Understanding Linux Device Model and Sysfs in Kernel Development

1. Linux Kernel Development Chpt17: kobjects and sysfs Tao Yang 2007-06-01

2. Overview • Device model • Kobject • Ktypes • Ksets • Subsystems • Reference Counts • Sysfs • The Kernel Events Layer

3. device model • Representing devices & describing topology • Benefits: • Minimization of code duplication • providing common facilities • enumerate all the devices • generate a complete tree of the entire device structure • link devices to their drivers • categorize devices by their class • The ability to walk the tree of devices from the leaves up to the root, poweringdown devices in the correct order

4. kobjects • Kobject: the heart of the device model

5. ktype • which the kobjects are associated with • describing default behavior for a family of kobjects • define default properties: deconstruction behavior, sysfs behavior, and default attributes

6. ksets • Collections of kobjects • Distinction of ksets and ktypes: • ksets collect kobjects into a set • ktypes describe properties shared by kobjects of a related type • allow kobjects of identical ktypes to be grouped into different ksets

7. Subsystem • high-level concepts in the kernel • a collection of one or more ksets • Ksets contain kobjects • subsystems contain ksets

8. Same ktype Relationship of these Structure • Kobject: the basic unit • Ktype: describe default properties • Ksets: Collections of kobjects

9. Managing and Manipulating kobjects • declaring and initializing • Before calling this function, the kobject must be zeroed • set the kobject's name

10. Reference Counts • To keep the kobject in memory or destroy it • Incrementing the reference count: • Decrementing the reference count: • Kref: provide the kobject reference counting

11. sysfs • In-memory virtual filesystem • View the kobject hierarchy • View the device topology as a simple filesystem

12. Adding and Removing kobjects from sysfs • Export a kobject to sysfs • Initializes the given kobject and adds to the object hierarchy • Removing a kobject's sysfs • Both kobject_del() and kobject_put():

13. Adding Files to sysfs • Default Attributes: • ktype field in kobjects and ksets provide default set of files • Attribute structures: map kernel data to files in sysfs • Creating New Attributes • Destroying New Attributes

14. sysfs Conventions • To keep sysfs clean and intuitive, conventions must be followed: • First, sysfs attributes should export one value per file • Second, organize data in sysfs in a clean hierarchy • Finally, keep in mind that sysfs provides a kernel-to-user service

15. The Kernel Events Layer • Implements a kernel-to-user notification system • Hard drive full • Processor is overheating • Partition mounted • send events out to user-space from kernel code:

16. Conclusion • Subject : device model, sysfs, kobjects, and the kernel event layer • Driver writers need only a peripheral view of these topics • Developers of kernel need a more in-depth understanding of kobjects

17. Thank you Q & A