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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