✨ Add write-only semantics

[elbeno]

Problem:

• Some hardware registers are write-only. This typically occurs when the hardware doesn't store the values but uses writes for edge-trigger effects.

Solution:

• Add write_only semantic that can be applied to write functions.

Notes:

• These write-only semantics do not take account of byte enables (which might alter written bits for a given bus). But at some level we have to assume hardware isn't doing intentionally weird things.
• Reading is not necessarily an error (as in, it will not produce a hardware fault).
• Field write functions take precedence over the containing register write function. So there are two straightforward possibilities where errors occur:
  • read from a register with write-only fields: a plain semantic error.
  • write to a write-only field where other fields in the same register are also marked write-only, but are not written: this would mean a RMW to preserve those fields, which is an error.
• And there are two less straightforward error possibilities:
  • read a (readable) field in a register marked as write-only where not all bits are defined in fields (therefore reading other bits is an error). This would be strange, but it is possible to mark a register write-only and yet fill it with readable fields.
  • write to field(s) where the register is marked as write-only and not all bits are covered by identity values: this would also mean a RMW to preserve the unwritten bits.

[mjcaisse-intel]

nice!