[PATCH 13/15] ARM: make mach/io.h include optional

On Tuesday 14 February 2012, Russell King - ARM Linux wrote:

quoted

Yes, at least in the long run. Note that this should make no difference
at all from a performance point of view, but it does impact code size a bit.

That depends whether the additional reloads of pci_io_base can be properly
scheduled by the compiler, and experience shows that you tend to end up
with the load delay slot not being filled on older processors.

Not only that, but the compiler will evaluate the entire:

        pci_io_base + (addr & IO_SPACE_LIMIT)

thing every time.  With a 64K mask, that will include reloading the
mask every single access.

So, we'll probably end up with about three additional loads per IO
operation, none of which would be scheduled particularly well.

"Yuck" and "not in my kernel" comes to mind.

I totally agree with the code size point, but my point above was that
from performance perspective all that you mentioned should be dwarfed
by the overhead of actually doing a synchronous operation on an
external bus. writel may be reasonably fast on a CPU internal bus,
but inb/outb implies a full bus synchronization and is only
used on older PCI hardware, typically those that date back to
ISA in some form.

	Arnd