UEFI 2.4 Review, Part 1: ResetSystem() Update

This series goes through the significant updates found in the UEFI 2.4 specification. Starting off minor, we look at the updates to the ResetSystem()  runtime service. Previously, there were three types: EfiResetCold, EfiResetWarm, and EfiResetShutdown. So what other types of reset do you need?

Well, there are resets and resets. In many platforms, there are actually many devices that can start a reset. The CPU itself can be reset. Or, the I/O controller can be reset, that resets the PCI bus and the CPU. Or, in some systems, an embedded controller can be reset, that resets the I/O controller, that resets the PCI bus and the CPU.

Now resets have all sorts of interesting side effects. Among them is the tendency to unlock all sorts of storage or locked configuration registers. In particular, some embedded controllers do not permit their internal flash to be updated unless they themselves are reset. But that kind of reset does not normally happen with either EfiResetWarm or EfiResetCold. Hmm...what to do?

Well, the UEFI 2.4 specification added a new reset type: EfiResetPlatformSpecific. This reset type can be further qualified by a GUID appended to the end of the ResetData after the null-terminated string.

There was nothing after the string in previous specifications. It is always a system-wide reset and, if the GUID is not recognized by the platform firmware, then it can choose the type of reset. But if the GUID is recognized, it can perform a platform-specific type of reset. For example, it could reset the embedded controller so that its on-chip flash would be unlocked and updated.

Useful, so that platform vendors don't need to use their own undocumented services or parameter values.

Next time: out-of-band key modification.