How to enable page flipping in AM3517 frame buffer driver?

I notice that AM3517 frame buffer driver uses single buffer, When I try to SET_VSCREENINFO to double the yres_virtual, it always failed, I look at the code and find that check_fb_size always shrink the height to single buffer.

I use AM35x-OMAP35x-PSP-SDK-03.00.00.03.