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.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
.. _device-manager:
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``.
Using Persistent and Pre-Populated ``/dev``
===========================================
To use the static method for device population, you need to set the
:term:`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
:term:`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 :term:`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:
Using ``devtmpfs`` and a Device Manager
=======================================
To use the dynamic method for device population, you need to use (or be
sure to set) the :term:`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
:term:`VIRTUAL-RUNTIME_dev_manager <VIRTUAL-RUNTIME>` 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"
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