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 to applications.
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/drm0 and can be accessed with
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
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-depedent 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
under the "DRI" product, component "libdrm"