Draw in a buffer we created ourselves

drawinbuffer.c

/* gcc -o drawinbuffer drawinbuffer.c $(pkg-config --cflags --libs glib-2.0 libdrm) */

#include <glib.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
#include <xf86drm.h>
#include <xf86drmMode.h>

int
main (int argc, char **argv)
{
  int i;
  int fd;
  drmModeRes *resources = NULL;
  drmModeConnector *connector = NULL;
  drmModeEncoder *encoder = NULL;
  drmModeCrtc *crtc = NULL;
  struct drm_mode_create_dumb create_dumb_buffer_request = { 0 };
  struct drm_mode_map_dumb map_dumb_buffer_request = { 0 };
  uint32_t buffer_id = 0;
  unsigned char *pixels;

  fd = open ("/dev/dri/card0", O_RDWR);
  if (fd < 0)
    {
      g_warning ("Unable to open DRI device");
      exit (1);
    }

  resources = drmModeGetResources (fd);

  /* Find the first active connector to display on. */
  for (i = 0; i < resources->count_connectors; i++)
    {
      connector = drmModeGetConnector (fd, resources->connectors[i]);
      if (connector == NULL)
        continue;

      if (connector->connection == DRM_MODE_CONNECTED &&
          connector->count_modes > 0)
        break;

      drmModeFreeConnector(connector);
    }

  if (i == resources->count_connectors)
    {
      g_warning ("Could not find an active connector");
      exit (1);
    }

  /* Find the CRTC. */
  encoder = drmModeGetEncoder (fd, connector->encoder_id);
  crtc = drmModeGetCrtc (fd, encoder->crtc_id);

  /* Create ourselves a new buffer on the card. */
  create_dumb_buffer_request.width = crtc->mode.hdisplay;
  create_dumb_buffer_request.height = crtc->mode.vdisplay;
  create_dumb_buffer_request.bpp = 32;
  create_dumb_buffer_request.flags = 0;

  if (drmIoctl (fd, DRM_IOCTL_MODE_CREATE_DUMB,
                &create_dumb_buffer_request) < 0)
    {
      g_warning ("Could not allocate frame buffer %m");
      exit (1);
    }

  /* Tell DRM that the buffer on the card is a framebuffer,
   * and its properties. */
  if (drmModeAddFB (fd,
                    create_dumb_buffer_request.width,
                    create_dumb_buffer_request.height,
                    24, /* depth */
                    create_dumb_buffer_request.bpp,
                    create_dumb_buffer_request.pitch,
                    create_dumb_buffer_request.handle,
                    &buffer_id) != 0)
    {
      g_warning ("Could not tell DRM about frame buffer %m");
      exit (1);
    }

  /* Draw on the buffer */
  map_dumb_buffer_request.handle = create_dumb_buffer_request.handle;
  if (drmIoctl (fd, DRM_IOCTL_MODE_MAP_DUMB,
                &map_dumb_buffer_request) < 0)
    {
      g_warning ("Could not map buffer %m");
      exit (1);
    }

  pixels = mmap (0, create_dumb_buffer_request.size,
                 PROT_READ | PROT_WRITE, MAP_SHARED,
                 fd, map_dumb_buffer_request.offset);

  if (pixels == MAP_FAILED)
    {
      g_warning ("Could not mmap buffer %m");
      exit (1);
    }

  {
    int x, y;
    int stride = create_dumb_buffer_request.pitch;

    /* Draw a red rectangle */
    for (y = 20; y < 100; y++)
      {
        for (x = 20; x < 100; x++)
          {
            unsigned char *ptr = &pixels[y * stride + x * 4];
            uint32_t *pixel = (uint32_t *) ptr;

            /* ARGB */
            *pixel = 0xFFFF0000;
          }
      }
  }

  /* Set the CRTC to show this buffer for five seconds. */
  if (drmModeSetCrtc (fd,
                      crtc->crtc_id,
                      buffer_id,
                      0, 0, /* x, y */
                      &connector->connector_id, 1, /* connector */
                      &crtc->mode) != 0)
    {
      g_warning ("Could not set CRTC to display buffer %m");
      exit (1);
    }

  sleep (5);

  /* After five seconds, revert back to the old buffer. */

  /* The CRTC structure has all the information that we
   * need to revert back to the old buffer. */
  if (drmModeSetCrtc (fd,
                      crtc->crtc_id,
                      crtc->buffer_id,
                      crtc->x,
                      crtc->y,
                      &connector->connector_id, 1, /* connector */
                      &crtc->mode) != 0)
    {
      g_warning ("Could not revert back to the old buffer");
      exit (1);
    }

  return 0;
}