DRM(7) Direct Rendering Manager DRM(7)
NAME
drm - Direct Rendering Manager
SYNOPSIS
#include <xf86drm.h>
DESCRIPTION
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 priviledged user-space processes which implement
all the hardware abstraction layers. But more and more tasks where 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.
Authentication
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.
Mode-Setting
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.
Memory Management
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).
REPORTING BUGS
Bugs in this manual should be reported to http://bugs.freedesktop.org under the "Mesa" product, with "Other" or "libdrm" as the component.
SEE ALSO
drm-kms(7), drm-memory(7), drmSetMaster(3), drmAuthMagic(3), drmAvailable(3), drmOpen(3)
libdrm September 2012 DRM(7)