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Linux Kernel — I/O Architecture & Device Drivers

How the kernel talks to hardware: character vs block vs network devices, major/minor & /dev, the device model (bus→device→driver, probe, hotplug) via sysfs & udev, and how a driver moves data — MMIO registers, interrupts, and DMA — plus the character-driver file_operations/ioctl interface.

How the kernel talks to hardware — the device model, the three device classes, and how a driver actually moves data (registers, interrupts, DMA). Original, interview-focused notes.

Devices are exposed as files under /dev and organized by the device model under /sys. A driver binds to a device on a bus, registers a set of operations, and moves data via memory-mapped registers + interrupts + DMA.

Three classes of device

Each device node has a major number (which driver) and minor (which instance). ls -l /dev shows c/b and the major,minor.

The device model: sysfs & udev

• The kernel models hardware as a tree of bus → device → driver, exposed under /sys. A driver registers with its bus; when a matching device appears, the bus calls the driver's probe() to bind them — that's hotplug.
• udev (userspace) listens for kernel uevents and creates /dev nodes, applies naming rules (persistent NIC/disk names), and runs actions on plug/unplug.
• Modules: drivers load on demand (modprobe); lsmod/modinfo inspect them.

How a driver moves data

• Register access: device registers are reached via MMIO (mapped into the kernel address space, ioremap) or legacy port I/O. The driver reads status / writes commands here.
• Interrupts: the device raises an IRQ when work is done; the handler (top half) acknowledges and schedules the rest (bottom half). See Interrupts.
• DMA: for anything sizeable the device does Direct Memory Access — it transfers data to/from RAM itself, and interrupts only when the whole transfer is done. The CPU sets up a DMA descriptor (a physically-addressed, mapped buffer via the DMA API / IOMMU) and stays out of the copy. This is why 10G+ NICs and NVMe don't melt the CPU.

The efficient pattern: DMA moves the bytes, one interrupt signals completion, a bottom half processes the result — minimal CPU per byte. PIO (CPU copies each word) is only for tiny/slow devices.

The character-driver interface

A char driver registers file_operations (open, read, write, ioctl, mmap) so its /dev node behaves like a file. ioctl is the catch-all for device-specific control that doesn't fit read/write.