Provided by: libdrm-dev_2.4.107-8ubuntu1~20.04.2_amd64 
NAME
drm-memory - DRM Memory Management
SYNOPSIS
#include <xf86drm.h>
DESCRIPTION
Many modern high-end GPUs come with their own memory managers. They even include several different caches
that need to be synchronized during access. Textures, framebuffers, command buffers and more need to be
stored in memory that can be accessed quickly by the GPU. Therefore, memory management on GPUs is highly
driver- and hardware-dependent.
However, there are several frameworks in the kernel that are used by more than one driver. These can be
used for trivial mode-setting without requiring driver-dependent code. But for hardware-accelerated
rendering you need to read the manual pages for the driver you want to work with.
Dumb-Buffers
Almost all in-kernel DRM hardware drivers support an API called Dumb-Buffers. This API allows to create
buffers of arbitrary size that can be used for scanout. These buffers can be memory mapped via mmap(2) so
you can render into them on the CPU. However, GPU access to these buffers is often not possible.
Therefore, they are fine for simple tasks but not suitable for complex compositions and renderings.
The DRM_IOCTL_MODE_CREATE_DUMB ioctl can be used to create a dumb buffer. The kernel will return a
32-bit handle that can be used to manage the buffer with the DRM API. You can create framebuffers with
drmModeAddFB(3) and use it for mode-setting and scanout. To access the buffer, you first need to retrieve
the offset of the buffer. The DRM_IOCTL_MODE_MAP_DUMB ioctl requests the DRM subsystem to prepare the
buffer for memory-mapping and returns a fake-offset that can be used with mmap(2).
The DRM_IOCTL_MODE_CREATE_DUMB ioctl takes as argument a structure of type struct drm_mode_create_dumb:
struct drm_mode_create_dumb {
__u32 height;
__u32 width;
__u32 bpp;
__u32 flags;
__u32 handle;
__u32 pitch;
__u64 size;
};
The fields height, width, bpp and flags have to be provided by the caller. The other fields are filled by
the kernel with the return values. height and width are the dimensions of the rectangular buffer that is
created. bpp is the number of bits-per-pixel and must be a multiple of 8. You most commonly want to pass
32 here. The flags field is currently unused and must be zeroed. Different flags to modify the behavior
may be added in the future. After calling the ioctl, the handle, pitch and size fields are filled by the
kernel. handle is a 32-bit gem handle that identifies the buffer. This is used by several other calls
that take a gem-handle or memory-buffer as argument. The pitch field is the pitch (or stride) of the new
buffer. Most drivers use 32-bit or 64-bit aligned stride-values. The size field contains the absolute
size in bytes of the buffer. This can normally also be computed with (height * pitch + width) * bpp / 4.
To prepare the buffer for mmap(2) you need to use the DRM_IOCTL_MODE_MAP_DUMB ioctl. It takes as argument
a structure of type struct drm_mode_map_dumb:
struct drm_mode_map_dumb {
__u32 handle;
__u32 pad;
__u64 offset;
};
You need to put the gem-handle that was previously retrieved via DRM_IOCTL_MODE_CREATE_DUMB into the
handle field. The pad field is unused padding and must be zeroed. After completion, the offset field will
contain an offset that can be used with mmap(2) on the DRM file-descriptor.
If you don't need your dumb-buffer, anymore, you have to destroy it with DRM_IOCTL_MODE_DESTROY_DUMB. If
you close the DRM file-descriptor, all open dumb-buffers are automatically destroyed. This ioctl takes as
argument a structure of type struct drm_mode_destroy_dumb:
struct drm_mode_destroy_dumb {
__u32 handle;
};
You only need to put your handle into the handle field. After this call, the handle is invalid and may be
reused for new buffers by the dumb-API.
TTM
TTM stands for Translation Table Manager and is a generic memory-manager provided by the kernel. It does
not provide a common user-space API so you need to look at each driver interface if you want to use it.
See for instance the radeon man pages for more information on memory-management with radeon and TTM.
GEM
GEM stands for Graphics Execution Manager and is a generic DRM memory-management framework in the kernel,
that is used by many different drivers. GEM is designed to manage graphics memory, control access to the
graphics device execution context and handle essentially NUMA environment unique to modern graphics
hardware. GEM allows multiple applications to share graphics device resources without the need to
constantly reload the entire graphics card. Data may be shared between multiple applications with gem
ensuring that the correct memory synchronization occurs.
GEM provides simple mechanisms to manage graphics data and control execution flow within the linux DRM
subsystem. However, GEM is not a complete framework that is fully driver independent. Instead, if
provides many functions that are shared between many drivers, but each driver has to implement most of
memory-management with driver-dependent ioctls. This manpage tries to describe the semantics (and if it
applies, the syntax) that is shared between all drivers that use GEM.
All GEM APIs are defined as ioctl(2) on the DRM file descriptor. An application must be authorized via
drmAuthMagic(3) to the current DRM-Master to access the GEM subsystem. A driver that does not support GEM
will return ENODEV for all these ioctls. Invalid object handles return EINVAL and invalid object names
return ENOENT.
Gem provides explicit memory management primitives. System pages are allocated when the object is
created, either as the fundamental storage for hardware where system memory is used by the graphics
processor directly, or as backing store for graphics-processor resident memory.
Objects are referenced from user-space using handles. These are, for all intents and purposes, equivalent
to file descriptors but avoid the overhead. Newer kernel drivers also support the drm-prime (7)
infrastructure which can return real file-descriptor for GEM-handles using the linux DMA-BUF API.
Objects may be published with a name so that other applications and processes can access them. The name
remains valid as long as the object exists. GEM-objects are reference counted in the kernel. The object
is only destroyed when all handles from user-space were closed.
GEM-buffers cannot be created with a generic API. Each driver provides its own API to create GEM-buffers.
See for example DRM_I915_GEM_CREATE, DRM_NOUVEAU_GEM_NEW or DRM_RADEON_GEM_CREATE. Each of these ioctls
returns a GEM-handle that can be passed to different generic ioctls. The libgbm library from the mesa3D
distribution tries to provide a driver-independent API to create GBM buffers and retrieve a GBM-handle to
them. It allows to create buffers for different use-cases including scanout, rendering, cursors and
CPU-access. See the libgbm library for more information or look at the driver-dependent man-pages (for
example drm-intel(7) or drm-radeon(7)).
GEM-buffers can be closed with the DRM_IOCTL_GEM_CLOSE ioctl. It takes as argument a structure of type
struct drm_gem_close:
struct drm_gem_close {
__u32 handle;
__u32 pad;
};
The handle field is the GEM-handle to be closed. The pad field is unused padding. It must be zeroed.
After this call the GEM handle cannot be used by this process anymore and may be reused for new GEM
objects by the GEM API.
If you want to share GEM-objects between different processes, you can create a name for them and pass
this name to other processes which can then open this GEM-object. Names are currently 32-bit integer IDs
and have no special protection. That is, if you put a name on your GEM-object, every other client that
has access to the DRM device and is authenticated via drmAuthMagic(3) to the current DRM-Master, can
guess the name and open or access the GEM-object. If you want more fine-grained access control, you can
use the new drm-prime(7) API to retrieve file-descriptors for GEM-handles. To create a name for a
GEM-handle, you use the DRM_IOCTL_GEM_FLINK ioctl. It takes as argument a structure of type struct
drm_gem_flink:
struct drm_gem_flink {
__u32 handle;
__u32 name;
};
You have to put your handle into the handle field. After completion, the kernel has put the new unique
name into the name field. You can now pass this name to other processes which can then import the name
with the DRM_IOCTL_GEM_OPEN ioctl. It takes as argument a structure of type struct drm_gem_open:
struct drm_gem_open {
__u32 name;
__u32 handle;
__u32 size;
};
You have to fill in the name field with the name of the GEM-object that you want to open. The kernel will
fill in the handle and size fields with the new handle and size of the GEM-object. You can now access the
GEM-object via the handle as if you created it with the GEM API.
Besides generic buffer management, the GEM API does not provide any generic access. Each driver
implements its own functionality on top of this API. This includes execution-buffers, GTT management,
context creation, CPU access, GPU I/O and more. The next higher-level API is OpenGL. So if you want to
use more GPU features, you should use the mesa3D library to create OpenGL contexts on DRM devices. This
does not require any windowing-system like X11, but can also be done on raw DRM devices. However, this is
beyond the scope of this man-page. You may have a look at other mesa3D man pages, including libgbm and
libEGL. 2D software-rendering (rendering with the CPU) can be achieved with the dumb-buffer-API in a
driver-independent fashion, however, for hardware-accelerated 2D or 3D rendering you must use OpenGL. Any
other API that tries to abstract the driver-internals to access GEM-execution-buffers and other GPU
internals, would simply reinvent OpenGL so it is not provided. But if you need more detailed information
for a specific driver, you may have a look into the driver-manpages, including drm-intel(7),
drm-radeon(7) and drm-nouveau(7). However, the drm-prime(7) infrastructure and the generic GEM API as
described here allow display-managers to handle graphics-buffers and render-clients without any deeper
knowledge of the GPU that is used. Moreover, it allows to move objects between GPUs and implement complex
display-servers that don't do any rendering on their own. See its man-page for more information.
EXAMPLES
This section includes examples for basic memory-management tasks.
Dumb-Buffers
This examples shows how to create a dumb-buffer via the generic DRM API. This is driver-independent (as
long as the driver supports dumb-buffers) and provides memory-mapped buffers that can be used for
scanout. This example creates a full-HD 1920x1080 buffer with 32 bits-per-pixel and a color-depth of 24
bits. The buffer is then bound to a framebuffer which can be used for scanout with the KMS API (see
drm-kms(7)).
struct drm_mode_create_dumb creq;
struct drm_mode_destroy_dumb dreq;
struct drm_mode_map_dumb mreq;
uint32_t fb;
int ret;
void *map;
/* create dumb buffer */
memset(&creq, 0, sizeof(creq));
creq.width = 1920;
creq.height = 1080;
creq.bpp = 32;
ret = drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &creq);
if (ret < 0) {
/* buffer creation failed; see "errno" for more error codes */
...
}
/* creq.pitch, creq.handle and creq.size are filled by this ioctl with
* the requested values and can be used now. */
/* create framebuffer object for the dumb-buffer */
ret = drmModeAddFB(fd, 1920, 1080, 24, 32, creq.pitch, creq.handle, &fb);
if (ret) {
/* frame buffer creation failed; see "errno" */
...
}
/* the framebuffer "fb" can now used for scanout with KMS */
/* prepare buffer for memory mapping */
memset(&mreq, 0, sizeof(mreq));
mreq.handle = creq.handle;
ret = drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &mreq);
if (ret) {
/* DRM buffer preparation failed; see "errno" */
...
}
/* mreq.offset now contains the new offset that can be used with mmap() */
/* perform actual memory mapping */
map = mmap(0, creq.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mreq.offset);
if (map == MAP_FAILED) {
/* memory-mapping failed; see "errno" */
...
}
/* clear the framebuffer to 0 */
memset(map, 0, creq.size);
REPORTING BUGS
Bugs in this manual should be reported to https://gitlab.freedesktop.org/mesa/drm/-/issues
SEE ALSO
drm(7), drm-kms(7), drm-prime(7), drmAvailable(3), drmOpen(3), drm-intel(7), drm-radeon(7), drm-nouveau(7) September 2012 DRM-MEMORY(7)