>    > 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.

   So, is the answer to include them via <machine/cpu.h>?

   And, if so, is it still mandatory to put them in <machine/psl.h>?

I already answered both questions in the text you quoted.