"Traditionally, Xorg drivers include several parts, with one being actually required:
*the DDX driver is the part that allows X to draw stuff on screen: setting up resolution and pixel buffer depth, and writing it on-screen. This driver is normally hosted entirely in user space, and traditionally included some accelerated functions such as fills. These acceleration methods were originally used through an API called XAA, which was later simplified and rewritten for most drivers to become EXA. This is the ‘required’ component.
*the DRI/DRM dual component is a later addition, dating back to Xfree86 4.0, which allowed X to talk to the graphics hardware directly: a kernel component (the DRM module) and an X component (the X DRI driver) will interact, following strict methods, and bypassing kernel memory copies for most functionalities. This is where extensions such as RENDER and Composite get interesting because they allow X to directly manipulate video RAM. EXA, coupled with RENDER, will make use of this kind of access to further accelerate some drawing operations. However, this component was never originally made to deal with massive amounts of data, and a memory manager had to be programmed specifically for each driver."
