Appendix F.
Additional Information for S/390 and zSeries
Users
F.1. The sysfs File System
The Linux 2.6 kernel introduces the sysfs file system. The sysfs file system is described as a union
of the proc, devfs, and devpty file systems. The sysfs file system enumerates the devices and
busses attached to the system into a file system hierarchy that can be accessed from user space. It
is designed to handle the device and driver specific options that have previously resided in /proc/,
and encompass the dynamic device addition previously offered by devfs. At this early point in the
implementation of sysfs, there are many drivers and utilities that still refer to the older proc entries.
However, it is understood that sysfs is the way of the future.
The sysfs file system is mounted at /sys/ and contains directories that organize the devices attached
to the system in several different ways. The /sysfs/ subdirectories include:
1. The /devices/ directory
This directory contains the /css0/ directory. Its subdirectories represent all the subchannels
detected by the Linux kernel. Subchannel directories are named in the form 0.0.nnnn where
nnnn is the subchannel number in hex between 0 and ffff. Subchannel directories in turn contain
status files and another subdirectory which represents the actual device. The device directory is
named 0.0.xxxx where xxxx is the unit address for the device. The /devices/ directory
also contains status information as well as configuration options for the device.
2. The /bus/ directory
This contains a /ccw/ subdirectory and a /ccwgroup/ subdirectory. CCW devices are accessed
using channel command words. Devices in the /ccw/ directory only use one subchannel on the
mainframe channel subsystem. CCW group devices are also accessed with channel command
words, but they use more than one subchannel per device. For example, a 3390-3 DASD device
uses one subchannel, while a QDIO network connection for an OSA adapter uses three sub-
channels. The /ccw/ and the /ccwgroup/ directories both contain directories called devices
and drivers:
The /devices/ directory contains a symbolic link to the device directories in the
/sys/devices/css0/ directory.
The /drivers/ directory contains directories for each device driver currently loaded on the
system. The zFCP driver has a directory here. The /driver/ directory contains settings for the
device driver, as well as symbolic links to the devices it is using (in the /sys/devices/css0/
directory).
3. The /class/ directory
This contains directories that group together similar devices such as ttys, SCSI tape drives,
network devices, and other miscellaneous devices.
4. The /block/ directory
This directory contains directories for each of the block devices on the system. These are mostly
disk type devices such as real DASD, loopback devices, and software raid block devices. The
noticeable difference between older Linux systems and ones that use sysfs is the need to refer
to devices by their sysfs name. On a 2.4 kernel image, the zFCP driver was passed as its device
addresses. On the 2.6 Kernel image system the driver is passed as 0.0.1600.
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