Re: [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase

[Peter Maydell]

On 20 May 2015 at 09:43, Kevin Wolf <address@hidden> wrote:
> Am 20.05.2015 um 10:06 hat Peter Maydell geschrieben:
>> That handles migration, which is good. But I still think that
>> storing the same information in two places in the device
>> state (phase field and the register fields) is error-prone.
>
> That's actually my point. The registers are accessed everywhere in the
> code, whereas phase transitions are in very few well-defined places
> (there are exactly four of them, iirc). If they get out of sync, chances
> are that the bug is in the register value, not in the phase. When we
> know what phase we're in, we can assert the bits and actually catch such
> bugs.
>
>> If we want to switch to having a phase field we should calculate
>> the relevant register bits on demand based on the phase, rather
>> than keeping both copies of the state in sync manually.
>
> That doesn't work, unfortunately. Some register bits imply a specific
> phase (assuming correct code), but you can't derive the exact bits just
> from the phase.

Having now dug out a copy of the 82078 spec, I agree that the state
isn't derivable purely from the register values in the general case.
The controller clearly has a state machine internally but it doesn't
surface that in the register state except indirectly.

-- PMM