Purpose of Sysfs

Accessing Kernel Data

  • There has always been a need to access kernel data from system programs
  • /proc was introduced to export process information from kernel
  • ps uses it to print the list of processes in the system

Hardware Information

  • As hardware grew complex, it became necessary to export hardware information to system programs
    • ifconfig
    • lspci
    • lsusb
    • parted

procfs limitations

  • procfs was designed to keep process related information.
  • Eventually got cluttered with lot of non-process information.
  • There was no standard format to represent information.
  • Information was human readable, but not programatically parsable.
  • Examples:
    • /proc/net/packet
    • /proc/net/wireless
    • /proc/asound/cards

Sysfs

  • An in-memory filesystem that exports information from kernel-space to user-space.
  • Exports Kernel datastructures, their attributes and relationships.
  • Examples: network, filesystems, hardware, drivers, memory.
  • Introduced by Patrick Mochel in the Linux 2.5 Kernel in 2003.

sysfs equivalent of /proc/asound/cards

$ cat /proc/asound/cards
 0 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                      HDA Intel HDMI at 0xf7d34000 irq 34
 1 [PCH            ]: HDA-Intel - HDA Intel PCH
                      HDA Intel PCH at 0xf7d30000 irq 32
$ ls /sys/class/sound
card0   card1
$ cat /sys/class/sound/card0/id
HDMI
$ cat /sys/class/sound/card0/device/irq
34

Charateristics of Sysfs

  • Exports information in the form of files and directories
  • Files may support read, write or both operations.
  • Provides user-friendly interfaces which follows the one-item per file rule.
  • Addition and removal of interfaces associated with devices.
figures/sys-char.png

Applications of Sysfs

  • Accessing system information by various user-space applications
  • Controlling and configuring hardware

/sys/ hierarchy

/sys/class

  • class - grouping devices based on functionality like audio cards, network cards, etc
  • Controlling and accessing information about these devices.
  • Similar to Device Manager in Windows.

/sys/class

$ tree /sys/class/
├── block
├── gpio
├── graphics
├── input
├── leds
├── printer
├── rtc
├── sound
├── spi_master
├── thermal
├── tty
figures/device-manager.jpg

/sys/bus

  • Groups devices based on their interface
    • USB
    • I2C
    • PCI
  • Just another way of viewing the devices in the system

Demo: Network Interfaces

  • Information available in /sys/class/net.
  • Download ifconfig.py
  • Output: 
$ python3 ifconfig.py
wlan0   HWaddr ac:d1:b8:09:ab:09
        MTU:1500
        RX packets:216089 errors:0
        TX packets:125334 errors:0
        RX bytes:116026931      TX bytes:20279921

Demo: Backlight

  • Backlight can be controlled via /sys/class/backlight. 
$ cat /sys/class/backlight/intel_backlight/max_brightness
4710
  • Brightness can be controlled by writing value into brightness file.
  • Invalid values are validated by sysfs. 
$ echo 500 > /sys/class/backlight/intel_backlight/brightness
$ echo 5000 > /sys/class/backlight/intel_backlight/brightness
bash: echo: write error: Invalid argument

Demo: RFKill

  • Radio interfaces can be turned on / off using RFKill Switch.
  • Device is accessible from /sys/class/rfkill.
  • Separate switch for WiFi, Bluetooth, etc. 
$ cd /sys/class/rfkill/rfkill0
$ cat name
$ echo 1 > soft   # Activate Kill Switch
$ echo 0 > soft   # Deactivate Kill Switch

ioctl vs sys

  • ioctl and sysfs provide overlapping functionality
  • Rule to choose between ioctl and sysfs
    • Do not create device file, just to provide ioctl
    • Use sysfs instead

Questions