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

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