How to allocate memory for DMA copy in Linux?

I allocated a video frame buffer which is used to copy video data from another video frame buffer which is get from FrameIn Link in linux. I want to use DMA to copy those video data.

The function dmaCopy2D() request the src and dst addresses are both physical address. But in my case I used malloc() to get the video frame buffer which only has virtual address.

So my question is how can I traslate the virtual address into physiscal address? Or how can I allocate a video frame buffer which has physiscal address?

Is there any demo code?

Thanks,

Dongming