Provided by: libdrm-dev_2.4.113-2~ubuntu0.22.04.1_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 drmCloseBufferHandle(3). It takes as argument the
GEM-handle to be closed. 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)