Re: RFC: New bus_space routine: bus_space_sync

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Fri, 20 Jan 2012, Mouse wrote:

> >> Even if originally intended for something else, [...]
> 
> > Why do you think BUS_SPACE_BARRIER_SYNC was intended for something
> > else ?  I can't see how a write barrier that doesn't ensure the write
> > has reached the target (main or device memory) can be usefull.
> 
> I can't comment on why someone else thinks something.  But barriers
> that have nothing to do with write completion to the target can still
> be useful.  There are algorithms that don't require that writes
> complete on any particular schedule, but do require that _this_ write
> complete before _that_ one.  When faced with write coalescing and
> reordering, a write barrier that does nothing but enforce ordering (in
> the sequence A-barrier-B, the barrier enforces the constraint that
> there is no time at which write B has completed but write A hasn't) can
> be useful.
> 
> For example, the standard double-buffering trick of "write inactive
> copy, then write variable indicating which is the active copy" does not
> work if the indicator's write can complete before the
> (formerly-)inactive copy's writes complete - but, in many uses, there
> is no requirement that those writes, as a sequence, be pushed to their
> target at any particular time.

That's not what the manpage documenting BUS_SPACE_BARRIER_SYNC says.  Read 
the manpage.

Eduardo