The coherent accelerator interface is designed to allow the coherent connection of accelerators (FPGAs and other devices) to a POWER system. Coherent in this context means that the accelerator and CPUs can both access system memory directly and with the same effective addresses. IBM refers to this as the Coherent Accelerator Processor Interface (CAPI). In the Linux world it is referred to by the name CXL to avoid confusion with the ISDN CAPI subsystem.
The Linux kernel interacts with the device POWER Service Layer (PSL). Userland interacts with the device Accelerator Function Unit (AFU). See the Linux kernel source file Documentation/powerpc/cxl.txt for a detailed description of the coherent accelerator interface.
The CXL library provides a userland API to coherently attached devices. CXL devices can be enumerated. Their capabilities can be queried. AFUs can be opened, attached to the current process, and started. Jobs, described by AFU specific Work Element Descriptors (WEDs), can be submitted and executed by AFUs. AFU MMIO space can be mapped into the current process memory, and AFUs can be configured and controlled via MMIO reads and writes.