devices
- The udev service enables user space programs to automatically configure and use new devices.
- The kernel presents many IO interfaces for devices as files to user space processes
- device files are in the
/devdirectory - to identify a device and view it’s properties, use
ls -l. Note the first character of each line in the output below. If you see b (block), c (character), p (pipe), or s (socket), the file is a device.-
ryan:notes/ |main ✓|$ ls -l /dev | head total 0 crw------- 1 root root 10, 107 Jan 6 15:14 acpi_thermal_rel crw-r--r-- 1 root root 10, 235 Jan 6 15:14 autofs drwxr-xr-x 2 root root 260 Jan 6 15:14 block/ crw-rw---- 1 root disk 10, 234 Jan 6 15:14 btrfs-control drwxr-xr-x 3 root root 60 Jan 6 15:14 bus/ drwxr-xr-x 2 root root 5960 Jan 7 08:29 char/ crw--w---- 1 root tty 5, 1 Jan 6 15:14 console lrwxrwxrwx 1 root root 11 Jan 6 15:14 core -> /proc/kcore drwxr-xr-x 10 root root 220 Jan 6 15:14 cpu/
-
device types
- block device = hard disks. Data is read in chunks
- character device = data is read in streams (monitors, printers, etc.)
- pipes = like character devices, except another process is at the end of the IO stream
- socket = special purpose interfaces that are typically used for inter-process communications
sysfs
- The sysfs filesystem is a pseudo-filesystem which provides an interface to kernel data structures.
- The sysfs filesystem is commonly mounted at /sys.
- Many of the files in the sysfs filesystem are read-only, but some files are writable, allowing kernel variables to be changed. To avoid redundancy, symbolic links are heavily used to connect entries across the filesystem tree.
ryan:~/ $ ll /sys/
total 4
1 0 dr-xr-xr-x 13 root root 0 Jan 6 15:14 ./
2 4 drwxr-xr-x 20 root root 4096 Dec 27 19:39 ../
8359 0 drwxr-xr-x 2 root root 0 Jan 6 15:14 block/
8 0 drwxr-xr-x 54 root root 0 Jan 6 15:14 bus/
10 0 drwxr-xr-x 87 root root 0 Jan 6 15:14 class/
5 0 drwxr-xr-x 4 root root 0 Jan 6 15:14 dev/
4 0 drwxr-xr-x 29 root root 0 Jan 6 15:14 devices/
11 0 drwxr-xr-x 6 root root 0 Jan 6 15:14 firmware/
2 0 drwxr-xr-x 10 root root 0 Jan 6 15:14 fs/
12 0 drwxr-xr-x 2 root root 0 Jan 6 15:14 hypervisor/
5235 0 drwxr-xr-x 17 root root 0 Jan 6 15:14 kernel/
6394 0 drwxr-xr-x 349 root root 0 Jan 6 15:14 module/
5247 0 drwxr-xr-x 3 root root 0 Jan 6 15:14 power/
- The most important directories within
/sysare:- block = contains info for every block device attached to the system
- bus = contains a directory for every bus type in the kernel
- hypervisor =
- class
- devices
- kernel
- firmware
- module
- power
https://docs.kernel.org/filesystems/sysfs.html https://www.kernel.org/doc/Documentation/filesystems/sysfs.txt
Hard Disks
-
Most block devices attached to a Linux system will have a device name with a prefix of /dev/sd*
- Example: /dev/sda
- The ‘sd’ portion stands for SCSI Disk
-
To list the SCSI devices on your system, use a tool that walks the SCSI device paths, such as
lsscsilsscsiis not commonly installed by default
udevd
- udevd is responsible for creating device files for attached devices
- The process is commonly
systemd-udevd - The kernel will send a notification to this process upon detecting a new device attached to the system. Udevd will then create a file in user-land for the device.
- This caused problems because some devices need to be available very early in the boot process, so devtmpfs was created
- devtmpfs filesytem is used by the kernel to create device files as necessary, but it also notifies udevd that a new device is available. Upon receiving this signal, udevd does not create a new device file, but it does perform device initialization along with setting permissions and notifying other processes that new devices are available.