An example, inspired by Rob Clark "kmscube.c" that uses Linux Direct Rendering Manager ( DRM ) and EGL to create an OpenGL ES 2 context. This is a standalone example, that just clears the screen with a blueish color. This is a fork of Miouyouyou's gist intended for use with the BeagleBone Black's IT PowerVR SGX530 GPU.

Linux_DRM_OpenGLES.c

// gcc -o drmgl Linux_DRM_OpenGLES.c `pkg-config --cflags --libs libdrm` -lgbm -lEGL -lGLESv2 -ldl -lsrv_um -I/usr/include/gbm -lIMGegl -ldrm_omap

/*
 * Copyright (c) 2012 Arvin Schnell <[email protected]>
 * Copyright (c) 2012 Rob Clark <[email protected]>
 * Copyright (c) 2013 Anand Balagopalakrishnan <[email protected]>
 * Copyright (c) 2017 Miouyouyou <Myy> <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sub license,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the
 * next paragraph) shall be included in all copies or substantial portions
 * of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

/* Based on a egl cube test app originally written by Arvin Schnell */
/* This is a port of Miouyouyou's blue screen */
/* DRM + GBM + EGL + OpenGLES test to GNU/Linux for */
/* the BeagleBone Black / TI AM335x based on TI's fork of the egl */
/* cube test app ("kmscube"). */

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>

#include <xf86drm.h>
#include <xf86drmMode.h>
#include <drm_fourcc.h>
#include <gbm.h>

#define GL_GLEXT_PROTOTYPES 1
#include <GLES2/gl2.h>
#include <GLES2/gl2ext.h>
#include <EGL/egl.h>
#include <EGL/eglext.h>

#include <assert.h>

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

#define MAX_DISPLAYS 	(4)
uint8_t DISP_ID = 0;
uint8_t all_display = 0;
int8_t connector_id = 30;
char* device = "/dev/dri/card1";

static struct {
	EGLDisplay display;
	EGLConfig config;
	EGLContext context;
	EGLSurface surface;
	GLuint program;
	GLint modelviewmatrix, modelviewprojectionmatrix, normalmatrix;
	GLuint vbo;
	GLuint positionsoffset, colorsoffset, normalsoffset;
       	GLuint vertex_shader, fragment_shader;
} gl;

static struct {
	struct gbm_device *dev;
	struct gbm_surface *surface;
} gbm;

static struct {
	int fd;
	uint32_t ndisp;
	uint32_t crtc_id[MAX_DISPLAYS];
	uint32_t connector_id[MAX_DISPLAYS];
	uint32_t resource_id;
	uint32_t encoder[MAX_DISPLAYS];
	uint32_t format[MAX_DISPLAYS];
	drmModeModeInfo *mode[MAX_DISPLAYS];
	drmModeConnector *connectors[MAX_DISPLAYS];
} drm;

struct drm_fb {
	struct gbm_bo *bo;
	uint32_t fb_id;
};

static uint32_t drm_fmt_to_gbm_fmt(uint32_t fmt)
{
	switch (fmt) {
		case DRM_FORMAT_XRGB8888:
			return GBM_FORMAT_XRGB8888;
		case DRM_FORMAT_ARGB8888:
			return GBM_FORMAT_ARGB8888;
		case DRM_FORMAT_RGB565:
			return GBM_FORMAT_RGB565;
		default:
			printf("Unsupported DRM format: 0x%x", fmt);
			return GBM_FORMAT_XRGB8888;
	}
}

static bool search_plane_format(uint32_t desired_format, int formats_count, uint32_t* formats)
{
	int i;

	for ( i = 0; i < formats_count; i++)
	{
		if (desired_format == formats[i])
			return true;
	}

	return false;
}

int get_drm_prop_val(int fd, drmModeObjectPropertiesPtr props,
	                 const char *name, unsigned int *p_val) {
	drmModePropertyPtr p;
	unsigned int i, prop_id = 0; /* Property ID should always be > 0 */

	for (i = 0; !prop_id && i < props->count_props; i++) {
		p = drmModeGetProperty(fd, props->props[i]);
		if (!strcmp(p->name, name)){
			prop_id = p->prop_id;
			break;
		}
		drmModeFreeProperty(p);
	}

	if (!prop_id) {
		printf("Could not find %s property\n", name);
		return(-1);
	}

	drmModeFreeProperty(p);
	*p_val = props->prop_values[i];
	return 0;
}

static bool set_drm_format(void)
{
	/* desired DRM format in order */
	static const uint32_t drm_formats[] = {DRM_FORMAT_XRGB8888, DRM_FORMAT_ARGB8888, DRM_FORMAT_RGB565};
	drmModePlaneRes *plane_res;
	bool found = false;
	int i,k;

	drmSetClientCap(drm.fd, DRM_CLIENT_CAP_UNIVERSAL_PLANES, 1);

	plane_res  = drmModeGetPlaneResources(drm.fd);

	if (!plane_res) {
		printf("drmModeGetPlaneResources failed: %s\n", strerror(errno));
		drmSetClientCap(drm.fd, DRM_CLIENT_CAP_UNIVERSAL_PLANES, 0);
		return false;
	}

	/*
	 * find the plane connected to crtc_id (the primary plane) and then find the desired pixel format
	 * from the plane format list
	 */
	for (i = 0; i < plane_res->count_planes; i++)
	{
		drmModePlane *plane = drmModeGetPlane(drm.fd, plane_res->planes[i]);
		drmModeObjectProperties *props;
		unsigned int plane_type;

		if(plane == NULL)
			continue;

		props = drmModeObjectGetProperties(drm.fd, plane->plane_id, DRM_MODE_OBJECT_PLANE);

		if(props == NULL){
			printf("plane (%d) properties not found\n",  plane->plane_id);
			drmModeFreePlane(plane);
			continue;
		}

		if(get_drm_prop_val(drm.fd, props, "type",  &plane_type) < 0)
		{
			printf("plane (%d) type value not found\n",  plane->plane_id);
			drmModeFreeObjectProperties(props);
			drmModeFreePlane(plane);
			continue;
		}

		if (plane_type != DRM_PLANE_TYPE_PRIMARY)
		{
			drmModeFreeObjectProperties(props);
			drmModeFreePlane(plane);
			continue;
		}
		else if (!plane->crtc_id)
		{
			plane->crtc_id = drm.crtc_id[drm.ndisp];
		}

		drmModeFreeObjectProperties(props);

		if (plane->crtc_id == drm.crtc_id[drm.ndisp])
		{
			for (k = 0; k < ARRAY_SIZE(drm_formats); k++)
			{
				if (search_plane_format(drm_formats[k], plane->count_formats, plane->formats))
				{
					drm.format[drm.ndisp] = drm_formats[k];
					drmModeFreePlane(plane);
					drmModeFreePlaneResources(plane_res);
					drmSetClientCap(drm.fd, DRM_CLIENT_CAP_UNIVERSAL_PLANES, 0);
					return true;
				}
			}
		}

		drmModeFreePlane(plane);
	}

	drmModeFreePlaneResources(plane_res);
	drmSetClientCap(drm.fd, DRM_CLIENT_CAP_UNIVERSAL_PLANES, 0);
	return false;
}

static int init_drm(void)
{
	drmModeRes *resources;
	drmModeConnector *connector = NULL;
	drmModeEncoder *encoder = NULL;
	drmModeCrtc *crtc = NULL;

	int i, j, k;
	uint32_t maxRes, curRes;

	/* Open default dri device */
	drm.fd = open(device, O_RDWR | O_CLOEXEC);
	if (drm.fd < 0) {
		printf("could not open drm device %s\n", device);
		return -1;
	}

	resources = drmModeGetResources(drm.fd);
	if (!resources) {
		printf("drmModeGetResources failed: %s\n", strerror(errno));
		return -1;
	}
	drm.resource_id = (uint32_t) resources;

	/* find a connected connector: */
	for (i = 0; i < resources->count_connectors; i++) {
		connector = drmModeGetConnector(drm.fd, resources->connectors[i]);
		if (connector->connection == DRM_MODE_CONNECTED) {

			/* find the matched encoders */
			for (j=0; j<connector->count_encoders; j++) {
				encoder = drmModeGetEncoder(drm.fd, connector->encoders[j]);

				/* Take the fisrt one, if none is assigned */
				if (!connector->encoder_id)
				{
					connector->encoder_id = encoder->encoder_id;
				}

				if (encoder->encoder_id == connector->encoder_id)
				{
					/* find the first valid CRTC if not assigned */
					if (!encoder->crtc_id)
					{
						for (k = 0; k < resources->count_crtcs; ++k) {
							/* check whether this CRTC works with the encoder */
							if (!(encoder->possible_crtcs & (1 << k)))
								continue;

							encoder->crtc_id = resources->crtcs[k];
							break;
						}

						if (!encoder->crtc_id)
						{
							printf("Encoder(%d): no CRTC find!\n", encoder->encoder_id);
							drmModeFreeEncoder(encoder);
							encoder = NULL;
							continue;
						}
					}

					break;
				}

				drmModeFreeEncoder(encoder);
				encoder = NULL;
			}

			if (!encoder) {
				printf("Connector (%d): no encoder!\n", connector->connector_id);
				drmModeFreeConnector(connector);
				continue;
			}

			/* choose the current or first supported mode */
			crtc = drmModeGetCrtc(drm.fd, encoder->crtc_id);
			for (j = 0; j < connector->count_modes; j++)
			{
				if (crtc->mode_valid)
				{
					if ((connector->modes[j].hdisplay == crtc->width) &&
					(connector->modes[j].vdisplay == crtc->height))
					{
						drm.mode[drm.ndisp] = &connector->modes[j];
						break;
					}
				}
				else
				{
					if ((connector->modes[j].hdisplay == crtc->x) &&
					   (connector->modes[j].vdisplay == crtc->y))
					{
						drm.mode[drm.ndisp] = &connector->modes[j];
						break;
					}
				}
			}

			if(j >= connector->count_modes)
				drm.mode[drm.ndisp] = &connector->modes[0];

			drm.connector_id[drm.ndisp] = connector->connector_id;

			drm.encoder[drm.ndisp]  = (uint32_t) encoder;
			drm.crtc_id[drm.ndisp] = encoder->crtc_id;
			drm.connectors[drm.ndisp] = connector;

			if (!set_drm_format())
			{
				// Error handling
				printf("No desired pixel format found!\n");
				return -1;
			}

			printf("### Display [%d]: CRTC = %d, Connector = %d, format = 0x%x\n", drm.ndisp, drm.crtc_id[drm.ndisp], drm.connector_id[drm.ndisp], drm.format[drm.ndisp]);
			printf("\tMode chosen [%s] : Clock => %d, Vertical refresh => %d, Type => %d\n", drm.mode[drm.ndisp]->name, drm.mode[drm.ndisp]->clock, drm.mode[drm.ndisp]->vrefresh, drm.mode[drm.ndisp]->type);
			printf("\tHorizontal => %d, %d, %d, %d, %d\n", drm.mode[drm.ndisp]->hdisplay, drm.mode[drm.ndisp]->hsync_start, drm.mode[drm.ndisp]->hsync_end, drm.mode[drm.ndisp]->htotal, drm.mode[drm.ndisp]->hskew);
			printf("\tVertical => %d, %d, %d, %d, %d\n", drm.mode[drm.ndisp]->vdisplay, drm.mode[drm.ndisp]->vsync_start, drm.mode[drm.ndisp]->vsync_end, drm.mode[drm.ndisp]->vtotal, drm.mode[drm.ndisp]->vscan);

			/* If a connector_id is specified, use the corresponding display */
			if ((connector_id != -1) && (connector_id == drm.connector_id[drm.ndisp]))
				DISP_ID = drm.ndisp;

			drm.ndisp++;
		}
                else {
			drmModeFreeConnector(connector);
		}
	}

	if (drm.ndisp == 0) {
		/* we could be fancy and listen for hotplug events and wait for
		 * a connector..
		 */
		printf("no connected connector!\n");
		return -1;
	}

	return 0;
}

static int init_gbm(void)
{
	gbm.dev = gbm_create_device(drm.fd);

	gbm.surface = gbm_surface_create(gbm.dev,
			drm.mode[DISP_ID]->hdisplay, drm.mode[DISP_ID]->vdisplay,
			drm_fmt_to_gbm_fmt(drm.format[DISP_ID]),
			GBM_BO_USE_SCANOUT | GBM_BO_USE_RENDERING);
	if (!gbm.surface) {
		printf("failed to create gbm surface\n");
		return -1;
	}

	return 0;
}

static int init_gl(void)
{
	EGLint major, minor, n;
	GLuint vertex_shader, fragment_shader;
	GLint ret;

	static const EGLint context_attribs[] = {
		EGL_CONTEXT_CLIENT_VERSION, 2,
		EGL_NONE
	};

	static const EGLint config_attribs[] = {
		EGL_SURFACE_TYPE, EGL_WINDOW_BIT,
		EGL_RED_SIZE, 1,
		EGL_GREEN_SIZE, 1,
		EGL_BLUE_SIZE, 1,
		EGL_ALPHA_SIZE, 0,
		EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
		EGL_NONE
	};

        gl.display = eglGetDisplay(gbm.dev);

	if (!eglInitialize(gl.display, &major, &minor)) {
		printf("failed to initialize\n");
		return -1;
	}

	printf("Using display %p with EGL version %d.%d\n",
			gl.display, major, minor);

	printf("EGL Version \"%s\"\n", eglQueryString(gl.display, EGL_VERSION));
	printf("EGL Vendor \"%s\"\n", eglQueryString(gl.display, EGL_VENDOR));
	printf("EGL Extensions \"%s\"\n", eglQueryString(gl.display, EGL_EXTENSIONS));

	if (!eglBindAPI(EGL_OPENGL_ES_API)) {
		printf("failed to bind api EGL_OPENGL_ES_API\n");
		return -1;
	}

	if (!eglChooseConfig(gl.display, config_attribs, &gl.config, 1, &n) || n != 1) {
		printf("failed to choose config: %d\n", n);
		return -1;
	}

	gl.context = eglCreateContext(gl.display, gl.config,
			EGL_NO_CONTEXT, context_attribs);
	if (gl.context == NULL) {
		printf("failed to create context\n");
		return -1;
	}

	gl.surface = eglCreateWindowSurface(gl.display, gl.config, gbm.surface, NULL);
	if (gl.surface == EGL_NO_SURFACE) {
		printf("failed to create egl surface\n");
		return -1;
	}

	/* connect the context to the surface */
	eglMakeCurrent(gl.display, gl.surface, gl.surface, gl.context);

	printf("GL Extensions: \"%s\"\n", glGetString(GL_EXTENSIONS));

	return 0;
}


static void exit_gbm(void)
{
        gbm_surface_destroy(gbm.surface);
        gbm_device_destroy(gbm.dev);
        return;
}

static void exit_gl(void)
{
        glDeleteProgram(gl.program);
        glDeleteBuffers(1, &gl.vbo);
        glDeleteShader(gl.fragment_shader);
        glDeleteShader(gl.vertex_shader);
        eglDestroySurface(gl.display, gl.surface);
        eglDestroyContext(gl.display, gl.context);
        eglTerminate(gl.display);
        return;
}

static void exit_drm(void)
{

        drmModeRes *resources;
        int i;

        resources = (drmModeRes *)drm.resource_id;
        for (i = 0; i < resources->count_connectors; i++) {
                drmModeFreeEncoder(drm.encoder[i]);
                drmModeFreeConnector(drm.connectors[i]);
        }
        drmModeFreeResources(drm.resource_id);
        close(drm.fd);
        return;
}

void cleanup(void)
{
	exit_gl();
	exit_gbm();
	exit_drm();
	printf("Cleanup of GL, GBM and DRM completed\n");
	return;
}

/* Draw code here */
static void draw(uint32_t i)
{
	glClear(GL_COLOR_BUFFER_BIT);
	glClearColor(0.2f, 0.3f, 0.5f, 1.0f);
}

static void
drm_fb_destroy_callback(struct gbm_bo *bo, void *data)
{
	struct drm_fb *fb = data;
	struct gbm_device *gbm = gbm_bo_get_device(bo);

	if (fb->fb_id)
		drmModeRmFB(drm.fd, fb->fb_id);

	free(fb);
}

static struct drm_fb * drm_fb_get_from_bo(struct gbm_bo *bo)
{
	struct drm_fb *fb = gbm_bo_get_user_data(bo);
	uint32_t width, height, format;
	uint32_t bo_handles[4] = {0}, offsets[4] = {0}, pitches[4] = {0};
	int ret;

	if (fb)
		return fb;

	fb = calloc(1, sizeof *fb);
	fb->bo = bo;

	width = gbm_bo_get_width(bo);
	height = gbm_bo_get_height(bo);
	pitches[0] = gbm_bo_get_stride(bo);
	bo_handles[0] = gbm_bo_get_handle(bo).u32;
	format = gbm_bo_get_format(bo);

	ret = drmModeAddFB2(drm.fd, width, height, format, bo_handles, pitches, offsets, &fb->fb_id, 0);
	if (ret) {
		printf("failed to create fb: %s\n", strerror(errno));
		free(fb);
		return NULL;
	}

	gbm_bo_set_user_data(bo, fb, drm_fb_destroy_callback);

	return fb;
}

static void page_flip_handler(int fd, unsigned int frame,
		  unsigned int sec, unsigned int usec, void *data)
{
	int *waiting_for_flip = data;
	*waiting_for_flip = 0;
}

int main(int argc, char *argv[])
{
	fd_set fds;
	drmEventContext evctx = {
			.version = DRM_EVENT_CONTEXT_VERSION,
			.page_flip_handler = page_flip_handler,
	};
	struct gbm_bo *bo;
	struct drm_fb *fb;
	uint32_t i = 0;
	int ret;

	ret = init_drm();
	if (ret) {
		printf("failed to initialize DRM\n");
		return ret;
	}
        printf("### Primary display => ConnectorId = %d, Resolution = %dx%d\n",
	    drm.connector_id[DISP_ID], drm.mode[DISP_ID]->hdisplay,
	    drm.mode[DISP_ID]->vdisplay);

	FD_ZERO(&fds);
	FD_SET(0, &fds);
	FD_SET(drm.fd, &fds);

	ret = init_gbm();
	if (ret) {
		printf("failed to initialize GBM\n");
		return ret;
	}

	ret = init_gl();
	if (ret) {
		printf("failed to initialize EGL\n");
		return ret;
	}

	/* clear the color buffer */
	glClearColor(0.5, 0.5, 0.5, 1.0);
	glClear(GL_COLOR_BUFFER_BIT);
	eglSwapBuffers(gl.display, gl.surface);
	bo = gbm_surface_lock_front_buffer(gbm.surface);
	fb = drm_fb_get_from_bo(bo);

	/* set mode: */
	ret = drmModeSetCrtc(drm.fd, drm.crtc_id[DISP_ID], fb->fb_id, 0, 0,
			&drm.connector_id[DISP_ID], 1, drm.mode[DISP_ID]);
	if (ret) {
		printf("failed to set mode: %s\n", strerror(errno));
		return ret;
	}

	while (true) {
		struct gbm_bo *next_bo;
		int waiting_for_flip = 1;

		draw(i++);

		eglSwapBuffers(gl.display, gl.surface);
		next_bo = gbm_surface_lock_front_buffer(gbm.surface);
		fb = drm_fb_get_from_bo(next_bo);

		/*
		 * Here you could also update drm plane layers if you want
		 * hw composition
		 */

		ret = drmModePageFlip(drm.fd, drm.crtc_id[DISP_ID], fb->fb_id,
				DRM_MODE_PAGE_FLIP_EVENT, &waiting_for_flip);
		if (ret) {
			printf("failed to queue page flip: %s\n", strerror(errno));
			return -1;
		}

		while (waiting_for_flip) {
			ret = select(drm.fd + 1, &fds, NULL, NULL, NULL);
			if (ret < 0) {
				printf("select err: %s\n", strerror(errno));
				return ret;
			}
                        else if (ret == 0) {
				printf("select timeout!\n");
				return -1;
			}
                        else if (FD_ISSET(0, &fds)) {
				printf("user interrupted!\n");
				break;
			}
			drmHandleEvent(drm.fd, &evctx);
		}

		/* release last buffer to render on again: */
		gbm_surface_release_buffer(gbm.surface, bo);
		bo = next_bo;
	}

        cleanup();
        printf("\n Exiting \n");

	return ret;
}