.. include:: <mmlalias.txt>

.. meta::
   :description: Slide deck: a demo module to show how a driver
                 creates a character device
   :keywords: schulung, training, linux, kernel, module, character
              device, cdev, device_create

Modern Character Devices (Slideshow)
====================================

.. contents::
   :local:

.. sidebar:: Source

   `See Github
   <https://github.com/jfasch/jf-kernel-course/tree/my_driver_cdev_dynamic_major/_morph>`__

Character Devices: Problems
---------------------------

* Manually creating device nodes is cumbersome
* *Number* conflicts are much more likely than *name* conflicts
* Want *names*, not *numbers*

**Linux way**: ``devtmpfs``

* File system that contains device nodes
* Automatically populated by the kernel
* ... with a little driver support

.. code-block:: console

   $ mount
   ...
   devtmpfs on /dev type devtmpfs (rw,relatime,...)
   ...

Dynamic Device Numbers
----------------------

jjj

* From ``register_chrdev_region()`` to ``alloc_chrdev_region()``

  * blah

https://www.kernel.org/doc/htmldocs/kernel-api/API-alloc-chrdev-region.html

.. code-block:: c

   int alloc_chrdev_region(
       dev_t * dev,
       unsigned baseminor,
       unsigned count,
       const char * name);


Device Classes
--------------



Devices
-------

* ``device_create()``: register character devices with sysfs

  https://www.kernel.org/doc/html/latest/driver-api/infrastructure.html?highlight=device_create#c.device_create

  * return value? |longrightarrow| ``BUG_ON()``
  * Module loading: ``ERROR: Unknown symbol in module``

    * Only exported to modules with GPL compatible license
      |longrightarrow| ``MODULE_LICENSE("GPL");``