DragonFly On-Line Manual Pages
DRM(7) Direct Rendering Manager DRM(7)
drm - Direct Rendering Manager
The Direct Rendering Manager (DRM) is a framework to manage Graphics
Processing Units (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
In earlier days, the kernel framework was solely used to provide raw
hardware access to privileged 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
graphics 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 called Mode-Setting. 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
Setting (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 Manager (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-dependent code is still needed. Many helpers are provided in
libgbm (Graphics Buffer Manager) from the Mesa project. For more
information on DRM memory management, see drm-memory(7).
Bugs in this manual should be reported to
drm-kms(7), drm-memory(7), drmSetMaster(3), drmAuthMagic(3),
September 2012 DRM(7)