1. WDF Custom Driver Design Pattern
Peter wieland
Software Design Engineer

2. Agenda What is the WDF Custom Driver Pattern? Example Components
Device
Custom Driver
App
Device Metadata
Design Guidance
Questions & Code Walkthrough

3. WDF Custom Driver Pattern
A device, such as a USB attached device, that doesn't fit into any of the other patterns.
Protocols: Any. A custom device driver provides access to the device from the app
Access Model: Restricted
API: Device Access: (CreateDeviceAccessInstance(), IDeviceIoControl())
Advantages
Disadvantages
Can meet needs of the broadest range of devices
Requires development, certification & maintenance of a custom device driver
Vendor controls which Metro style apps access their device
Only vertical solutions are supported, and only a small # of apps can have privileged access to the device
Existing custom drivers can be easily adapted for this pattern
This is the most complicated pattern with the most components and moving parts.
Communications between app and driver must be written in C++
Not supported for external devices on WinRT

4. WDF Custom Driver Pattern
Example
Custom USB Device : USB credit-card reader with app to perform CC transactions
Acquisition Scenarios: Automatic Acquisition, Windows Store Download
Device Discovery: Windows.Devices.Enumeration
Device Access: CreateDeviceAccessInstance(), IDeviceIoControl
Device
Device driver
Device metadata
Metro style app +
meta
app

5. Components Custom WDF Driver pattern can be applied to any device
Other patterns may be a better fit, depending on the device
Device must have a unique hardware ID
Device must be pass Windows Hardware Certification
For use with real, external devices
Not software or virtualized devices
Device

6. Components Written using Windows Driver Framework (WDF)
Device Driver
Written using Windows Driver Framework (WDF)
Driver can be user-mode or kernel-mode WDF driver
See Using the Windows Driver Framework to build better drivers
Driver provides communications between app & device
Registers a custom device interface for the app to open
Sets restricted property on interface to grant privileged app access
App sends I/O controls to the driver to control device
Driver should be signed and deployed to Windows Update
Custom driver can use other drivers to talk to device
e.g. USB HID drivers
Device driver

7. Components
app
Declares the custom device interface class as a device capability in app manifest
Discovers device interface using DeviceInformation’s FindAllAsync() method
Opens a handle to the device using CreateDeviceAccessInstance()
Communicates with device using IDeviceIoControl interface
Last two steps can only be done in C++ code
Recommend creating a C++ WinRT component for this
app
Metro style app

8. meta Components For the purposes of a device app, Device Metadata:
Declares your app as a privileged app
Set AccessCustomDriver attribute to “true”
Declares your app as an automatically acquired app for the device
Permission applies to the device container
App can access any restricted interface it has declared access to in the container
meta
Device metadata

9. Design Guidance Authoring the Device Driver
Write custom drivers in user-mode WDF where possible
Drivers must be certified with the Windows Hardware Certification Kit (WHCK)
Check Windows Hardware Certification requirements for your device type
May want to create new device interface specifically for device app
Can provide different capabilities to admin running desktop app vs. normal user running device app
Many existing device interfaces assume administrator access
App to driver interface can only use IOCTLs

10. Updating with Custom WDF Pattern
Loose coupling between app, device metadata, and drivers.
Components are distributed through different services with update rhythms:
App updates – Available sometime after acceptance. Users are notified of an update and must initiate update.
Device metadata updates – 2 days after acceptance, auto-download within 8 days
Driver updates – available sometime after acceptance. Driver notification & update dependent on Windows Update client settings.
Components need to behave gracefully when all proper components are not present. (maintain forward and backward compatibility)

11. Sample Walkthrough

12. Summary Very flexible in terms of devices supported
Only supports restricted app model
Certified custom driver makes this a costly pattern
Beware of compatibility between components