Dumb-Buffers

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

GEM

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.

Dumb-Buffers

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);