Re: [PATCH] MdeModulePkg/PciBusDxe: GetBarAttributes() returns Host address

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 12 September 2017 at 06:01, Ni, Ruiyu <ruiyu.ni@intel.com> wrote:
> Laszlo,
> Your understanding is: DeviceAddress = HostAddress + AddressTranslationOffset
> But my patch assumes: HostAddress = DeviceAddress + AddressTranslationOffset
>
> They are totally different. If I follow your understanding, the patch is wrong!
> Since UEFI spec doesn't describe "apply to" in sentence " Offset to apply to the
> Starting address of a BAR to convert it to a PCI address" very clearly, I quoted
> the statement from ACPI spec.
> Your understanding to "apply to" is "add", my understanding is "minus".
>

Even though we are stretching the ACPI definition of a QWord
descriptor beyond its original meaning, I don't think there is a lot
of ambiguity here, to be honest. The AddrRangeMin field contains the
address on the secondary side of a bridge, and the primary value can
be obtained by 'applying' the ATO. In my opinion, applying a (positive
or negative) offset implies addition, not subtraction, as subtraction
involves negating the second addend before applying it. And the
secondary side of the host bridge is clearly the PCI side. It does
mean the offset field is signed, though.

So I don't agree with the conclusion that no clarification is
required. We need to make sure the spec is crystal clear in this
regard. But I do agree with the change, I think it is the only
solution that makes sense.