1. Viewing Devices from the /dev Directory

In Linux systems, the vast majority of devices are exposed in the /dev directory in the form of device files. These device files are essentially "entry points for accessing devices", not ordinary files that actually store data.

Execute ls -l /dev on the development board and you will see output similar to the following:

lckfb@linaro-alip:~$ ls -l /dev
total 8
drwxr-xr-x  3 root  root         480 Dec 9 19:02 block
drwxr-xr-x  1 root  root          60 Jan 1 1970 bus
crw-rw----  1 root  video   248,   0 Dec 9 19:02 cec0
drwxr-xr-x  1 root  root        5300 Dec 9 19:02 char
crw--w----  1 root  tty       5,   1 Dec 9 19:03 console
crw-------  1 root  root     10, 124 Dec 9 19:02 cpu_dma_latency
crw-rw-rw-  1 root  root     10, 126 Dec 9 19:02 crypto
drwxr-xr-x  9 root  root         180 Dec 9 19:02 disk
drwxr-xr-x  1 root  root         100 Jan 1 1970 dma_heap
drwxr-xr-x  1 root  root         140 Dec 9 19:02 dri
crw-------  1 root  root    240,   0 Dec 9 19:02 drm_dp_aux0
lrwxrwxrwx  1 root  root          13 Dec 9 19:02 fd -> /proc/self/fd
crw-rw-rw-  1 root  root      1,   7 Dec 9 19:02 full
crw-rw-rw-  1 root  root     10, 229 Dec 9 19:02 fuse
crw-------  1 root  root    254,   0 Dec 9 19:02 gpiochip0
crw-------  1 root  root    254,   1 Dec 9 19:02 gpiochip1
..........

crw-------  1 root  root     10, 183 Dec 9 19:02 hwrng
crw-rw----  1 root  i2c      89,   1 Dec 9 19:02 i2c-1
crw-rw----  1 root  i2c      89,  10 Dec 9 19:02 i2c-10
.........

crw-------  1 root  root    245,   0 Dec 9 19:02 iio:device0
lrwxrwxrwx  1 root  root          12 Dec 9 19:02 initctl -> /run/initctl
drwxr-xr-x  2 root  root         180 Dec 9 19:02 input
crw-r--r--  1 root  root      1,  11 Dec 9 19:02 kmsg
lrwxrwxrwx  1 root  root          28 Dec 9 19:02 log -> /run/systemd/journal/dev-log
brw-rw----  1 root  disk      7,   0 Dec 9 19:02 loop0
................

crw-rw----  1 root  disk     10, 237 Dec 9 19:02 loop-control
crw-rw----  1 root  video    10, 122 Dec 9 19:02 mali0
crw-rw----+  1 root  video   249,   0 Dec 9 19:02 media0
crw-rw----+  1 root  video   249,   1 Dec 9 19:02 media1
.....................

crw-r-----  1 root  kmem      1,   1 Dec 9 19:02 mem
brw-rw----  1 root  disk    179,   0 Dec 9 19:02 mmcblk0
brw-rw----  1 root  disk    179,  32 Dec 9 19:02 mmcblk0boot0
brw-rw----  1 root  disk    179,  64 Dec 9 19:02 mmcblk0boot1
brw-rw----  1 root  disk    179,   1 Dec 9 19:02 mmcblk0p1
brw-rw----  1 root  disk    179,   2 Dec 9 19:02 mmcblk0p2
brw-rw----  1 root  disk    179,   3 Dec 9 19:02 mmcblk0p3
brw-rw----  1 root  disk    179,   4 Dec 9 19:02 mmcblk0p4
brw-rw----  1 root  disk    179,   5 Dec 9 19:02 mmcblk0p5
brw-rw----  1 root  disk    179,   6 Dec 9 19:02 mmcblk0p6
brw-rw----  1 root  disk    179,   7 Dec 9 19:02 mmcblk0p7
brw-rw----  1 root  disk    179,   8 Dec 9 19:02 mmcblk0p8
crw-------  1 root  root    235,   0 Dec 9 19:02 mmcblk0rpmb
crw-rw----  1 root  video   241,   0 Dec 9 19:02 mpp_service
crw-rw-rw-  1 root  root      1,   3 Dec 9 19:02 null
crw-r-----  1 root  kmem      1,   4 Dec 9 19:02 port
crw-rw-rw-  1 root  tty       5,   2 Dec 12 14:34 ptmx
crw-------  1 root  root    246,   0 Dec 9 19:02 ptp0
drwxr-xr-x  2 root  root           0 Dec 9 19:02 pts
brw-rw----  1 root  disk      1,   0 Dec 9 19:02 ram0
crw-rw-rw-  1 root  root      1,   8 Dec 9 19:02 random
crw-rw-r--+  1 root  netdev   10, 242 Dec 9 19:02 rfkill
crw-rw----  1 root  video    10, 123 Dec 9 19:02 rga
lrwxrwxrwx  1 root  root           4 Dec 9 19:02 rtc -> rtc0
crw-------  1 root  root    250,   0 Dec 9 19:02 rtc0
drwxr-xrwt  2 root  root          40 Dec 9 19:02 shm
drwxr-xr-x  3 root  root         260 Dec 9 19:02 snd
lrwxrwxrwx  1 root  root          15 Dec 9 19:02 stderr -> /proc/self/fd/2
lrwxrwxrwx  1 root  root          15 Dec 9 19:02 stdin -> /proc/self/fd/0
lrwxrwxrwx  1 root  root          15 Dec 9 19:02 stdout -> /proc/self/fd/1
crw-------  1 root  root     10, 127 Dec 9 19:02 sw_sync
crw-------  1 root  root    244,   0 Dec 9 19:02 tee0
crw-------  1 root  root    244,  16 Dec 9 19:02 teepriv0
crw-rw-rw-  1 root  tty       5,   0 Dec 9 19:02 tty
crw--w----  1 root  tty       4,   0 Dec 9 19:02 tty0
crw--w----  1 root  tty       4,   1 Dec 9 19:02 tty1
..................

crw-------  1 root  root     10, 121 Dec 9 19:02 ubi_ctrl
crw-------  1 root  root     10, 239 Dec 9 19:02 uhid
crw-------  1 root  root     10, 223 Dec 9 19:02 uinput
crw-rw-rw-  1 root  root      1,   9 Dec 9 19:02 urandom
drwxr-xr-x  3 root  root          60 Dec 9 19:02 usb-ffs
crw-------  1 root  root    237,   0 Dec 9 19:02 usbmon0
crw-------  1 root  root    237,   1 Dec 9 19:02 usbmon1
crw-------  1 root  root    237,   2 Dec 9 19:02 usbmon2
drwxr-xr-x  3 root  root          60 Dec 9 19:02 v4l
crw-rw----+  1 root  video    81,  65 Dec 9 19:02 v4l-subdev0
crw-rw----+  1 root  video    81,  70 Dec 9 19:02 v4l-subdev1
..................

crw-rw----  1 root  tty       7,   0 Dec 9 19:02 vcs
crw-rw----  1 root  tty       7,   1 Dec 9 19:02 vcs1
.................

crw-rw----  1 root  tty       7,  70 Dec 9 19:02 vcsu6
crw-rw----  1 root  tty       7,  71 Dec 9 19:02 vcsu7
crw-------  1 root  root     10, 125 Dec 9 19:02 vendor_storage
crw-------  1 root  root     10, 137 Dec 9 19:02 vhci
crw-rw----+  1 root  video    81,   0 Dec 9 19:02 video0
crw-rw----+  1 root  video    81,   1 Dec 9 19:02 video1
.................

crw-------  1 root  root     10, 130 Dec 9 19:02 watchdog
crw-------  1 root  root    243,   0 Dec 9 19:02 watchdog0
crw-rw-rw-  1 root  root      1,   5 Dec 9 19:02 zero
brw-rw----  1 root  disk    254,   0 Dec 9 19:02 zram0

The most important ones are :

• First column letters (device type):

crw-rw-rw-  1 root  root      1,   3 Dec 9 19:02 null
^
|
File type: c = character device

brw-rw----  1 root  disk    179,   0 Dec 9 19:02 mmcblk0
^
|
File type: b = block device

• c: Character device
• b: Block device
• d: Directory
• l: Symbolic link
• Other types are not elaborated here

For driver developers, the most common devices to work with are:

• Devices starting with c character devices (such as /dev/null, /dev/ttyS0, /dev/gpiochip0, etc.)
• Devices starting with b block devices (such as /dev/mmcblk0, /dev/zram0, etc.)

• The pair of numbers in the 3rd and 2nd from last columns (major and minor device numbers):

crw-rw-rw-  1 root  root    1,   3 Dec 9 19:02 null
                            ^    ^
                            |    |
                       Major number  Minor number

These two numbers together form the device number, used in the kernel to uniquely identify a device and its corresponding driver. For example:

• /dev/null: 1, 3
• /dev/mmcblk0: 179, 0

Subsequent chapters will specifically introduce the meaning of major and minor device numbers, allocation methods, and how driver programs register and use them.

2. Classification by Access Method

The most fundamental classification in Linux is by access method, dividing devices into:

1. Character Device
2. Block Device

2.1 Character Device

Character devices perform sequential read/write in byte stream (stream) mode:

• Generally do not require alignment to fixed-size "blocks".
• Usually do not have complex buffering or random access optimization.

Typical examples:

• Serial ports: /dev/ttyS0, /dev/ttyUSB0
• Console: /dev/console
• Some simple control or debugging devices: /dev/null, /dev/zero, /dev/random
• User-defined character devices: /dev/mychar, etc.

In ls -l output:

• Starts with c:

crw-rw---- 1 dialout 4, 64 ... /dev/ttyS0

User space typically uses system calls like open, read, write, ioctl to access character devices:

int fd = open("/dev/ttyS0", O_RDWR);
write(fd, buf, len);
read(fd, buf, len);
close(fd);

Subsequent chapters will focus on this type of device.

2.2 Block Device

Block devices read and write in units of data blocks, with block sizes typically being 512 bytes or multiples thereof. Characteristics:

• Support efficient random access and caching mechanisms.
• Mainly used for various storage media.

Typical examples:

• eMMC / SD cards: /dev/mmcblk0, /dev/mmcblk0p1
• USB drives, SATA/USB disks: /dev/sda, /dev/sdb

In ls -l output:

• Starts with b:

brw-rw---- 1 root disk 179, 0 Dec 9 19:02 /dev/mmcblk0

Usually, high-level data read/write is not performed directly on block devices, but rather:

1. Create a file system (such as ext4, vfat) on the block device;
2. Mount it to a certain directory:

mount /dev/mmcblk0p1 /mnt

3. Access files under /mnt through ordinary file interfaces.

Block device drivers work with file systems, involving more content. This chapter does not focus on this.

3. Network Devices and Other Types

Character devices and block devices are primarily exposed to user space through /dev.

In addition, there are some device types that do not present as traditional device files, primarily used through specialized subsystems.

3.1 Network Device

Network devices, such as Ethernet, WiFi, etc., usually appear as network interfaces:

• For example: eth0, end0, wlan0, lo, etc.
• Can be viewed with:

ip link show

Network devices:

• Do not correspond to ordinary /dev device files.
• Interact with the kernel network subsystem through socket interfaces, for example:

socket(AF_INET, SOCK_STREAM, 0);

Network drivers typically implement struct net_device related interfaces, belonging to a separate subsystem. This tutorial will not expand on this for now.

3.2 Other Devices

Some device types have dedicated frameworks in the kernel and are exposed to user space in specific ways:

• Input devices (keyboards, touch screens, etc.):
  • Bottom layer is usually character devices: /dev/input/eventX
  • User space often uses upper-level libraries like libinput, Xorg, Wayland.
• Audio devices:
  • ALSA framework: such as /dev/snd/*
  • Provides PCM, control, and other interfaces.
• Graphics devices (GPU / graphics cards):
  • DRM/KMS system: such as /dev/dri/card0

These devices are generally still based on character devices or other abstractions inside the kernel, but externally use their respective subsystems and APIs.

4. Device Classification from sysfs Perspective

In addition to /dev, modern Linux kernels also organize and display devices hierarchically through sysfs (usually mounted at /sys).

Use ls /sys/class/ on the development board:

lckfb@linaro-alip:~$ ls /sys/class/
android_usb  devcoredump     dvb          input      net             pwm          scsi_disk      tpm                 usb_role
ata_device   devfreq         extcon       iommu      nvme            regulator    scsi_host      tpmrm               vc
ata_link     devfreq-event   gpio         leds       nvme-generic    retimer      sound          tty                 video4linux
ata_port     devlink         hdmirx       mdio_bus   nvme-subsystem  rfkill       spidev         typec               vtconsole
backlight    dma             hidraw       mem        pci_bus         rk_modem     spi_host       typec_mux           wakeup
bdi          dma_heap        hwmon        misc       phy             rkwifi       spi_master     ubi                 watchdog
block        drm             i2c-adapter  mpp_class  power_supply    rpmsg        spi_transport  udc                 zram-control
bluetooth    drm_dp_aux_dev  i2c-dev      mtd        ptp             rtc          tee            usbmon
bsg          dsmc            ieee80211    mtd        ptp             scsi_device  thermal        usb_power_delivery

Visible typical directories, such as:

• block : Block device class
• tty : Terminal/serial port device class
• net : Network interface
• input : Input device class
• mmc_host : MMC host controller

The class under these directories represents a type of device or subsystem; each entry under a class corresponds to a specific device instance and is linked via symbolic links to physical devices in /sys/devices.

Subsequent chapters will provide detailed introductions to this.

6. Summary

1. Common device classifications in Linux:
   • Character device: Accessed in byte/stream mode, such as serial ports, some control devices;
   • Block device: Accessed in fixed-size data blocks, mainly used for various storage media;
   • There are also network devices, input devices, audio devices, etc., used through dedicated subsystems and interfaces.
2. In /dev:
   • Device files starting with c represent character devices;
   • Device files starting with b represent block devices;
   • The subsequent major and minor device numbers are used to uniquely identify a device and its corresponding driver.
3. In /sys/class, you can observe the organization of different types of devices from a device model perspective.