.c
Workqueues and Kevents
!Ekernel/workqueue.c
Internal Functions
!Ikernel/exit.c
!Ikernel/signal.c
!Iinclude/linux/kthread.h
!Ekernel/kthread.c
Kernel objects manipulation
!Elib/kobject.c
Kernel utility functions
!Iinclude/linux/kernel.h
!Ekernel/printk.c
!Ekernel/panic.c
!Ekernel/sys.c
!Ekernel/rcupdate.c
Device Resource Management
!Edrivers/base/devres.c
Data Types
Doubly Linked Lists
!Iinclude/linux/list.h
Basic C Library Functions
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.
String Conversions
!Ilib/vsprintf.c
!Elib/vsprintf.c
String Manipulation
!Elib/string.c
Bit Operations
!Iinclude/asm-x86/bitops.h
Basic Kernel Library Functions
The Linux kernel provides more basic utility functions.
Bitmap Operations
!Elib/bitmap.c
!Ilib/bitmap.c
Command-line Parsing
!Elib/cmdline.c
CRC Functions
!Elib/crc7.c
!Elib/crc16.c
!Elib/crc-itu-t.c
!Elib/crc32.c
!Elib/crc-ccitt.c
Memory Management in Linux
The Slab Cache
!Iinclude/linux/slab.h
!Emm/slab.c
User Space Memory Access
!Iinclude/asm-x86/uaccess_32.h
!Earch/x86/lib/usercopy_32.c
More Memory Management Functions
!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
Kernel IPC facilities
IPC utilities
!Iipc/util.c
FIFO Buffer
kfifo interface
!Iinclude/linux/kfifo.h
!Ekernel/kfifo.c
relay interface support
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.
relay interface
!Ekernel/relay.c
!Ikernel/relay.c
Module Support
Module Loading
!Ekernel/kmod.c
Inter Module support
Refer to the file kernel/module.c for more information.
Hardware Interfaces
Interrupt Handling
!Ekernel/irq/manage.c
DMA Channels
!Ekernel/dma.c
Resources Management
!Ikernel/resource.c
!Ekernel/resource.c
MTRR Handling
!Earch/x86/kernel/cpu/mtrr/main.c
PCI Support Library
!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
!Edrivers/pci/probe.c
!Edrivers/pci/rom.c
PCI Hotplug Support Library
!Edrivers/pci/hotplug/pci_hotplug_core.c
MCA Architecture
MCA Device Functions
Refer to the file arch/x86/kernel/mca_32.c for more information.
MCA Bus DMA
!Iinclude/asm-x86/mca_dma.h
Firmware Interfaces
DMI Interfaces
!Edrivers/firmware/dmi_scan.c
EDD Interfaces
!Idrivers/firmware/edd.c
Security Framework
!Isecurity/security.c
Audit Interfaces
!Ekernel/audit.c
!Ikernel/auditsc.c
!Ikernel/auditfilter.c
Accounting Framework
!Ikernel/acct.c
Device drivers infrastructure
Device Drivers Base
!Edrivers/base/driver.c
!Edrivers/base/core.c
!Edrivers/base/class.c
!Edrivers/base/firmware_class.c
!Edrivers/base/transport_class.c
!Edrivers/base/sys.c
!Edrivers/base/platform.c
!Edrivers/base/bus.c
Device Drivers Power Management
!Edrivers/base/power/main.c
Device Drivers ACPI Support
!Edrivers/acpi/scan.c
!Idrivers/acpi/scan.c
Device drivers PnP support
!Idrivers/pnp/core.c
!Edrivers/pnp/card.c
!Idrivers/pnp/driver.c
!Edrivers/pnp/manager.c
!Edrivers/pnp/support.c
Userspace IO devices
!Edrivers/uio/uio.c
!Iinclude/linux/uio_driver.h
Block Devices
!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
Char devices
!Efs/char_dev.c
Miscellaneous Devices
!Edrivers/char/misc.c
Parallel Port Devices
!Iinclude/linux/parport.h
!Edrivers/parport/ieee1284.c
!Edrivers/parport/share.c
!Idrivers/parport/daisy.c
Message-based devices
Fusion message devices
!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
I2O message devices
!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
Sound Devices
!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
16x50 UART Driver
!Iinclude/linux/serial_core.h
!Edrivers/serial/serial_core.c
!Edrivers/serial/8250.c
Frame Buffer Library
The frame buffer drivers depend heavily on four data structures.
These structures are declared in include/linux/fb.h. They are
fb_info, fb_var_screeninfo, fb_fix_screeninfo and fb_monospecs.
The last three can be made available to and from userland.
fb_info defines the current state of a particular video card.
Inside fb_info, there exists a fb_ops structure which is a
collection of needed functions to make fbdev and fbcon work.
fb_info is only visible to the kernel.
fb_var_screeninfo is used to describe the features of a video card
that are user defined. With fb_var_screeninfo, things such as
depth and the resolution may be defined.
The next structure is fb_fix_screeninfo. This defines the
properties of a card that are created when a mode is set and can't
be changed otherwise. A good example of this is the start of the
frame buffer memory. This "locks" the address of the frame buffer
memory, so that it cannot be changed or moved.
The last structure is fb_monospecs. In the old API, there was
little importance for fb_monospecs. This allowed for forbidden things
such as setting a mode of 800x600 on a fix frequency monitor. With
the new API, fb_monospecs prevents such things, and if used
correctly, can prevent a monitor from being cooked. fb_monospecs
will not be useful until kernels 2.5.x.
Frame Buffer Memory
!Edrivers/video/fbmem.c
Frame Buffer Colormap
!Edrivers/video/fbcmap.c
Frame Buffer Video Mode Database
!Idrivers/video/modedb.c
!Edrivers/video/modedb.c
Frame Buffer Macintosh Video Mode Database
!Edrivers/video/macmodes.c
Frame Buffer Fonts
Refer to the file drivers/video/console/fonts.c for more information.
Input Subsystem
!Iinclude/linux/input.h
!Edrivers/input/input.c
!Edrivers/input/ff-core.c
!Edrivers/input/ff-memless.c
Serial Peripheral Interface (SPI)
SPI is the "Serial Peripheral Interface", widely used with
embedded systems because it is a simple and efficient
interface: basically a multiplexed shift register.
Its three signal wires hold a clock (SCK, often in the range
of 1-20 MHz), a "Master Out, Slave In" (MOSI) data line, and
a "Master In, Slave Out" (MISO) data line.
SPI is a full duplex protocol; for each bit shifted out the
MOSI line (one per clock) another is shifted in on the MISO line.
Those bits are assembled into words of various sizes on the
way to and from system memory.
An additional chipselect line is usually active-low (nCS);
four signals are normally used for each peripheral, plus
sometimes an interrupt.
The SPI bus facilities listed here provide a generalized
interface to declare SPI busses and devices, manage them
according to the standard Linux driver model, and perform
input/output operations.
At this time, only "master" side interfaces are supported,
where Linux talks to SPI peripherals and does not implement
such a peripheral itself.
(Interfaces to support implementing SPI slaves would
necessarily look different.)
The programming interface is structured around two kinds of driver,
and two kinds of device.
A "Controller Driver" abstracts the controller hardware, which may
be as simple as a set of GPIO pins or as complex as a pair of FIFOs
connected to dual DMA engines on the other side of the SPI shift
register (maximizing throughput). Such drivers bridge between
whatever bus they sit on (often the platform bus) and SPI, and
expose the SPI side of their device as a
struct spi_master.
SPI devices are children of that master, represented as a
struct spi_device and manufactured from
struct spi_board_info descriptors which
are usually provided by board-specific initialization code.
A struct spi_driver is called a
"Protocol Driver", and is bound to a spi_device using normal
driver model calls.
The I/O model is a set of queued messages. Protocol drivers
submit one or more struct spi_message
objects, which are processed and completed asynchronously.
(There are synchronous wrappers, however.) Messages are
built from one or more struct spi_transfer
objects, each of which wraps a full duplex SPI transfer.
A variety of protocol tweaking options are needed, because
different chips adopt very different policies for how they
use the bits transferred with SPI.
!Iinclude/linux/spi/spi.h
!Fdrivers/spi/spi.c spi_register_board_info
!Edrivers/spi/spi.c
I<superscript>2</superscript>C and SMBus Subsystem
I2C (or without fancy typography, "I2C")
is an acronym for the "Inter-IC" bus, a simple bus protocol which is
widely used where low data rate communications suffice.
Since it's also a licensed trademark, some vendors use another
name (such as "Two-Wire Interface", TWI) for the same bus.
I2C only needs two signals (SCL for clock, SDA for data), conserving
board real estate and minimizing signal quality issues.
Most I2C devices use seven bit addresses, and bus speeds of up
to 400 kHz; there's a high speed extension (3.4 MHz) that's not yet
found wide use.
I2C is a multi-master bus; open drain signaling is used to
arbitrate between masters, as well as to handshake and to
synchronize clocks from slower clients.
The Linux I2C programming interfaces support only the master
side of bus interactions, not the slave side.
The programming interface is structured around two kinds of driver,
and two kinds of device.
An I2C "Adapter Driver" abstracts the controller hardware; it binds
to a physical device (perhaps a PCI device or platform_device) and
exposes a struct i2c_adapter representing
each I2C bus segment it manages.
On each I2C bus segment will be I2C devices represented by a
struct i2c_client. Those devices will
be bound to a struct i2c_driver,
which should follow the standard Linux driver model.
(At this writing, a legacy model is more widely used.)
There are functions to perform various I2C protocol operations; at
this writing all such functions are usable only from task context.
The System Management Bus (SMBus) is a sibling protocol. Most SMBus
systems are also I2C conformant. The electrical constraints are
tighter for SMBus, and it standardizes particular protocol messages
and idioms. Controllers that support I2C can also support most
SMBus operations, but SMBus controllers don't support all the protocol
options that an I2C controller will.
There are functions to perform various SMBus protocol operations,
either using I2C primitives or by issuing SMBus commands to
i2c_adapter devices which don't support those I2C operations.
!Iinclude/linux/i2c.h
!Fdrivers/i2c/i2c-boardinfo.c i2c_register_board_info
!Edrivers/i2c/i2c-core.c
Clock Framework
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 struct clk 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.
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 retention
mode.
This mode still incurs leakage currents, especially with finer
circuit geometries, but for CMOS circuits power is mostly used
by clocked state changes.
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.
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.
!Iinclude/linux/clk.h