transfer data from DMA into file

Hi everybody!

I'm new in the complex task of writing modules for Linux.
I've already read Linux Device Drivers from O'Reilly, but I'm not sure
how I can transfer Data from DMA to a file.
With __get_free_pages, I reserved 1Mb and have a hardware device
writing to it. After transfer is complete, the hardwaredevice will
generate an interrupt.
In this moment I want to write the data from the reserved memory
region to a file.
Can anybody give some hints please, how to do this the best way?
.