Sysfs

{{Short description|Pseudo file system provided by the Linux kernel}}

sysfs is a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files.{{cite web|url=https://www.kernel.org/doc/Documentation/filesystems/sysfs.txt|title=sysfs - _The_ filesystem for exporting kernel objects|author=Patrick Mochel and Mike Murphy|publisher=kernel.org}} In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuration.

sysfs provides functionality similar to 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, and that, in Linux, sysctl configuration parameters are made available at /proc/sys/ as part of procfs, not sysfs which is mounted at /sys/.{{cite web|quote=sysctl is used to modify kernel parameters at runtime. The parameters available are those listed under /proc/sys/. Procfs is required for sysctl support in Linux. You can use sysctl to both read and write sysctl data.|url=https://www.freebsd.org/cgi/man.cgi?query=sysctl&sektion=8&manpath=SuSE+Linux%2fi386+11.3|publisher=FreeBSD|author=SUSE|title=sysctl man page}}

History

During the 2.5 development cycle, the Linux driver model was introduced to fix the following shortcomings of version 2.4:

No unified method of representing driver-device relationships existed.
There was no generic hotplug mechanism.
procfs was cluttered with non-process information.

Sysfs was designed to export the information present in the device tree which would then no longer clutter up procfs. It was written by Patrick Mochel.{{cite web |first=Linus |last=Torvalds |title=Linux v2.5.44 - and offline for a week |date=18 October 2002 |url=https://lkml.org/lkml/2002/10/19/8}}{{cite web |first=Linus |last=Torvalds |title=Linux v2.5.46 |date=4 November 2002 |url=https://lkml.org/lkml/2002/11/4/213}} 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 began to prove useful to other subsystems.{{cite web |first=Patrick |last=Mochel |title=[RFC] New Driver Model for 2.5 |date=17 October 2001 |url=https://lkml.org/lkml/2001/10/17/147}}{{cite web |first=Tim |last=Jansen |title=Re: [PATCH] 2.5 PROPOSAL: Replacement for current /proc of shit. |date=1 November 2001 |url=https://lkml.org/lkml/2001/11/1/38}} 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 {{mono|/sys}} mount point. If it is not mounted automatically during initialization, it can be mounted manually using the mount command: mount -t sysfs sysfs /sys.

Supported buses

; ACPI

: Exports information about ACPI devices.

; PCI

: Exports information about PCI and PCI Express devices.

; PCI Express

: Exports information about PCI Express devices.

; USB

: Exports information about USB devices.

; SCSI

: Exports information about mass storage devices, including USB, SATA and NVMe interfaces.{{Cite web|title=SCSI Interfaces Guide — The Linux Kernel documentation|url=https://www.kernel.org/doc/html/latest/driver-api/scsi.html|access-date=2020-11-13|website=www.kernel.org}}

; 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.

References

External links

[https://lwn.net/Articles/31185/ Driver model overview from the LWN porting to 2.6 series]
[https://lwn.net/Articles/54651/ kobjects and sysfs from the LWN porting to 2.6 series]
[http://wiki.debian.org/ramfs Ramfs]
[https://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf The sysfs Filesystem, OLS'05]
[http://kernel.org/doc/Documentation/filesystems/sysfs.txt Documentation/filesystems/sysfs.txt] Linux kernel documentation for sysfs

Category:Free special-purpose file systems

Category:Interfaces of the Linux kernel

Category:Linux kernel features

Category:Pseudo file systems supported by the Linux kernel