Overview

  • Kernel Space: This is where the Linux kernel executes and provides low-level access to hardware, system memory management, process scheduling, and device drivers. Kernel space has privileged access to system resources and is protected from direct user interference. For example, when a user requests data from a hardware sensor, the kernel driver handles communication with the hardware, processes the request, and returns the data to user space through system calls.
  • User Space: This is where applications and system utilities run. User-space processes operate with restricted privileges and interact with the kernel via system calls, libraries, and IPC mechanisms. For example, a user-space daemon may monitor the watchdog status by writing to /dev/watchdog, or a mobile app may read light intensity from /sys/bus/i2c/devices/1-0039/lux.

Communication Methods between Kernel and User Space

There are several ways to facilitate communication between user space and kernel space in an embedded Linux environment:

  1. System Calls
    • The primary interface between user space and kernel space.
    • Standard POSIX APIs like open(), read(), write(), mmap(), etc.
    • Example: A watchdog daemon calls write(fd, &data, sizeof(data)) to reset the watchdog timer.
  2. ioctl (Input/Output Control)
    • Used for sending custom commands to kernel drivers.
    • Allows direct communication with device drivers.
    • Example: ioctl(fd, CMD, arg);
  3. Proc Filesystem (/proc)
    • A virtual filesystem providing a mechanism to read/write kernel information.
    • Example: /proc/sys/kernel/ for system configurations.
  4. Sysfs (/sys)
    • A virtual filesystem primarily used for exporting kernel object attributes.
    • Example: /sys/class/gpio/ for GPIO control.
  5. Character and Block Device Drivers
    • User space can interact with device files (/dev/xyz) using standard file operations.
    • Example: echo "data" > /dev/watchdog to prevent system reset.
  6. Netlink Sockets
    • Provides full-duplex communication between kernel and user space.
    • Used for networking-related operations, system monitoring, etc.
    • Example: A kernel module sending system event notifications to a user-space monitoring application.
  7. Message Queues (POSIX & System V)
    • IPC mechanism for exchanging messages between processes.
    • Can be used between a user-space application and kernel-space service.
    • Example: A watchdog driver sending failure notifications to a monitoring daemon.
  8. Shared Memory (mmap())
    • Allows user space and kernel space to share memory regions.
    • Used in high-speed data transfer applications.
    • Example: A video processing pipeline using shared memory for fast frame exchange between kernel-space drivers and user-space applications.
  9. FIFOs (Named Pipes)
    • A simple IPC mechanism that allows data transfer between user and kernel space through files.
  10. Signals
    • Used to notify user-space processes about kernel events asynchronously.
    • Example: kill -SIGUSR1 <pid>.
  11. UPROBE and KPROBE
    • Kernel probes allowing user-space debugging and performance monitoring.
    • Example: Debugging an I2C driver by probing read/write operations.
  12. eBPF (Extended Berkeley Packet Filter)
    • Used for tracing and monitoring without modifying kernel code.

Choosing the Right Communication Mechanism

  • System Health Monitoring: Use Character Devices or ioctl (e.g., /dev/watchdog).
  • Sensor Interfaces: Use Sysfs for simple reads, ioctl for advanced configs.
  • High-Speed Data Transfer: Use Shared Memory (**mmap()**).
  • Event Notifications: Use Netlink Sockets.
  • Device Control: Use ioctl for structured commands.
  • Debugging & Performance Monitoring: Use eBPF.