Field._cachedvalue not initialised leading to errors when reading registers e.g. main_id

[Bouteillebleu]

Reproduction steps:

1. makepp seeds/pycorn/pycorn.uimage (to build the Pycorn image)
2. makepp seeds/pycorn/run (to launch it in qemu)
3. Type the following into the interpreter:

from metal import cpu
cpu.SystemControlCoprocessor.main_id.implementor

You get the following error:
Traceback (most recent call last):
File "", line 1, in
File "/initrd/metal/bits.py", line 65, in get
File "/initrd/metal/bits.py", line 46, in extract
TypeError: unsupported operand type(s) for &: 'NoneType' and 'long'

From looking into this, this is because:

• Field.get returns self.bits.extract(obj._cachedvalue) where obj is the instance.
• Bits.extract does a bitwise and between the bitmask for the field and the contents of Field._objvalue.
• But Field.init doesn't set self._cachedvalue, only Field.set does.

So if I'd set the values of a field first I'd expect this to be okay.

But in a read-only register like main_id, it doesn't make sense to do that - and yet nowhere in setting up the register does self._cachedvalue gets set, so any attempt at get on any of main_id's fields will cause a TypeError.

[tornewuff]

Yeah, you can't access the fields of a register outside of a "with" block, because of the read-modify-write requirements. It shouldn't just crash, obviously :)

For read-only registers it may be okay to support directly reading, but there's still a potential problem for registers that are read-only to the CPU but not constant - allowing them to be read outside of a with block means you can inadvertently read an inconsistent snapshot of two fields that should be read atomically.