Re: [PATCH] MdeModulePkg/MdeModulePkg.dec: Fix EBC build failure of PciBus driver

[Laszlo Ersek <lersek@redhat.com>]

On 10/11/16 07:01, Ruiyu Ni wrote:
> When PciBus is built as EBC, PcdPciDegradeResourceForOptionRom does
> not have associated value resulting build failure.
> The patch sets the default value to TRUE, covering the EBC ARCH.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  MdeModulePkg/MdeModulePkg.dec | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/MdeModulePkg/MdeModulePkg.dec b/MdeModulePkg/MdeModulePkg.dec
> index f870b83..42fef75 100644
> --- a/MdeModulePkg/MdeModulePkg.dec
> +++ b/MdeModulePkg/MdeModulePkg.dec
> @@ -746,7 +746,6 @@ [PcdsFeatureFlag]
>    # @Prompt Turn on PS2 Mouse Extended Verification
>    gEfiMdeModulePkgTokenSpaceGuid.PcdPs2MouseExtendedVerification|TRUE|BOOLEAN|0x00010075
>  
> -[PcdsFeatureFlag.X64]
>    ## Indicates whether 64-bit PCI MMIO BARs should degrade to 32-bit in the presence of an option ROM
>    #  On X64 platforms, Option ROMs may contain code that executes in the context of a legacy BIOS (CSM),
>    #  which requires that all PCI MMIO BARs are located below 4 GB
> 

Hmmmm, I wonder if this is the right thing to do. As far as I understand
the original patch (commit 065ae7d717f9e), it added
PcdPciDegradeResourceForOptionRom twice to the DEC file expressly for
the purpose of providing different defaults (per arch), without having
to update the DSC files of existing platforms.

The original patch didn't name EBC in either of the sections -- which is
why the EBC compilation would fail --, but I don't think that including
EBC in either section (manually or implicitly, as illustrated by the
patch) would be correct.

Namely, EBC is an instruction set that is independent of the platform
that executes it. The suggested patch is correct if the EBC build of
PciBusDxe is expected to run on x64 platforms, but it is incorrect if
the exact same binary is expected to run on aarch64 platforms.

Meaning that for EBC, *both* default values (TRUE and FALSE) are
incorrect on some platforms.

With that in mind, I propose that we declare
PcdPciDegradeResourceForOptionRom only once in MdeModulePkg.dec,
regardless of architecture -- that is, in the plain [PcdsFeatureFlag]
section --, and require all platforms that include PciBusDxe to set the
feature flag in their DSCs if they disagree with the (now centralized)
default.

In practical terms, this would turn this patch into a series of patches,
first adding the DSC changes -- for platforms that are in-tree --, and
then unifying the declaration.

I expect this will create some churn for out-of-tree modules, but that
seems justified -- considering EBC, the PCD would have to be customized
in some platform DSCs *anyway*, regardless of what default we picked for
EBC.

The following DSC files include PciBusDxe.inf:

ArmVirtPkg/ArmVirtQemu.dsc
ArmVirtPkg/ArmVirtQemuKernel.dsc
CorebootPayloadPkg/CorebootPayloadPkgIa32.dsc
CorebootPayloadPkg/CorebootPayloadPkgIa32X64.dsc
EmulatorPkg/EmulatorPkg.dsc
MdeModulePkg/MdeModulePkg.dsc
Nt32Pkg/Nt32Pkg.dsc
OvmfPkg/OvmfPkgIa32.dsc
OvmfPkg/OvmfPkgIa32X64.dsc
OvmfPkg/OvmfPkgX64.dsc
QuarkPlatformPkg/Quark.dsc
QuarkPlatformPkg/QuarkMin.dsc
Vlv2TbltDevicePkg/PlatformPkgGccX64.dsc
Vlv2TbltDevicePkg/PlatformPkgIA32.dsc
Vlv2TbltDevicePkg/PlatformPkgX64.dsc

Just my two cents.

Thanks
Laszlo