About mmap in frame buffer

Hi,

I met some problem in fb_mmap(). When fb_mmap() call my mmap function, it displayed as this:
mapping /dev/fb0: Invalid argument

Error: failed to map framebuffer device to memory

I want to know how this happened?

And another question, if I want to draw some picture on the LCD through the interface of fb_mmap(),  where should I add the function of transferring data in memory to LCD Graphic RAM?
In another word, which function the fb_mmap() call and is called?

Thanks a lot

Sincerely yours
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