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. Jan
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