[RFA]: hw-glue-bus-mux selection behavior

[Dave Brolley]

Frank Ch. Eigler wrote:

> Hi -
> 
> brolley wrote:
> : [...]
> : > To me, this seems like an unnecessary feature; I'd let the client pick
> : > exactly 0 or 1.
> : 
> : ok -- just trying to be too clever, I guess. How about 0 vs non-zero? 
> : I think there should be some defined behavior for all values.  [...]
> 
> No doubt about that.  Not that I want to get you to go all the way
> on this now, but ideally, setting the selector to any index that does
> not have an configured downstream accessor should be okay, merely cause
> sid::bus::unmapped for all future accesses.  The 0/1 case falls out of
> that automatically, as does future expansion to 2..N downstream
> accessors.

OK -- new patch attached

Right now mux_passthrough_bus returns sid::bus::unpermitted if the 
target bus is NULL. Shall I change that to sid::bus::unmapped?

Dave