> The following constants must be defined in <machine/psl.h>, if _KERNEL
   > is defined:

   How are they brought into scope by drivers?

Based on current practice, it seems that including either of
<machine/psl.h> or <machine/cpu.h> should do it.  Whether or not
including both should be allowed is outside the scope of my message.
I find it bad form to gratuitously #include extra files, so I'd
recommend not making psl.h idempotent unless there is a pressing need
to.

   (1) how can drivers or busses get strings describing the constants, to
       print?  Or is this considered unnecessary?

I did not define the driver interface.  If I were to define it, I'd
say that this is not necessary.

   (2) what 'form factor' must the constants fit in?  I would assume
       'int,' but i'd like to have it written down some place.

int