extensions/KHR/EGL_KHR_platform_gbm.txt

Name

    KHR_platform_gbm

Name Strings

    EGL_KHR_platform_gbm

Contributors

    Chad Versace <chad.versace@intel.com>
    Jon Leech (oddhack 'at' sonic.net)
    Kristian Høgsberg <krh@bitplanet.org>

Contacts

    Chad Versace <chad.versace@intel.com>

Status

    Complete.
    Approved by the EGL Working Group on January 31, 2014.
    Ratified by the Khronos Board of Promoters on March 14, 2014.

Version

    Version 3, 2016/01/04

Number

    EGL Extension #69

Extension Type

    EGL client extension

Dependencies

    EGL 1.5 is required.

    This extension is written against the EGL 1.5 Specification (draft
    20140122).

Overview

    This extension defines how to create EGL resources from native GBM
    resources using the EGL 1.5 platform functionality (GBM is a Generic
    Buffer Manager for Linux).

New Types

    None

New Procedures and Functions

    None

New Tokens

    Accepted as the <platform> argument of eglGetPlatformDisplay:

        EGL_PLATFORM_GBM_KHR                     0x31D7

Additions to the EGL Specification

    None.

New Behavior

    To determine if the EGL implementation supports this extension, clients
    should query the EGL_EXTENSIONS string of EGL_NO_DISPLAY.

    To obtain an EGLDisplay from an GBM device, call eglGetPlatformDisplay with
    <platform> set to EGL_PLATFORM_GBM_KHR. The <native_display> parameter
    specifies the GBM device to use and must either point to a `struct
    gbm_device` or be EGL_DEFAULT_DISPLAY. If <native_display> is
    EGL_DEFAULT_DISPLAY, then the resultant EGLDisplay will be backed by some
    implementation-chosen GBM device.

    For each EGLConfig that belongs to the GBM platform, the
    EGL_NATIVE_VISUAL_ID attribute is a GBM color format, such as
    GBM_FORMAT_XRGB8888.

    To obtain a rendering surface from a GBM surface, call
    eglCreatePlatformWindowSurface with a <dpy> that belongs to the GBM
    platform and a <native_window> that points to a `struct gbm_surface`.  If
    <native_window> was created without the GBM_BO_USE_RENDERING flag, or if
    the color format of <native_window> differs from the EGL_NATIVE_VISUAL_ID
    of <config>, then the function fails and generates EGL_BAD_MATCH.

    It is not valid to call eglCreatePlatformPixmapSurface with a <dpy> that
    belongs to the GBM platform. Any such call fails and generates
    an EGL_BAD_PARAMETER error.

Issues

    1. Should this extension permit EGL_DEFAULT_DISPLAY as input to
       eglGetPlatformDisplay?

       RESOLUTION: Yes. When given EGL_DEFAULT_DISPLAY, eglGetPlatformDisplay
       returns an EGLDisplay backed by an implementation-chosen GBM device.

Example Code

    // This example program creates an EGL surface from a GBM surface.
    //
    // If the macro EGL_KHR_platform_gbm is defined, then the program
    // creates the surfaces using the methods defined in this specification.
    // Otherwise, it uses the methods defined by the EGL 1.4 specification.
    //
    // Compile with `cc -std=c99 example.c -lgbm -lEGL`.

    #include <stdlib.h>
    #include <string.h>

    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>

    #include <EGL/egl.h>
    #include <gbm.h>

    struct my_display {
        struct gbm_device *gbm;
        EGLDisplay egl;
    };

    struct my_config {
        struct my_display dpy;
        EGLConfig egl;
    };

    struct my_window {
        struct my_config config;
        struct gbm_surface *gbm;
        EGLSurface egl;
    };

    static void
    check_extensions(void)
    {
    #ifdef EGL_KHR_platform_gbm
        const char *client_extensions = eglQueryString(EGL_NO_DISPLAY, EGL_EXTENSIONS);

        if (!client_extensions) {
            // No client extensions string available
            abort();
        }
        if (!strstr(client_extensions, "EGL_KHR_platform_gbm")) {
            abort();
        }
    #endif
    }

    static struct my_display
    get_display(void)
    {
        struct my_display dpy;

        int fd = open("/dev/dri/card0", O_RDWR | FD_CLOEXEC);
        if (fd < 0) {
            abort();
        }

        dpy.gbm = gbm_create_device(fd);
        if (!dpy.gbm) {
            abort();
        }


    #ifdef EGL_KHR_platform_gbm
        dpy.egl = eglGetPlatformDisplay(EGL_PLATFORM_GBM_KHR, dpy.gbm, NULL);
    #else
        dpy.egl = eglGetDisplay(dpy.gbm);
    #endif

        if (dpy.egl == EGL_NO_DISPLAY) {
            abort();
        }

        EGLint major, minor;
        if (!eglInitialize(dpy.egl, &major, &minor)) {
            abort();
        }

        return dpy;
    }

    static struct my_config
    get_config(struct my_display dpy)
    {
        struct my_config config = {
            .dpy = dpy,
        };

        EGLint egl_config_attribs[] = {
            EGL_BUFFER_SIZE,        32,
            EGL_DEPTH_SIZE,         EGL_DONT_CARE,
            EGL_STENCIL_SIZE,       EGL_DONT_CARE,
            EGL_RENDERABLE_TYPE,    EGL_OPENGL_ES2_BIT,
            EGL_SURFACE_TYPE,       EGL_WINDOW_BIT,
            EGL_NONE,
        };

        EGLint num_configs;
        if (!eglGetConfigs(dpy.egl, NULL, 0, &num_configs)) {
            abort();
        }

        EGLConfig *configs = malloc(num_configs * sizeof(EGLConfig));
        if (!eglChooseConfig(dpy.egl, egl_config_attribs,
                             configs, num_configs, &num_configs)) {
            abort();
        }
        if (num_configs == 0) {
            abort();
        }

        // Find a config whose native visual ID is the desired GBM format.
        for (int i = 0; i < num_configs; ++i) {
            EGLint gbm_format;

            if (!eglGetConfigAttrib(dpy.egl, configs[i],
                                    EGL_NATIVE_VISUAL_ID, &gbm_format)) {
                abort();
            }

            if (gbm_format == GBM_FORMAT_XRGB8888) {
                config.egl = configs[i];
                free(configs);
                return config;
            }
        }

        // Failed to find a config with matching GBM format.
        abort();
    }

    static struct my_window
    get_window(struct my_config config)
    {
        struct my_window window = {
            .config = config,
        };

        window.gbm = gbm_surface_create(config.dpy.gbm,
                                        256, 256,
                                        GBM_FORMAT_XRGB8888,
                                        GBM_BO_USE_RENDERING);
        if (!window.gbm) {
            abort();
        }

    #ifdef EGL_KHR_platform_gbm
        window.egl = eglCreatePlatformWindowSurface(config.dpy.egl,
                                                    config.egl,
                                                    window.gbm,
                                                    NULL);
    #else
        window.egl = eglCreateWindowSurface(config.dpy.egl,
                                            config.egl,
                                            window.gbm,
                                            NULL);
    #endif

        if (window.egl == EGL_NO_SURFACE) {
            abort();
        }

        return window;
    }

    int
    main(void)
    {
        check_extensions();

        struct my_display dpy = get_display();
        struct my_config config = get_config(dpy);
        struct my_window window = get_window(config);

        return 0;
    }

Revision History

    Version 3, 2016-01-04 (Jon Leech)
        - Free config memory allocated in sample code (Public Bug 1445).

    Version 2, 2014/02/12 (Chad Versace)
        - Change resolution of issue #1 from "no" to "yes". Now
          eglGetPlatformDisplay accepts EGL_DEFAULT_DISPLAY for GBM.

    Version 1, 2014/01/22 (Jon Leech)
        - Promote EGL_MESA_platform_gbm to KHR to go with EGL 1.5.