RF_CACHEABLE flag

[Konstantin Belousov]

On Tue, Feb 23, 2016 at 02:19:57PM -0600, Justin Hibbits wrote:
> This really isn't much different from bus_space_map() conceptually.
> The work involved is pretty much the same, and if this route is deemed
> the Wrong Way(TM), I'll go that route.
> 
> Grepping through sys/, only x86 currently implements
> pmap_change_attr(), and arm has the declaration but no definition that
> I could see.  Writing it wouldn't be difficult of course, there's just
> not much compelling case for it right now.  But, yes, either of these
> alternatives are acceptable, and I had explored it.  Seeing John
> Baldwin's comment on the phab review, it looks like he wants to go a
> different (parallel) direction.

If this was not clear from my reply, I did not objected against RF_CACHEABLE,
but was more to highlight weird needs of seemingly simple architecture,
which are close to RF_CACHEABLE stuff.  And, I think that architectures
that care about caching modes, should do provide real pmap_change_attr()
implementation.  This might be important e.g. if drm is brought up on
these platforms.