What kind of situation trigger the alloc_surface & free_surface function in video.c (video-driver)

[Cui, Hunk]

Hi, guys,
	Now I am researching about allocate the video overlay memory,
some confuse to inquire. 
	In xf86-video-geode -> lx_video.c -> LXInitOffscreenImages:
		alloc_surface = LXAllocateSurface;
		free_surface  = LXFreeSurface;

	In xf86-video-ati	  -> radeon_video.c ->
RADEONInitOffscreenImages:
		alloc_surface = RADEONAllocateSurface;
		free_surface  = RADEONFreeSurface;

	Inquire: what kind of situation trigger above functions? What
are the mainly effect?

Thanks,
Hunk Cui