Hello,

I am writing a device driver and would like to know when it is appropriate
to use the bus_space_vaddr() function. I need to guarentee the ordering of
my read-modify-writes for the control registers I am manipulating. It would
be nice to setup a few register pointers via bus_space_vaddr() and use
regular C operators. For example, given that sc->iot and sc->ioh are mapped
to the base address of a set of control registers:

	#define rCTRLREG(b)	(*(b + REG_OFFSET))

	vaddr_t b;

	b = (vaddr_t) bus_space_vaddr(sc->sc_iot, sc->ioh);
	rCTRLREG(b) |= BITMSK;

Seems preferable to:

	u_int32_t value;

	value = bus_space_read_4(sc->sc_iot, sc->ioh, REG_OFFSET);
	value |= BITMSK;
      bus_space_write_4(sc->sc_iot, sc->ioh, offset, REG_OFFSET);

Are these equivalent? Do I need to specify a barrier after my
read-modify-write? I have searched through several existing drivers and
bus_space_vaddr() seems used rarely. There is a warning about its use in the
bus_space(9) man page so I am cautious. Is there any convention for how
bus_space_vaddr() is used?

Any help is appcreciated,
Sean