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Updated: Thursday, June 13, 2019
 
 

drm-ttm (7)

Name

drm-ttm - DRM Memory Management

Synopsis

#include <xf86drm.h>

Description

DRM-MEMORY(7)              Direct Rendering Manager              DRM-MEMORY(7)



NAME
       drm-memory, drm-mm, drm-gem, drm-ttm - 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 32bit 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
       32bit 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 32bit or 64bit 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 manpages 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 32bit 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 manpages, 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://bugs.freedesktop.org/enter_bug.cgi?product=DRI&component=libdrm
       under the "DRI" product, component "libdrm"


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+--------------------+
       |ATTRIBUTE TYPE |  ATTRIBUTE VALUE   |
       +---------------+--------------------+
       |Availability   | x11/library/libdrm |
       +---------------+--------------------+
       |Stability      | Volatile           |
       +---------------+--------------------+
SEE ALSO
       drm(7), drm-kms(7), drm-prime(7), drmAvailable(3), drmOpen(3), drm-
       intel(7), drm-radeon(7), drm-nouveau(7)



NOTES
       This software was built from source available at
       https://github.com/oracle/solaris-userland.  The original community
       source was downloaded from
       https://dri.freedesktop.org/libdrm/libdrm-2.4.97.tar.bz2

       Further information about this software can be found on the open source
       community website at https://dri.freedesktop.org/.



libdrm                          September 2012                   DRM-MEMORY(7)