Linux kernel device driver to DMA from a device into user-space memory

Question

I want to get data from a DMA enabled, PCIe hardware device into user-space as quickly as possible.

Q: How do I combine \"direct I/O to user-space with/and/via a DMA

Answer 1

first_page_offset = udata & PAGE_MASK; 

It seems wrong. It should be either:

first_page_offset = udata & ~PAGE_MASK;

or

first_page_offset = udata & (PAGE_SIZE - 1)