tl;dr

  • Character devices
  • Major and Minor Numbers
  • File operations
  • Bringing device IDs and file operations together
  • A character device driver example
  • Testing the simple_char driver

Commands

cat /proc/devices
stat <file>
mknod <file_name> <type> <major_num> <minor_num>
dmesg -w

@host
aarch64-linux-gnu-gcc write_prog.c -o write_prog
scp write_prog root@<VM-IP-ADDRESS>:~/
@VM
root@localhost:~# ./write_prog simple_char_node

Notes

  • Character devices
    • Abstraction provided by Linux to support read/write devices with relatively small data transfers (one or a few bytes size).
    • Abstracted as files in the file system and accessed through file access system calls.
    • Example of devices: serial ports, keyboards, mice, etc.
    • Example of character device files: /dev/ttyS0, dev/input/mouse0, /dev/kmsg, /dev/zero.
  • Major and Minor Numbers
    • Files associated with character devices are special types of files.
    • Allow users to interface with devices from the user space.
    • Under the hood, device files are the device drivers that handle the system calls for them.
    • The association between device files and devices is made with a device ID that consists of two parts: a major and a minor number
    • The device ID (major/minor number) is used to choose which driver to run to support a particular device.
  • File operations
    • Character device drivers may implement functions to handle file access system calls.
    • The syscalls implemented by a device driver are set into a struct file_operations object.
    • There are several different file operations a character driver can implement.
    • Basic system calls: open, close, read, write.
    • Handled by open, release, read, and write functions stored in struct file_operations objects.

Tutorial steps

The following steps were executed to test the simple_char driver. This could be considered a basic example of a workflow for adding a module to the kernel, building, and installing it.

  • Create an example character device
  • Enable the device: make -C "$IIO_TREE" menuconfig
  • Build linux image: kw build --clean && kw build
  • Install the module: 
    mkdir "${VM_DIR}/arm64_rootfs"
sudo guestmount --rw --add "${VM_DIR}/arm64_img.qcow2" --mount /dev/vda2 "${VM_DIR}/arm64_rootfs"
sudo --preserve-env make -C "${IIO_TREE}" INSTALL_MOD_PATH="${VM_DIR}/arm64_rootfs" modules_install
sudo guestunmount "${VM_DIR}/arm64_rootfs"
  • Start the VM: sudo virsh start arm64
  • Connect to the VM: ssh root@$(sudo virsh net-dhcp-leases default | grep -oE '\b([0-9]{1,3}\.){3}[0-9]{1,3}\b')
  • Get information about the module: modinfo simple_char
  • Load the module: modprobe simple_char
  • See kernel logs: dmesg | tail

Initially the device wasn’t writing to the buffer, giving me the following error:

root@localhost:~# ./write_prog simple_char_node
Error: 9wrote -1 bytes to buffer

But this was caused by a misconfiguration with the device interface (simple_char_node). By deleting and recreating it with the correct major number (cat /proc/devices | grep simp) I was able to write and read using the device.

root@localhost:~# ./write_prog simple_char_node
wrote 256 bytes to buffer
root@localhost:~# ./read_prog simple_char_node 
Read buffer: A new message for simple_char.

Reference

Introduction to Linux kernel Character Device Drivers