drm - Direct Rendering Manager
The Direct Rendering Manager
(DRM) is a framework to manage Graphics
(GPUs). It is designed to support the needs of complex
graphics devices, usually containing programmable pipelines well suited to 3D
graphics acceleration. Furthermore, it is responsible for memory management,
interrupt handling and DMA to provide a uniform interface to applications.
In earlier days, the kernel framework was solely used to provide raw hardware
access to priviledged user-space processes which implement all the hardware
abstraction layers. But more and more tasks were moved into the kernel. All
these interfaces are based on ioctl
(2) commands on the DRM character
device. The libdrm
library provides wrappers for these system-calls and
many helpers to simplify the API.
When a GPU is detected, the DRM system loads a driver for the detected hardware
type. Each connected GPU is then presented to user-space via a
character-device that is usually available as /dev/dri/card0 and can be
accessed with open
(2) and close
(2). However, it still depends on
the grapics driver which interfaces are available on these devices. If an
interface is not available, the syscalls will fail with EINVAL.
All DRM devices provide authentication mechanisms. Only a DRM-Master is allowed
to perform mode-setting or modify core state and only one user can be
DRM-Master at a time. See drmSetMaster
(3) for information on how to
become DRM-Master and what the limitations are. Other DRM users can be
authenticated to the DRM-Master via drmAuthMagic
(3) so they can perform
buffer allocations and rendering.
Managing connected monitors and displays and changing the current modes is
. This is restricted to the current DRM-Master.
Historically, this was implemented in user-space, but new DRM drivers
implement a kernel interface to perform mode-setting called Kernel Mode
(KMS). If your hardware-driver supports it, you can use the KMS
API provided by DRM. This includes allocating framebuffers, selecting modes
and managing CRTCs and encoders. See drm-kms
(7) for more.
The most sophisticated tasks for GPUs today is managing memory objects.
Textures, framebuffers, command-buffers and all other kinds of commands for
the GPU have to be stored in memory. The DRM driver takes care of managing all
memory objects, flushing caches, synchronizing access and providing CPU access
to GPU memory. All memory management is hardware driver dependent. However,
two generic frameworks are available that are used by most DRM drivers. These
are the Translation Table Manager
(TTM) and the Graphics Execution
(GEM). They provide generic APIs to create, destroy and access
buffers from user-space. However, there are still many differences between the
drivers so driver-depedent code is still needed. Many helpers are provided in
(Graphics Buffer Manager) from the mesa-project
. For more
information on DRM memory-management, see drm-memory
Bugs in this manual should be reported to
under the "DRI" product, component "libdrm"