8 Private Context Data Structures

UEFI drivers managing more than one controller need to be designed with reentrancy in mind. This means that global variables should not be used to track information about individual controllers. Instead, data structures should be allocated with the UEFI memory services for each controller, and those data structures should contain all the information that the driver requires to manage each individual controller.
This chapter introduces some object-oriented programming techniques that can be applied to drivers managing controllers. These techniques can simplify driver design and implementation. The concept of a private context data structure containing all the information required to manage a controller is introduced. This data structure contains the public data fields, public services, private data fields, and private services a UEFI driver may require to manage a controller.
Some categories of UEFI drivers do not require the use of these data structures. If a UEFI driver only produces a single protocol, or it manages, at most, one device, the techniques presented here are not required. An initializing driver does not produce any services and does not manage any devices, so it does not use this technique. A service driver that produces a single protocol and does not manage any devices does not likely use this technique. A root bridge driver that manages a single root bridge device does not likely use this technique, but a root bridge driver that manages more than one root bridge device may use this technique.
Finally, all UEFI drivers that follow the UEFI driver model should use this technique. Even if the driver writer is convinced that the UEFI driver manages only a single device in a platform, this technique should still be used because it simplifies the process of updating the driver to manage more than one device. The driver writer should make as few device and platform assumptions as possible when designing a new driver.
Implementations of Hybrid drivers that follow the UEFI Driver Model may define two different private context data structures―one for the bus controller and another one for the child controllers it produces.
It is possible to use other techniques to track the information required to manage multiple controllers in a re-entrant-safe manner, but those techniques likely require more overhead in the driver itself to manage this information. The techniques presented here are intended to produce small driver executables. These techniques are used throughout the drivers in EDK II.