get vsync using libdrm

vsync.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/time.h>
#include "xf86drm.h"

// gcc -I/usr/include/libdrm/ -ldrm vsync.c

int64_t get_time(void)
{
  struct timeval now_tv;
  gettimeofday(&now_tv, NULL);
  return now_tv.tv_sec * 1000000LL + now_tv.tv_usec;
}

int main()
{
  drm_wait_vblank_t blank;
  int fd;
  int i;

  fd = open("/dev/dri/card0", O_RDWR);
  if (fd < 0) {
    fprintf(stderr, "open\n");
    return -1;
  }

  i = 0;
  while (i++ < 100) {
    int64_t before = get_time();

    blank.request.type = DRM_VBLANK_RELATIVE;
    blank.request.sequence = 1;
    if (drmWaitVBlank(fd, (drmVBlankPtr) &blank)) {
      fprintf(stderr, "drmWaitVBlank\n");
      return -1;
    }

    printf("%ld\n", get_time() - before);
  }

  close(fd);

  return 0;
}