sysfs is a virtual file system provided by the Linux kernel. By using virtual files, sysfs exports information about various kernel subsystems, hardware devices and associated device drivers from the kernel's device model to user space. In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuring.
sysfs provides similar functionality as the sysctl mechanism found in BSD operating systems, with the difference that sysfs is implemented as a virtual file system instead of being a purpose-built kernel mechanism.
During the 2.5 development cycle, the Linux driver model was introduced to fix several shortcomings of version 2.4:
- No unified method of representing driver-device relationships existed.
- There was no generic hotplug mechanism.
- procfs was cluttered with lots of non-process information.
Sysfs is designed to export the information present in the device tree which would then no longer clutter up procfs. It was written by Patrick Mochel. Maneesh Soni later wrote the sysfs backing store patch to reduce memory usage on large systems.
During the next year of 2.5 development, the infrastructural capabilities of the driver model and driverfs, formerly called ddfs, began to prove useful to other subsystems. kobjects were developed to provide a central object management mechanism and driverfs was renamed to sysfs to represent its subsystem agnosticism.
Sysfs is mounted under the /sys mount point.
- Exports information about PCI devices.
- Contains both USB devices and USB hosts. USB core is also very important.
- S/390 buses
- As the S/390 architecture contains devices not found elsewhere, special buses have been created:
- css: Contains subchannels (currently the only driver provided is for I/O subchannels).
- ccw: Contains channel attached devices (driven by CCWs).
- ccwgroup: Artificial devices, created by the user and consisting of ccw devices. Replaces some of the 2.4 chandev functionality.
- iucv: Artificial devices like netiucv devices which use VM's IUCV interface.
Sysfs and userspace
Sysfs is used by several utilities to access information about hardware and its driver (kernel modules) such as udev or HAL. Scripts have been written to access information previously obtained via procfs, and some scripts configure device drivers and devices via their attributes.
- Torvalds, Linus (18 October 2002). "Linux v2.5.44 - and offline for a week".
- Torvalds, Linus (4 November 2002). "Linux v2.5.46".
- Mochel, Patrick (17 October 2001). "[RFC] New Driver Model for 2.5".
- Jansen, Tim (1 November 2001). "Re: [PATCH] 2.5 PROPOSAL: Replacement for current /proc of shit.".
- Driver model overview from the LWN porting to 2.6 series
- kobjects and sysfs from the LWN porting to 2.6 series
- The sysfs Filesystem, OLS'05
- Documentation/filesystems/sysfs.txt Linux kernel documentation for sysfs