# The Linux Kernel API

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.

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.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

For more details see the file COPYING in the source distribution of Linux.

## 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_32.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

## Power Management

!Ekernel/power/pm.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

## 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

## I2C 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