man/drm-memory.xml - libdrm-imx - Git at Google

 <?xml version='1.0'?> <!--*-nxml-*-->
 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
           "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

 <!--
   Written 2012 by David Herrmann <dh.herrmann@googlemail.com>
   Dedicated to the Public Domain
 -->

 <refentry id="drm-memory">
   <refentryinfo>
     <title>Direct Rendering Manager</title>
     <productname>libdrm</productname>
     <date>September 2012</date>
     <authorgroup>
       <author>
         <contrib>Developer</contrib>
         <firstname>David</firstname>
         <surname>Herrmann</surname>
         <email>dh.herrmann@googlemail.com</email>
       </author>
     </authorgroup>
   </refentryinfo>

   <refmeta>
     <refentrytitle>drm-memory</refentrytitle>
     <manvolnum>7</manvolnum>
   </refmeta>

   <refnamediv>
     <refname>drm-memory</refname>
     <refname>drm-mm</refname>
     <refname>drm-gem</refname>
     <refname>drm-ttm</refname>
     <refpurpose>DRM Memory Management</refpurpose>
   </refnamediv>

   <refsynopsisdiv>
     <funcsynopsis>
       <funcsynopsisinfo>#include &lt;xf86drm.h&gt;</funcsynopsisinfo>
     </funcsynopsis>
   </refsynopsisdiv>

   <refsect1>
     <title>Description</title>
       <para>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.</para>

       <para>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.</para>

     <refsect2>
       <title>Dumb-Buffers</title>
       <para>Almost all in-kernel DRM hardware drivers support an API called
             <emphasis>Dumb-Buffers</emphasis>. This API allows to create buffers
             of arbitrary size that can be used for scanout. These buffers can be
             memory mapped via
             <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
             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.</para>

       <para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> 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
             <citerefentry><refentrytitle>drmModeAddFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>
             and use it for mode-setting and scanout. To access the buffer, you
             first need to retrieve the offset of the buffer. The
             <constant>DRM_IOCTL_MODE_MAP_DUMB</constant> ioctl requests the DRM
             subsystem to prepare the buffer for memory-mapping and returns a
             fake-offset that can be used with
             <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>.</para>

       <para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> ioctl takes as
             argument a structure of type
             <structname>struct drm_mode_create_dumb</structname>:

 <programlisting>
 struct drm_mode_create_dumb {
 	__u32 height;
 	__u32 width;
 	__u32 bpp;
 	__u32 flags;

 	__u32 handle;
 	__u32 pitch;
 	__u64 size;
 };
 </programlisting>

             The fields <structfield>height</structfield>,
             <structfield>width</structfield>, <structfield>bpp</structfield> and
             <structfield>flags</structfield> have to be provided by the caller.
             The other fields are filled by the kernel with the return values.
             <structfield>height</structfield> and
             <structfield>width</structfield> are the dimensions of the
             rectangular buffer that is created. <structfield>bpp</structfield>
             is the number of bits-per-pixel and must be a multiple of
             <literal>8</literal>. You most commonly want to pass
             <literal>32</literal> here. The <structfield>flags</structfield>
             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 <structfield>handle</structfield>,
             <structfield>pitch</structfield> and <structfield>size</structfield>
             fields are filled by the kernel. <structfield>handle</structfield>
             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 <structfield>pitch</structfield> field is the
             pitch (or stride) of the new buffer. Most drivers use 32bit or 64bit
             aligned stride-values. The <structfield>size</structfield> field
             contains the absolute size in bytes of the buffer. This can normally
             also be computed with
             <emphasis>(height * pitch + width) * bpp / 4</emphasis>.</para>

       <para>To prepare the buffer for
             <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
             you need to use the <constant>DRM_IOCTL_MODE_MAP_DUMB</constant>
             ioctl. It takes as argument a structure of type
             <structname>struct drm_mode_map_dumb</structname>:

 <programlisting>
 struct drm_mode_map_dumb {
 	__u32 handle;
 	__u32 pad;

 	__u64 offset;
 };
 </programlisting>

             You need to put the gem-handle that was previously retrieved via
             <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> into the
             <structfield>handle</structfield> field. The
             <structfield>pad</structfield> field is unused padding and must be
             zeroed. After completion, the <structfield>offset</structfield>
             field will contain an offset that can be used with
             <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
             on the DRM file-descriptor.</para>

       <para>If you don't need your dumb-buffer, anymore, you have to destroy it
             with <constant>DRM_IOCTL_MODE_DESTROY_DUMB</constant>. If you close
             the DRM file-descriptor, all open dumb-buffers are automatically
             destroyed. This ioctl takes as argument a structure of type
             <structname>struct drm_mode_destroy_dumb</structname>:

 <programlisting>
 struct drm_mode_destroy_dumb {
 	__u32 handle;
 };
 </programlisting>

             You only need to put your handle into the
             <structfield>handle</structfield> field. After this call, the handle
             is invalid and may be reused for new buffers by the dumb-API.</para>

     </refsect2>

     <refsect2>
       <title>TTM</title>
       <para><emphasis>TTM</emphasis> stands for
             <emphasis>Translation Table Manager</emphasis> 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.</para>
     </refsect2>

     <refsect2>
       <title>GEM</title>
       <para><emphasis>GEM</emphasis> stands for
             <emphasis>Graphics Execution Manager</emphasis> 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.</para>

       <para>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.</para>

       <para>All GEM APIs are defined as
             <citerefentry><refentrytitle>ioctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
             on the DRM file descriptor. An application must be authorized via
             <citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
             to the current DRM-Master to access the GEM subsystem. A driver that
             does not support gem will return <constant>ENODEV</constant> for all
             these ioctls. Invalid object handles return
             <constant>EINVAL</constant> and invalid object names return
             <constant>ENOENT</constant>.</para>

       <para>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.</para>

       <para>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
             <citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
             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.</para>

       <para>Gem-buffers cannot be created with a generic API. Each driver
             provides its own API to create gem-buffers. See for example
             <constant>DRM_I915_GEM_CREATE</constant>,
             <constant>DRM_NOUVEAU_GEM_NEW</constant> or
             <constant>DRM_RADEON_GEM_CREATE</constant>. Each of these ioctls
             returns a gem-handle that can be passed to different generic ioctls.
             The <emphasis>libgbm</emphasis> library from the
             <emphasis>mesa3D</emphasis> 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
             <citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>
             or
             <citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>

       <para>Gem-buffers can be closed with the
             <constant>DRM_IOCTL_GEM_CLOSE</constant> ioctl. It takes as argument
             a structure of type <structname>struct drm_gem_close</structname>:

 <programlisting>
 struct drm_gem_close {
 	__u32 handle;
 	__u32 pad;
 };
 </programlisting>

             The <structfield>handle</structfield> field is the gem-handle to be
             closed. The <structfield>pad</structfield> 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.</para>

       <para>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
             <citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
             to the current DRM-Master, can <emphasis>guess</emphasis> the name
             and open or access the gem-object. If you want more fine-grained
             access control, you can use the new
             <citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
             API to retrieve file-descriptors for gem-handles. To create a name
             for a gem-handle, you use the
             <constant>DRM_IOCTL_GEM_FLINK</constant> ioctl. It takes as argument
             a structure of type <structname>struct drm_gem_flink</structname>:

 <programlisting>
 struct drm_gem_flink {
 	__u32 handle;
 	__u32 name;
 };
 </programlisting>

             You have to put your handle into the
             <structfield>handle</structfield> field. After completion, the
             kernel has put the new unique name into the
             <structfield>name</structfield> field. You can now pass this name to
             other processes which can then import the name with the
             <constant>DRM_IOCTL_GEM_OPEN</constant> ioctl. It takes as argument
             a structure of type <structname>struct drm_gem_open</structname>:

 <programlisting>
 struct drm_gem_open {
 	__u32 name;

 	__u32 handle;
 	__u32 size;
 };
 </programlisting>

             You have to fill in the <structfield>name</structfield> field with
             the name of the gem-object that you want to open. The kernel will
             fill in the <structfield>handle</structfield> and
             <structfield>size</structfield> 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.</para>

       <para>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 <emphasis>OpenGL</emphasis>. So if you want to
             use more GPU features, you should use the
             <emphasis>mesa3D</emphasis> library to create OpenGL contexts on DRM
             devices. This does <emphasis>not</emphasis> 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
             <citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
             <citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
             and
             <citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
             However, the
             <citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
             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.</para>
     </refsect2>
   </refsect1>

   <refsect1>
     <title>Examples</title>
       <para>This section includes examples for basic memory-management
             tasks.</para>

     <refsect2>
       <title>Dumb-Buffers</title>
         <para>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
               <citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>

 <programlisting>
 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(&amp;creq, 0, sizeof(creq));
 creq.width = 1920;
 creq.height = 1080;
 creq.bpp = 32;
 ret = drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &amp;creq);
 if (ret &lt; 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, &amp;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(&amp;mreq, 0, sizeof(mreq));
 mreq.handle = creq.handle;
 ret = drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &amp;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);
 </programlisting>

     </refsect2>

   </refsect1>

   <refsect1>
     <title>Reporting Bugs</title>
     <para>Bugs in this manual should be reported to
       https://bugs.freedesktop.org/enter_bug.cgi?product=DRI&amp;component=libdrm
       under the "DRI" product, component "libdrm"</para>
   </refsect1>

   <refsect1>
     <title>See Also</title>
     <para>
       <citerefentry><refentrytitle>drm</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drmAvailable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drmOpen</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>
     </para>
   </refsect1>
 </refentry>
	<?xml version='1.0'?> <!---nxml--->
	<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

	<!--
	Written 2012 by David Herrmann <dh.herrmann@googlemail.com>
	Dedicated to the Public Domain
	-->

	<refentry id="drm-memory">
	<refentryinfo>
	<title>Direct Rendering Manager</title>
	<productname>libdrm</productname>
	<date>September 2012</date>
	<authorgroup>
	<author>
	<contrib>Developer</contrib>
	<firstname>David</firstname>
	<surname>Herrmann</surname>
	<email>dh.herrmann@googlemail.com</email>
	</author>
	</authorgroup>
	</refentryinfo>

	<refmeta>
	<refentrytitle>drm-memory</refentrytitle>
	<manvolnum>7</manvolnum>
	</refmeta>

	<refnamediv>
	<refname>drm-memory</refname>
	<refname>drm-mm</refname>
	<refname>drm-gem</refname>
	<refname>drm-ttm</refname>
	<refpurpose>DRM Memory Management</refpurpose>
	</refnamediv>

	<refsynopsisdiv>
	<funcsynopsis>
	<funcsynopsisinfo>#include <xf86drm.h></funcsynopsisinfo>
	</funcsynopsis>
	</refsynopsisdiv>

	<refsect1>
	<title>Description</title>
	<para>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.</para>

	<para>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.</para>

	<refsect2>
	<title>Dumb-Buffers</title>
	<para>Almost all in-kernel DRM hardware drivers support an API called
	<emphasis>Dumb-Buffers</emphasis>. This API allows to create buffers
	of arbitrary size that can be used for scanout. These buffers can be
	memory mapped via
	<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
	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.</para>

	<para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> 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
	<citerefentry><refentrytitle>drmModeAddFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>
	and use it for mode-setting and scanout. To access the buffer, you
	first need to retrieve the offset of the buffer. The
	<constant>DRM_IOCTL_MODE_MAP_DUMB</constant> ioctl requests the DRM
	subsystem to prepare the buffer for memory-mapping and returns a
	fake-offset that can be used with
	<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>.</para>

	<para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> ioctl takes as
	argument a structure of type
	<structname>struct drm_mode_create_dumb</structname>:

	<programlisting>
	struct drm_mode_create_dumb {
	__u32 height;
	__u32 width;
	__u32 bpp;
	__u32 flags;

	__u32 handle;
	__u32 pitch;
	__u64 size;
	};
	</programlisting>

	The fields <structfield>height</structfield>,
	<structfield>width</structfield>, <structfield>bpp</structfield> and
	<structfield>flags</structfield> have to be provided by the caller.
	The other fields are filled by the kernel with the return values.
	<structfield>height</structfield> and
	<structfield>width</structfield> are the dimensions of the
	rectangular buffer that is created. <structfield>bpp</structfield>
	is the number of bits-per-pixel and must be a multiple of
	<literal>8</literal>. You most commonly want to pass
	<literal>32</literal> here. The <structfield>flags</structfield>
	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 <structfield>handle</structfield>,
	<structfield>pitch</structfield> and <structfield>size</structfield>
	fields are filled by the kernel. <structfield>handle</structfield>
	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 <structfield>pitch</structfield> field is the
	pitch (or stride) of the new buffer. Most drivers use 32bit or 64bit
	aligned stride-values. The <structfield>size</structfield> field
	contains the absolute size in bytes of the buffer. This can normally
	also be computed with
	<emphasis>(height * pitch + width) * bpp / 4</emphasis>.</para>

	<para>To prepare the buffer for
	<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
	you need to use the <constant>DRM_IOCTL_MODE_MAP_DUMB</constant>
	ioctl. It takes as argument a structure of type
	<structname>struct drm_mode_map_dumb</structname>:

	<programlisting>
	struct drm_mode_map_dumb {
	__u32 handle;
	__u32 pad;

	__u64 offset;
	};
	</programlisting>

	You need to put the gem-handle that was previously retrieved via
	<constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> into the
	<structfield>handle</structfield> field. The
	<structfield>pad</structfield> field is unused padding and must be
	zeroed. After completion, the <structfield>offset</structfield>
	field will contain an offset that can be used with
	<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
	on the DRM file-descriptor.</para>

	<para>If you don't need your dumb-buffer, anymore, you have to destroy it
	with <constant>DRM_IOCTL_MODE_DESTROY_DUMB</constant>. If you close
	the DRM file-descriptor, all open dumb-buffers are automatically
	destroyed. This ioctl takes as argument a structure of type
	<structname>struct drm_mode_destroy_dumb</structname>:

	<programlisting>
	struct drm_mode_destroy_dumb {
	__u32 handle;
	};
	</programlisting>

	You only need to put your handle into the
	<structfield>handle</structfield> field. After this call, the handle
	is invalid and may be reused for new buffers by the dumb-API.</para>

	</refsect2>

	<refsect2>
	<title>TTM</title>
	<para><emphasis>TTM</emphasis> stands for
	<emphasis>Translation Table Manager</emphasis> 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.</para>
	</refsect2>

	<refsect2>
	<title>GEM</title>
	<para><emphasis>GEM</emphasis> stands for
	<emphasis>Graphics Execution Manager</emphasis> 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.</para>

	<para>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.</para>

	<para>All GEM APIs are defined as
	<citerefentry><refentrytitle>ioctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
	on the DRM file descriptor. An application must be authorized via
	<citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
	to the current DRM-Master to access the GEM subsystem. A driver that
	does not support gem will return <constant>ENODEV</constant> for all
	these ioctls. Invalid object handles return
	<constant>EINVAL</constant> and invalid object names return
	<constant>ENOENT</constant>.</para>

	<para>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.</para>

	<para>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
	<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	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.</para>

	<para>Gem-buffers cannot be created with a generic API. Each driver
	provides its own API to create gem-buffers. See for example
	<constant>DRM_I915_GEM_CREATE</constant>,
	<constant>DRM_NOUVEAU_GEM_NEW</constant> or
	<constant>DRM_RADEON_GEM_CREATE</constant>. Each of these ioctls
	returns a gem-handle that can be passed to different generic ioctls.
	The <emphasis>libgbm</emphasis> library from the
	<emphasis>mesa3D</emphasis> 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
	<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	or
	<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>

	<para>Gem-buffers can be closed with the
	<constant>DRM_IOCTL_GEM_CLOSE</constant> ioctl. It takes as argument
	a structure of type <structname>struct drm_gem_close</structname>:

	<programlisting>
	struct drm_gem_close {
	__u32 handle;
	__u32 pad;
	};
	</programlisting>

	The <structfield>handle</structfield> field is the gem-handle to be
	closed. The <structfield>pad</structfield> 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.</para>

	<para>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
	<citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
	to the current DRM-Master, can <emphasis>guess</emphasis> the name
	and open or access the gem-object. If you want more fine-grained
	access control, you can use the new
	<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	API to retrieve file-descriptors for gem-handles. To create a name
	for a gem-handle, you use the
	<constant>DRM_IOCTL_GEM_FLINK</constant> ioctl. It takes as argument
	a structure of type <structname>struct drm_gem_flink</structname>:

	<programlisting>
	struct drm_gem_flink {
	__u32 handle;
	__u32 name;
	};
	</programlisting>

	You have to put your handle into the
	<structfield>handle</structfield> field. After completion, the
	kernel has put the new unique name into the
	<structfield>name</structfield> field. You can now pass this name to
	other processes which can then import the name with the
	<constant>DRM_IOCTL_GEM_OPEN</constant> ioctl. It takes as argument
	a structure of type <structname>struct drm_gem_open</structname>:

	<programlisting>
	struct drm_gem_open {
	__u32 name;

	__u32 handle;
	__u32 size;
	};
	</programlisting>

	You have to fill in the <structfield>name</structfield> field with
	the name of the gem-object that you want to open. The kernel will
	fill in the <structfield>handle</structfield> and
	<structfield>size</structfield> 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.</para>

	<para>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 <emphasis>OpenGL</emphasis>. So if you want to
	use more GPU features, you should use the
	<emphasis>mesa3D</emphasis> library to create OpenGL contexts on DRM
	devices. This does <emphasis>not</emphasis> 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
	<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	and
	<citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
	However, the
	<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	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.</para>
	</refsect2>
	</refsect1>

	<refsect1>
	<title>Examples</title>
	<para>This section includes examples for basic memory-management
	tasks.</para>

	<refsect2>
	<title>Dumb-Buffers</title>
	<para>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
	<citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>

	<programlisting>
	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);
	</programlisting>

	</refsect2>

	</refsect1>

	<refsect1>
	<title>Reporting Bugs</title>
	<para>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"</para>
	</refsect1>

	<refsect1>
	<title>See Also</title>
	<para>
	<citerefentry><refentrytitle>drm</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drmAvailable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drmOpen</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	</para>
	</refsect1>
	</refentry>