vchi

VCHI is an acronym used to denote the VideoCore Host Interface, a low-level communication protocol between the ARM CPU and the VideoCore GPU on Raspberry Pi devices. The term appears in both the kernel and user-space components of the Raspberry Pi multimedia stack and is sometimes seen in lowercase as "vchi" in code and documentation.

Origins and scope: VCHI originated in Broadcom's VideoCore architecture to enable efficient messaging between host and video cores. The commonly used variant is VCHIQ (VideoCore Host Interface for Queue), which provides a channel-based, request-response IPC mechanism and shared memory buffers.

Functionality: VCHI allows user-space codecs and drivers (such as MMAL, the Raspberry Pi Multimedia Abstraction Layer, and V4L2-based components) to issue commands, transfer buffers, and receive events from the VideoCore. It supports service discovery, channel creation, and asynchronous messaging.

Implementation: In Linux-based Raspberry Pi systems, the VCHIQ driver is part of the kernel, while user-space libraries expose APIs to create and manage channels. The combination underpins components like the MMAL framework, OpenMAX IL, and the camera stack used on Pi devices.

Note: The term is not universally standardized; "vchi" may refer to the broader concept of the VideoCore Host Interface or specifically to VCHIQ. Documentation and examples vary by distribution and firmware version.