How to detect VSYNC?

From: Chris G (cgrebeld_at_hotmail.com)
Date: 06/24/04 

Date: 24 Jun 2004 07:34:27 -0700

Is there a way for me to access the vsync signal through userspace?
(Like through one of the X-Windows APIs)

And failing that, is there a way to do it from a custom kernel
module?, ( without having to modify the video driver )

I know I can set the graphics APIs to 'wait-on-vsync', but I need to
know exactly when the video buffer will be sent to the monitor,
relative to the time when I issued the buffer-blt command. I would
like to be able to do the following:

set_wait_on_vsync( true )
while( true ) {
  draw_stuff( my_buffer )
  back_buffer_swap( my_buffer )

  block_until_vsync() <----- how do i do this?

  save_timestamp( time() )
}

Chris G