Kernel Terminology Glossary

Quick reference for terms used throughout this guide.

A

Atomic context

Code path where sleeping is not allowed (interrupt handlers, spinlock-held regions). Use GFP_ATOMIC for allocations.

Autosuspend

Runtime PM feature that delays suspend after the last pm_runtime_put(), avoiding rapid suspend/resume cycles.

B

BAR (Base Address Register)

PCI configuration register that defines a memory or I/O region the device exposes to the system.

bio

Block I/O structure representing a segment of contiguous sectors. Multiple bios may be merged into a request.

Bottom half

Deferred work from an interrupt handler (tasklets, workqueues, threaded IRQs). Runs with interrupts enabled.

Bus mastering

Ability of a PCI device to initiate DMA transfers. Enabled via pci_set_master().

C

Carrier

Network link status. netif_carrier_on() indicates link is up; netif_carrier_off() indicates link is down.

cdev

Character device structure. Registered with cdev_add() to associate a device number with file operations.

Coherent DMA

DMA memory that is automatically synchronized between CPU and device. Allocated with dma_alloc_coherent().

Completion

Synchronization primitive for waiting until another thread signals an event. See wait_for_completion().

container_of

Macro to get the containing structure from a pointer to one of its members.

D

Deferred probe

When probe() returns -EPROBE_DEFER, the driver will be retried later after other drivers load.

Device Tree (DT)

Data structure describing hardware topology. Parsed at boot to create platform devices.

devres (Device-managed resources)

Resources allocated with devm_* functions that are automatically freed when the device is removed.

DMA (Direct Memory Access)

Hardware mechanism allowing devices to transfer data to/from memory without CPU involvement.

E

Endpoint

USB communication channel. Each endpoint has a direction (IN/OUT) and type (bulk, interrupt, control, isochronous).

EXPORT_SYMBOL

Macro that makes a kernel symbol available to loadable modules.

F

file_operations (fops)

Structure containing function pointers for file operations (read, write, ioctl, etc.) on a character device.

Fixed-link

Network configuration without a PHY chip, used for MAC-to-MAC or MAC-to-switch connections.

G

gendisk

Structure representing a block device (disk). Contains capacity, name, and link to request queue.

GFP flags

Get Free Pages flags controlling memory allocation behavior (e.g., GFP_KERNEL, GFP_ATOMIC).

gpio_chip

Structure for GPIO controller drivers, defining operations for requesting, setting, and reading GPIOs.

H

HZ

Kernel timer frequency (jiffies per second). Typically 100, 250, or 1000.

HWMON

Hardware monitoring subsystem for temperature, voltage, and fan sensors.

I

IIO (Industrial I/O)

Subsystem for ADCs, DACs, accelerometers, and other data acquisition devices.

Interface (USB)

Logical function within a USB device. Drivers bind to interfaces, not devices.

ioremap

Function to map physical device memory into kernel virtual address space.

IRQ

Interrupt Request. Hardware signal that interrupts CPU to handle an event.

J

Jiffies

Kernel time counter incremented HZ times per second.

K

kmap_local_page

Temporarily maps a page to kernel address space. Use for highmem pages.

kmalloc

Kernel memory allocator for small, physically contiguous allocations.

kobject

Base object in the kernel device model. Provides reference counting and sysfs representation.

kref

Kernel reference counter. Calls a release function when count reaches zero.

L

Lockdep

Kernel lock validator that detects potential deadlocks at runtime.

M

Major/minor number

Device number pair. Major identifies the driver; minor identifies specific device.

MMIO (Memory-Mapped I/O)

Hardware registers accessed through memory read/write operations.

MSI/MSI-X (Message Signaled Interrupts)

Modern interrupt mechanism for PCI devices, eliminating shared IRQ issues.

Mutex

Sleeping lock for protecting critical sections. Cannot be held in atomic context.

N

NAPI (New API)

Network polling mechanism that switches from interrupts to polling under high load.

net_device

Structure representing a network interface (eth0, wlan0, etc.).

net_device_ops

Operations structure for network drivers (open, stop, start_xmit, etc.).

O

of_* functions

Device Tree parsing functions (of_property_read_u32, of_get_named_gpio, etc.).

P

PHY

Physical layer transceiver chip in Ethernet networks. Managed via PHYLIB.

Platform device

Non-discoverable device described in Device Tree or board files.

Probe

Driver callback called when a matching device is found. Initializes the device.

R

RCU (Read-Copy-Update)

Synchronization mechanism optimized for read-heavy workloads.

Regmap

API for accessing device registers over I2C, SPI, or memory-mapped I/O.

Request (block)

Block layer structure representing an I/O operation. Contains one or more bios.

Runtime PM

Power management for individual devices while the system is running.

S

Scatter-gather

DMA technique using multiple non-contiguous memory segments.

Sector

Basic unit of block device addressing (historically 512 bytes).

sk_buff (skb)

Socket buffer structure representing a network packet.

Spinlock

Busy-wait lock for short critical sections. Can be held in interrupt context.

Streaming DMA

DMA mapping that requires explicit sync operations. More efficient than coherent for one-time transfers.

Sysfs

Virtual filesystem exposing kernel objects and device attributes.

T

Tag set

blk-mq structure defining hardware queue configuration.

Tasklet

Soft IRQ mechanism for deferring work from interrupt handlers.

Top half

The interrupt handler itself. Runs with interrupts disabled, should be fast.

U

URB (USB Request Block)

Structure for USB data transfers. Submitted asynchronously, completed via callback.

udev

User-space device manager that creates device nodes and handles hotplug events.

V

VID/PID

Vendor ID and Product ID. Used to identify USB and PCI devices.

vmalloc

Kernel memory allocator for large, virtually contiguous (but physically scattered) allocations.

W

Wakeup source

Device capable of waking the system from suspend.

Work queue

Mechanism for deferring work to a kernel thread context.

Further Reading

• Kernel Glossary - Official kernel glossary
• Linux Kernel Documentation - Comprehensive documentation