31 Selecting a Device Manager

The Yocto Project provides multiple ways to manage the device manager (/dev):

  • Persistent and Pre-Populated /dev: For this case, the /dev directory is persistent and the required device nodes are created during the build.

  • Use devtmpfs with a Device Manager: For this case, the /dev directory is provided by the kernel as an in-memory file system and is automatically populated by the kernel at runtime. Additional configuration of device nodes is done in user space by a device manager like udev or busybox-mdev.

31.1 Using Persistent and Pre-Populated /dev

To use the static method for device population, you need to set the USE_DEVFS variable to “0” as follows:

USE_DEVFS = "0"

The content of the resulting /dev directory is defined in a Device Table file. The IMAGE_DEVICE_TABLES variable defines the Device Table to use and should be set in the machine or distro configuration file. Alternatively, you can set this variable in your local.conf configuration file.

If you do not define the IMAGE_DEVICE_TABLES variable, the default device_table-minimal.txt is used:

IMAGE_DEVICE_TABLES = "device_table-mymachine.txt"

The population is handled by the makedevs utility during image creation:

31.2 Using devtmpfs and a Device Manager

To use the dynamic method for device population, you need to use (or be sure to set) the USE_DEVFS variable to “1”, which is the default:

USE_DEVFS = "1"

With this setting, the resulting /dev directory is populated by the kernel using devtmpfs. Make sure the corresponding kernel configuration variable CONFIG_DEVTMPFS is set when building you build a Linux kernel.

All devices created by devtmpfs will be owned by root and have permissions 0600.

To have more control over the device nodes, you can use a device manager like udev or busybox-mdev. You choose the device manager by defining the VIRTUAL-RUNTIME_dev_manager variable in your machine or distro configuration file. Alternatively, you can set this variable in your local.conf configuration file:

VIRTUAL-RUNTIME_dev_manager = "udev"

# Some alternative values
# VIRTUAL-RUNTIME_dev_manager = "busybox-mdev"
# VIRTUAL-RUNTIME_dev_manager = "systemd"