[IA64] ia64: add ar.k0 usage note

static void __init

io_port_init (void)

{

	extern unsigned long ia64_iobase;

	unsigned long phys_iobase;

	/*

	 *  Set `iobase' to the appropriate address in region 6 (uncached access range).

	 * Set `iobase' based on the EFI memory map or, failing that, the

	 * value firmware left in ar.k0.

	 *

	 *  The EFI memory map is the "preferred" location to get the I/O port space base,

	 *  rather the relying on AR.KR0. This should become more clear in future SAL

	 *  specs. We'll fall back to getting it out of AR.KR0 if no appropriate entry is

	 *  found in the memory map.

	 * Note that in ia32 mode, IN/OUT instructions use ar.k0 to compute

	 * the port's virtual address, so ia32_load_state() loads it with a

	 * user virtual address.  But in ia64 mode, glibc uses the

	 * *physical* address in ar.k0 to mmap the appropriate area from

	 * /dev/mem, and the inX()/outX() interfaces use MMIO.  In both

	 * cases, user-mode can only use the legacy 0-64K I/O port space.

	 *

	 * ar.k0 is not involved in kernel I/O port accesses, which can use

	 * any of the I/O port spaces and are done via MMIO using the

	 * virtual mmio_base from the appropriate io_space[].

	 */

	phys_iobase = efi_get_iobase();

	if (phys_iobase)

		/* set AR.KR0 since this is all we use it for anyway */

		ia64_set_kr(IA64_KR_IO_BASE, phys_iobase);

	else {

	if (!phys_iobase) {

		phys_iobase = ia64_get_kr(IA64_KR_IO_BASE);

		printk(KERN_INFO "No I/O port range found in EFI memory map, falling back "

		       "to AR.KR0\n");

		printk(KERN_INFO "I/O port base = 0x%lx\n", phys_iobase);

		printk(KERN_INFO "No I/O port range found in EFI memory map, "

			"falling back to AR.KR0 (0x%lx)\n", phys_iobase);

	}

	ia64_iobase = (unsigned long) ioremap(phys_iobase, 0);

	ia64_set_kr(IA64_KR_IO_BASE, __pa(ia64_iobase));

	/* setup legacy IO port space */

	io_space[0].mmio_base = ia64_iobase;