The DRM is a kernel module that gives direct hardware access to DRI clients.

This module deals with DMA, AGP memory management, resource locking,
and secure hardware access. In order to support multiple, simultaneous
3D applications the 3D graphics hardware must be treated as a shared
resource. Locking is required to provide mutual exclusion. DMA
transfers and the AGP interface are used to send buffers of graphics
commands to the hardware. Finally, there must be security to prevent
out-of-control clients from crashing the hardware.