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