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

[Jan Kiszka]

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.

Jan

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