Why it is not possible to use ioremap then remap_pfn_range?

Question

In my driver, I am trying to map an address returned from ioremap to a userspace address.

1. What kind of an address is returned from ioremap?
2. How is it different from a kmalloc address ?
3. How can I map an address returned from ioremap?
4. Which address should be inserted to remap_pfn_range?

Answer 1

You don't need ioremap() if you're using remap_pfn_range(). ioremap() maps a physical address into a kernel virtual address. remap_pfn_range() maps physical addresses directly to user space. Just pass your physical address (downshifted by PAGE_SHIFT to produce a pfn) directly to remap_pfn_range(). Your questions in order:

1. kernel virtual address
2. kmalloc returns kernel virtual, but guarantees contiguous memory See question 116343
3. you could do this if you call virt_to_phys() first, to convert kernel virtual address to physical. But skip a step if you don't actually need kernel access to this memory range
4. physical address, downshifted by PAGE_SHIFT to produce a pfn