English

简体中文

English

简体中文

Appearance

1. Why Do We Need to Know the Device Location in sysfs? ​

When learning driver development, you often need to locate "your device" in the sysfs (/sys) directory tree. Understanding the device mapping relationship helps you:

  • Observe where the device you are registering or debugging appears;
  • Understand device organization, parent-child relationships, and physical/logical classification;
  • Proficiently read and manipulate various device attribute files.

2. Main Entry Points for Device Mapping in sysfs ​

The Linux device model automatically generates dedicated sysfs directories and attributes for each device, primarily appearing in the following paths:

  • /sys/devices/

    Maps the real device topology (tree structure, representing physical or virtual connections).

  • /sys/class/

    Logical device directories organized by functional category, such as net (network devices), block (block devices), leds (LEDs), input (input devices), etc.

  • /sys/bus/<bus-name>/devices/

    Device list grouped by bus membership. For example, all devices under the platform bus are listed in /sys/bus/platform/devices/.

  • /sys/bus/<bus-name>/drivers/

    Registered drivers grouped by bus membership.

3. Practical Examples ​

1. Real Topology Structure ​

  • Linux organizes all devices in a hierarchical manner under /sys/devices/ based on physical or virtual "bus relationships". For example:

    bash
    /sys/devices/
    platform/
        soc/
            1c28000.i2c/
                1c28000.i2c:devicexx/
    pci0000:00/
        0000:00:14.0/
            0000:01:00.0/
    virtual/
        net/
            lo/

  • You can recursively view all registered physical (or virtual) devices.

  • Each device is a directory containing numerous attribute files (such as uevent, power, subsystem, driver, resource registers, etc.).

2. Grouped by Function ​

  • Under /sys/class/, devices are categorized by "functional type", for example:
bash
/sys/class/net/       # Network devices
/sys/class/leds/      # LED devices
/sys/class/input/     # Input devices
/sys/class/block/     # Block devices (disks/partitions)
  • The "devices" under these directories are often symlinks — pointing to the real device directories in /sys/devices/, facilitating unified management by function.

3. Grouped by Bus ​

  • /sys/bus/usb/devices/: All recognized devices under the USB bus.
  • /sys/bus/platform/devices/: All devices under the platform bus (embedded CPU/chip internal resources, etc.).
  • These directories are also usually symlinks, pointing to actual device nodes under /sys/devices/.

For example:

4. Quickly Locating Your Device ​

1. Determine Device Type ​

  • For example, when developing an LED driver or looking for LED nodes:
bash
ls /sys/class/leds/
# List all LED names

2. Trace Symlinks to Physical Devices ​

  • Taking LED "work" as an example:
bash
ls -l /sys/class/leds/work

Output:

bash
lrwxrwxrwx 1 root root 0 Dec 16 18:42 /sys/class/leds/work -> ../../devices/platform/leds/leds/work

This indicates that /sys/class/leds/work actually points to /sys/devices/platform/leds/leds/work

  • You can cd into that directory to view all detailed attributes, drivers, parent-child relationships bound to that physical device.

3. Search by Bus ​

  • If you are developing an I2C device, you can use:
bash
ls /sys/bus/i2c/devices/
  • Each entry is an I2C device node link, pointing to the corresponding location in /sys/devices/.

5. Practical Examples with Commands ​

  • Find all character devices and quickly locate parent devices/drivers:
bash
# Find /dev/tty0
udevadm info -a -p $(udevadm info -q path -n /dev/tty0)
  • Check which driver a device belongs to:
bash
ls -l /sys/class/net/eth0/device/driver

As shown in the figure, it is the rk_gmac-dwmac driver

6. Diagram ​

  • /sys
    • Root node, i.e., the mount point of the sysfs filesystem.
  • /sys/devices/...
    • Represents the "real device topology tree"
    • Path: /sys/devices/platform/soc/.../work
    • This shows all real devices and their physical/virtual hierarchy.
    • For example, platform devices common in embedded devices, peripherals connected internally to the SoC (System on Chip), etc.
  • /sys/class/...
    • Represents the "functional classification index"
    • Path: /sys/class/leds/work
    • This is classified by the device's "functional type" — for example, all LED devices, all network devices (net), all block devices (block), etc.
    • Device names in these directories are usually symlinks, actually pointing to real device nodes under /devices.
    • Convenient for users to find devices by purpose.
  • /sys/bus/...
    • Represents "bus organization and device-driver relationships"
    • Example path: /sys/bus/platform/devices/work
    • Buses are a management structure in Linux, such as platform, i2c, PCI, USB, etc.
    • Each bus has its own devices and drivers subdirectories.
    • /bus/platform/devices/work is also a symlink, pointing to the actual device at /devices/.../work.

7. Summary ​

  • A device usually has multiple entry paths in sysfs (physical topology, functional category, bus membership).
  • /sys/devices/ is the "real physical" existence of devices, while class/bus are classifications or indexes (symlinks).
  • Proficiently finding the full path of your device in sysfs is a fundamental skill for driver development and debugging.

Last updated: