4.2.5 Never Directly Access Hardware Resources

A UEFI driver should also never directly access any system chipset resources. Directly accessing these resources limits the compatibility of the UEFI driver to systems only with that specific chipset.

Instead, the UEFI boot services, UEFI runtime services, and various protocol services should be used to access the system resources that are required by a UEFI driver. The driver should look only for services to hook into-a capability, such as a PCI bus. The driver then consumes the protocols necessary for accessing that capability. It is the chipset's responsibility to get that capability ready for use.

TIP: The general rule is to only access the hardware that the UEFI Driver supports and use abstractions for other hardware. If there is not an abstraction for a system device, do not use the device. That device may change interface and functionality in the future.

This recommendation serves several purposes. By using the software abstractions provided by the platform vendor, the UEFI driver maximizes its platform compatibility. The platform vendor can also optimize the services that are provided by the platform, so the performance of the UEFI driver improves by using these services. Chapter 29 in this guide discusses the EBC porting considerations, and one of the most important considerations is the performance of an EBC driver because EBC code is interpreted. The performance of an EBC driver can be greatly improved by calling system services instead of using internal functions.

Putting effort into source code portability helps maximize future platform compatibility.