mmap slower than ioremap

Question

I am developing for an ARM device running Linux 2.6.37. I am trying to toggle an IO pin as fast as possible. I made a little kernel module and a user space application. I tried

Answer 1

My guess would be that since mmap has to check to make sure you're writing to memory you're allowed to write to, it's going to be slower than the kernel version (which I believe/assume doesn't do that kind of checking--with a kernel module you're responsible for testing until you're very sure you're not breaking things).

Try using do_mmap (I believe that's the one) to use mmap from kernel space, and see how that compares. If it's comparably faster, then I'm right. If it's not, it's something else.

Answer 2

This is because ioremap_nocache() still enables the CPU write buffer in your VM mapping whereas pgprot_noncached() disables both bufferability and cacheability.

Apples to apples comparison would be to use ioremap_strongly_ordered() instead.