431 lines
20 KiB
XML
431 lines
20 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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Written 2012 by David Herrmann <dh.herrmann@googlemail.com>
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Dedicated to the Public Domain
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-->
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<refentry id="drm-memory">
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<refentryinfo>
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<title>Direct Rendering Manager</title>
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<productname>libdrm</productname>
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<date>September 2012</date>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>David</firstname>
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<surname>Herrmann</surname>
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<email>dh.herrmann@googlemail.com</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>drm-memory</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>drm-memory</refname>
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<refname>drm-mm</refname>
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<refname>drm-gem</refname>
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<refname>drm-ttm</refname>
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<refpurpose>DRM Memory Management</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <xf86drm.h></funcsynopsisinfo>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>Many modern high-end GPUs come with their own memory managers. They
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even include several different caches that need to be synchronized
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during access. Textures, framebuffers, command buffers and more need
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to be stored in memory that can be accessed quickly by the GPU.
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Therefore, memory management on GPUs is highly driver- and
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hardware-dependent.</para>
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<para>However, there are several frameworks in the kernel that are used by
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more than one driver. These can be used for trivial mode-setting
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without requiring driver-dependent code. But for
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hardware-accelerated rendering you need to read the manual pages for
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the driver you want to work with.</para>
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<refsect2>
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<title>Dumb-Buffers</title>
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<para>Almost all in-kernel DRM hardware drivers support an API called
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<emphasis>Dumb-Buffers</emphasis>. This API allows to create buffers
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of arbitrary size that can be used for scanout. These buffers can be
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memory mapped via
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<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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so you can render into them on the CPU. However, GPU access to these
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buffers is often not possible. Therefore, they are fine for simple
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tasks but not suitable for complex compositions and
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renderings.</para>
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<para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> ioctl can be
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used to create a dumb buffer. The kernel will return a 32bit handle
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that can be used to manage the buffer with the DRM API. You can
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create framebuffers with
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<citerefentry><refentrytitle>drmModeAddFB</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and use it for mode-setting and scanout. To access the buffer, you
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first need to retrieve the offset of the buffer. The
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<constant>DRM_IOCTL_MODE_MAP_DUMB</constant> ioctl requests the DRM
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subsystem to prepare the buffer for memory-mapping and returns a
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fake-offset that can be used with
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<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>.</para>
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<para>The <constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> ioctl takes as
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argument a structure of type
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<structname>struct drm_mode_create_dumb</structname>:
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<programlisting>
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struct drm_mode_create_dumb {
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__u32 height;
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__u32 width;
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__u32 bpp;
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__u32 flags;
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__u32 handle;
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__u32 pitch;
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__u64 size;
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};
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</programlisting>
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The fields <structfield>height</structfield>,
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<structfield>width</structfield>, <structfield>bpp</structfield> and
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<structfield>flags</structfield> have to be provided by the caller.
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The other fields are filled by the kernel with the return values.
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<structfield>height</structfield> and
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<structfield>width</structfield> are the dimensions of the
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rectangular buffer that is created. <structfield>bpp</structfield>
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is the number of bits-per-pixel and must be a multiple of
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<literal>8</literal>. You most commonly want to pass
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<literal>32</literal> here. The <structfield>flags</structfield>
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field is currently unused and must be zeroed. Different flags to
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modify the behavior may be added in the future. After calling the
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ioctl, the <structfield>handle</structfield>,
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<structfield>pitch</structfield> and <structfield>size</structfield>
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fields are filled by the kernel. <structfield>handle</structfield>
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is a 32bit gem handle that identifies the buffer. This is used by
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several other calls that take a gem-handle or memory-buffer as
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argument. The <structfield>pitch</structfield> field is the
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pitch (or stride) of the new buffer. Most drivers use 32bit or 64bit
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aligned stride-values. The <structfield>size</structfield> field
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contains the absolute size in bytes of the buffer. This can normally
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also be computed with
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<emphasis>(height * pitch + width) * bpp / 4</emphasis>.</para>
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<para>To prepare the buffer for
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<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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you need to use the <constant>DRM_IOCTL_MODE_MAP_DUMB</constant>
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ioctl. It takes as argument a structure of type
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<structname>struct drm_mode_map_dumb</structname>:
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<programlisting>
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struct drm_mode_map_dumb {
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__u32 handle;
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__u32 pad;
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__u64 offset;
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};
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</programlisting>
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You need to put the gem-handle that was previously retrieved via
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<constant>DRM_IOCTL_MODE_CREATE_DUMB</constant> into the
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<structfield>handle</structfield> field. The
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<structfield>pad</structfield> field is unused padding and must be
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zeroed. After completion, the <structfield>offset</structfield>
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field will contain an offset that can be used with
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<citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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on the DRM file-descriptor.</para>
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<para>If you don't need your dumb-buffer, anymore, you have to destroy it
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with <constant>DRM_IOCTL_MODE_DESTROY_DUMB</constant>. If you close
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the DRM file-descriptor, all open dumb-buffers are automatically
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destroyed. This ioctl takes as argument a structure of type
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<structname>struct drm_mode_destroy_dumb</structname>:
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<programlisting>
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struct drm_mode_destroy_dumb {
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__u32 handle;
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};
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</programlisting>
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You only need to put your handle into the
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<structfield>handle</structfield> field. After this call, the handle
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is invalid and may be reused for new buffers by the dumb-API.</para>
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</refsect2>
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<refsect2>
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<title>TTM</title>
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<para><emphasis>TTM</emphasis> stands for
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<emphasis>Translation Table Manager</emphasis> and is a generic
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memory-manager provided by the kernel. It does not provide a common
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user-space API so you need to look at each driver interface if you
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want to use it. See for instance the radeon manpages for more
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information on memory-management with radeon and TTM.</para>
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</refsect2>
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<refsect2>
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<title>GEM</title>
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<para><emphasis>GEM</emphasis> stands for
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<emphasis>Graphics Execution Manager</emphasis> and is a generic DRM
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memory-management framework in the kernel, that is used by many
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different drivers. Gem is designed to manage graphics memory,
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control access to the graphics device execution context and handle
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essentially NUMA environment unique to modern graphics hardware. Gem
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allows multiple applications to share graphics device resources
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without the need to constantly reload the entire graphics card. Data
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may be shared between multiple applications with gem ensuring that
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the correct memory synchronization occurs.</para>
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<para>Gem provides simple mechanisms to manage graphics data and control
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execution flow within the linux DRM subsystem. However, gem is not a
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complete framework that is fully driver independent. Instead, if
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provides many functions that are shared between many drivers, but
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each driver has to implement most of memory-management with
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driver-dependent ioctls. This manpage tries to describe the
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semantics (and if it applies, the syntax) that is shared between all
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drivers that use gem.</para>
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<para>All GEM APIs are defined as
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<citerefentry><refentrytitle>ioctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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on the DRM file descriptor. An application must be authorized via
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<citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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to the current DRM-Master to access the GEM subsystem. A driver that
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does not support gem will return <constant>ENODEV</constant> for all
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these ioctls. Invalid object handles return
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<constant>EINVAL</constant> and invalid object names return
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<constant>ENOENT</constant>.</para>
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<para>Gem provides explicit memory management primitives. System pages are
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allocated when the object is created, either as the fundamental
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storage for hardware where system memory is used by the graphics
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processor directly, or as backing store for graphics-processor
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resident memory.</para>
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<para>Objects are referenced from user-space using handles. These are, for
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all intents and purposes, equivalent to file descriptors but avoid
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the overhead. Newer kernel drivers also support the
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<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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infrastructure which can return real file-descriptor for gem-handles
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using the linux dma-buf API. Objects may be published with a name so
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that other applications and processes can access them. The name
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remains valid as long as the object exists. Gem-objects are
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reference counted in the kernel. The object is only destroyed when
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all handles from user-space were closed.</para>
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<para>Gem-buffers cannot be created with a generic API. Each driver
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provides its own API to create gem-buffers. See for example
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<constant>DRM_I915_GEM_CREATE</constant>,
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<constant>DRM_NOUVEAU_GEM_NEW</constant> or
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<constant>DRM_RADEON_GEM_CREATE</constant>. Each of these ioctls
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returns a gem-handle that can be passed to different generic ioctls.
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The <emphasis>libgbm</emphasis> library from the
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<emphasis>mesa3D</emphasis> distribution tries to provide a
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driver-independent API to create gbm buffers and retrieve a
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gbm-handle to them. It allows to create buffers for different
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use-cases including scanout, rendering, cursors and CPU-access. See
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the libgbm library for more information or look at the
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driver-dependent man-pages (for example
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<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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or
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<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>
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<para>Gem-buffers can be closed with the
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<constant>DRM_IOCTL_GEM_CLOSE</constant> ioctl. It takes as argument
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a structure of type <structname>struct drm_gem_close</structname>:
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<programlisting>
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struct drm_gem_close {
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__u32 handle;
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__u32 pad;
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};
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</programlisting>
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The <structfield>handle</structfield> field is the gem-handle to be
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closed. The <structfield>pad</structfield> field is unused padding.
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It must be zeroed. After this call the gem handle cannot be used by
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this process anymore and may be reused for new gem objects by the
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gem API.</para>
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<para>If you want to share gem-objects between different processes, you
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can create a name for them and pass this name to other processes
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which can then open this gem-object. Names are currently 32bit
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integer IDs and have no special protection. That is, if you put a
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name on your gem-object, every other client that has access to the
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DRM device and is authenticated via
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<citerefentry><refentrytitle>drmAuthMagic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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to the current DRM-Master, can <emphasis>guess</emphasis> the name
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and open or access the gem-object. If you want more fine-grained
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access control, you can use the new
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<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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API to retrieve file-descriptors for gem-handles. To create a name
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for a gem-handle, you use the
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<constant>DRM_IOCTL_GEM_FLINK</constant> ioctl. It takes as argument
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a structure of type <structname>struct drm_gem_flink</structname>:
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<programlisting>
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struct drm_gem_flink {
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__u32 handle;
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__u32 name;
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};
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</programlisting>
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You have to put your handle into the
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<structfield>handle</structfield> field. After completion, the
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kernel has put the new unique name into the
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<structfield>name</structfield> field. You can now pass this name to
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other processes which can then import the name with the
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<constant>DRM_IOCTL_GEM_OPEN</constant> ioctl. It takes as argument
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a structure of type <structname>struct drm_gem_open</structname>:
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<programlisting>
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struct drm_gem_open {
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__u32 name;
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__u32 handle;
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__u32 size;
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};
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</programlisting>
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You have to fill in the <structfield>name</structfield> field with
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the name of the gem-object that you want to open. The kernel will
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fill in the <structfield>handle</structfield> and
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<structfield>size</structfield> fields with the new handle and size
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of the gem-object. You can now access the gem-object via the handle
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as if you created it with the gem API.</para>
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<para>Besides generic buffer management, the GEM API does not provide any
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generic access. Each driver implements its own functionality on top
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of this API. This includes execution-buffers, GTT management,
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context creation, CPU access, GPU I/O and more. The next
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higher-level API is <emphasis>OpenGL</emphasis>. So if you want to
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use more GPU features, you should use the
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<emphasis>mesa3D</emphasis> library to create OpenGL contexts on DRM
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devices. This does <emphasis>not</emphasis> require any
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windowing-system like X11, but can also be done on raw DRM devices.
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However, this is beyond the scope of this man-page. You may have a
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look at other mesa3D manpages, including libgbm and libEGL. 2D
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software-rendering (rendering with the CPU) can be achieved with the
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dumb-buffer-API in a driver-independent fashion, however, for
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hardware-accelerated 2D or 3D rendering you must use OpenGL. Any
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other API that tries to abstract the driver-internals to access
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GEM-execution-buffers and other GPU internals, would simply reinvent
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OpenGL so it is not provided. But if you need more detailed
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information for a specific driver, you may have a look into the
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driver-manpages, including
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<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
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However, the
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<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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infrastructure and the generic gem API as described here allow
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display-managers to handle graphics-buffers and render-clients
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without any deeper knowledge of the GPU that is used. Moreover, it
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allows to move objects between GPUs and implement complex
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display-servers that don't do any rendering on their own. See its
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man-page for more information.</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>This section includes examples for basic memory-management
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tasks.</para>
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<refsect2>
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<title>Dumb-Buffers</title>
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<para>This examples shows how to create a dumb-buffer via the generic
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DRM API. This is driver-independent (as long as the driver
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supports dumb-buffers) and provides memory-mapped buffers that can
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be used for scanout. This example creates a full-HD 1920x1080
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buffer with 32 bits-per-pixel and a color-depth of 24 bits. The
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buffer is then bound to a framebuffer which can be used for
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scanout with the KMS API (see
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<citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>
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<programlisting>
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struct drm_mode_create_dumb creq;
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struct drm_mode_destroy_dumb dreq;
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struct drm_mode_map_dumb mreq;
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uint32_t fb;
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int ret;
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void *map;
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/* create dumb buffer */
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memset(&creq, 0, sizeof(creq));
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creq.width = 1920;
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creq.height = 1080;
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creq.bpp = 32;
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ret = drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &creq);
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if (ret < 0) {
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/* buffer creation failed; see "errno" for more error codes */
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...
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}
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/* creq.pitch, creq.handle and creq.size are filled by this ioctl with
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* the requested values and can be used now. */
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/* create framebuffer object for the dumb-buffer */
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ret = drmModeAddFB(fd, 1920, 1080, 24, 32, creq.pitch, creq.handle, &fb);
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if (ret) {
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/* frame buffer creation failed; see "errno" */
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...
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}
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/* the framebuffer "fb" can now used for scanout with KMS */
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/* prepare buffer for memory mapping */
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memset(&mreq, 0, sizeof(mreq));
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mreq.handle = creq.handle;
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ret = drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &mreq);
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if (ret) {
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/* DRM buffer preparation failed; see "errno" */
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...
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}
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/* mreq.offset now contains the new offset that can be used with mmap() */
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/* perform actual memory mapping */
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map = mmap(0, creq.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mreq.offset);
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if (map == MAP_FAILED) {
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/* memory-mapping failed; see "errno" */
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...
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}
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/* clear the framebuffer to 0 */
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memset(map, 0, creq.size);
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</programlisting>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Reporting Bugs</title>
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<para>Bugs in this manual should be reported to
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http://bugs.freedesktop.org under the "Mesa" product, with "Other" or
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"libdrm" as the component.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>drm</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-kms</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-prime</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drmAvailable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drmOpen</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-intel</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-radeon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>drm-nouveau</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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