Re: [Qemu-devel] Re: [PATCH] hpet: Clean up initial hpet counter

[Gleb Natapov]

On Thu, Jun 17, 2010 at 10:59:01AM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Thu, Jun 17, 2010 at 10:42:34AM +0200, Jan Kiszka wrote:
> >> Gleb Natapov wrote:
> >>> On Thu, Jun 17, 2010 at 10:30:15AM +0200, Jan Kiszka wrote:
> >>>>> Sorry, I lost you here. What "works for IO-based fw-cfg, but not for
> >>>>> MMIO-based".
> >>>> Undefined IO ports return -1, undefined (/wrt read access) MMIO 0. So
> >>>> you need to select a key that is different from both.
> >>>>
> >>> But can we rely on it? Is this defined somewhere or if it happens to be
> >>> the case in current qemu for x86 arch.
> >> For x86 with its port-based access, we are on the safe side as (pre-pnp)
> >> device probing used to work this way. Can't tell for the other archs
> >> that support fw-cfg.
> >>
> >>>>> Can you write pseudo logic of how you think it
> >>>>> all should work?
> >>>> The firmware should do this:
> >>>>
> >>>> write(CTL_BASE, FW_CFG_ID);
> >>>> if (read(CTL_BASE) != FW_CFG_ID)
> >>>>  deal_with_old_qemu();
> >>>> else
> >>>>  check_for_supported_keys();
> >>>>
> >>> Ah, I thought about read() returning 0/1, not key itself, so any key that
> >>> always existed would do.
> >> Yes, read-back would mean returning FWCfgState::cur_entry. And that will
> >> be -1 when selected an invalid one.
> >>
> > Heh, actually I have better idea. Why not advance FW_CFG_ID to version 2.
> 
> If that is supposed to be a version number - yeah, good idea.
> 
That was the idea behind it. I just forgot it exists.

--
                        Gleb.