8 Stage VI: Advanced Feature Selection
Advanced features are non-essential features. Essential features are defined as being support required to meet earlier stage boot objectives. An advanced feature must be implemented as highly cohesive and stand-alone software to only support a specific feature. Modularizing such features, reducing dependencies on other advanced features, and eliminating dependencies on specific implementations of other advanced features is critical and results in a variety of benefits:
- The minimum platform serves as a basic enabling vehicle ready to support various roles for a given hardware platform. This yields a minimum platform solution that is open to extension but closed for modification.
- System power-on is simplified because unnecessary code paths and silicon paths can be avoided or deferred.
- Platforms can be composed in a more modular and portable manner allowing generic advanced features to be readily shared among participants.
- Feature adoption benefits from modular design that is simple to maintain.
Organizing advanced features in the platform architecture enables better realization of the benefits in UEFI specification compliant firmware with highly cohesive and lowly coupled component interactions.
This chapter provides guidance on how to design and integrate advanced features. The source code layout and other maintenance details are outside the scope of this specification.
The core advanced feature requirements that must be met:
- Cohesive, the feature should not contain any functionality unrelated to the feature.
- Complete, the feature must have a complete design that minimizes dependencies. A feature package cannot directlydepend on another feature package.
- Easy to Integrate, the feature should expose well-defined software interfaces to use and configure the feature.
- It should also present a set of simple and well-documented standard EDK II configuration options such as PCDs toconfigure the feature.
- In general, features should be self-contained and started by the dispatcher. The board firmware shouldbe required to perform as few steps as possible to enable the feature.
- All features are required to have a feature enable PCD (
PcdFeatureEnable). Any effort to enable the featurebesides this PCD should be carefully considered. Default configuration values should apply to the common case.
- Portable, the feature is not allowed to depend on other advanced feature or board source code packages. For example,if Feature A depends on output Feature B, a board integration module should use a generic interface in Feature Ato get the output and pass it to a generic interface in Feature B. Structures should not be shared between featurepackages. Most structures should be defined in a common package such as MdePkg if the structure is industry standard,IntelSiliconPkg if the structure is specific to Intel silicon initialization, etc. Feature-specific structures areof course allowed to be defined within a feature package and used by libraries and modules in that package.
- Self Documenting, the feature should follow software best practices to allow others to debug the code andcontribute changes. In addition to source code, advanced features must have a Readme.md with sufficientinformation for a newcomer to understand the feature.
- Single Instance, the feature should not have more than one instance of a source solution. If an existing featurepackage does not solve a specific instance of a problem for the feature, the feature package should be re-workedto consider new requirements instead of duplicating feature code.
The number of embedded features must be minimized in order to support the broadest compatibility of the minimal platform. Features should be designed to define an API that can be used to integrate the feature into generic platform configurations. The feature source code should never be modified to absorb details of a specific platform or board.