Device Drivers Concluded

Technology develops, but the ideas often remain the same. In the old ISA world, peripherals located their buffers at the “very high end of address space”--above 640 KB. Many PCI-cards now do the same, but nowadays, this is something more like the end of a 32-bit address space (like 0xF0100000).

If you want to access a buffer at these addresses, you have to use vremap() as defined in linux/mm.h to remap the same pages of this physical memory into your own virtual address space.

vremap() works a little bit like the mmap() user call in nasty, but it's much easier:

void * vremap (unsigned long offset,
               unsigned long size);

You just pass the start address of your buffer and its length. Remember, we always map pages; therefore offset and size have to be page length-aligned. If your buffer is smaller or does not start on a page boundary, map the whole page and try to avoid accessing invalid addresses.

I personally have not tried this, and I'm not sure if the tricks I described above on how to map buffers to user space work with PCI high memory buffers. If you want to give it a try, you definitely have to remove the “brute force” manipulation of the mem_map array, as mem_map is only for physical RAM. Try to replace the kmalloc() and kfree() stuff with the analogous vremap() calls and then perform a second remapping with do_mmap() to user space.

But as you might realize, we've come to an end of this series, and now it is up to you to boldly go where no Linuxer has gone before...

Good Luck!