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
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
drm-kms(7), drm-memory(7), drmSetMaster(3),
drmAuthMagic(3), drmAvailable(3), drmOpen(3)