Subdirectories are organized as follows:

kernel: Architecture-independent kernel code
ppe42:  Kernel code that is common for all ppe42 instances
std:    External interrupt support for standard ppe42 instances (SBE, CME, etc.)
gpe:    External interrupt support for ppe42 instances residing in the OCC complex
ppe:    Support for ppe42 instances that do not require or have external interrupts
trace:  Kernel trace support