[PATCH] MdeModulePkg/PciBusDxe: make OPROM BAR degradation X64 specific

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

The 'universal' PCI bus driver in MdeModulePkg contains a quirk to
degrade 64-bit PCI MMIO BARs to 32-bit in the presence of an option
ROM on the same PCI controller.

This quirk is highly specific to not just the X64 architecture in general,
but to the PC platform in particular, given that only X64 platforms that
require legacy PC BIOS compatibility require it. However, making the
quirk dependent on the presence of the legacy BIOS protocol met with
resistance, due to the fact that it introduces a dependency on the
IntelFrameworkModulePkg package.

So instead, make the quirk dependent on whether we are compiling for the
X64 architecture, by putting the #ifdef MDE_CPU_X64/#endif preprocessor
directives around it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---

Since nobody proposed any alternatives to the solution I proposed,
let's just make the quirk disappear on architectures that have no
use for it.

Note that forcing non-X64 architectures to install a driver that
produces the IncompatiblePciDevice protocol only to make the PCI
bus driver behave normally (and which, incidentally, can only be
produced once per platform, making it difficult to provide a default
implementation that can be widely reused) is not acceptable IMO.

If anything, this should require an IncompatiblePlatform protocol
that informs the PCI bus driver that a special quirk is required,
but I am aware that this is difficult to accomplish without breaking
backward compatibility. Hence this compromise.
 
 MdeModulePkg/Bus/Pci/PciBusDxe/PciResourceSupport.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/MdeModulePkg/Bus/Pci/PciBusDxe/PciResourceSupport.c b/MdeModulePkg/Bus/Pci/PciBusDxe/PciResourceSupport.c
index b0632d53b82b..b4771a822d65 100644
--- a/MdeModulePkg/Bus/Pci/PciBusDxe/PciResourceSupport.c
+++ b/MdeModulePkg/Bus/Pci/PciBusDxe/PciResourceSupport.c
@@ -1052,6 +1052,7 @@ DegradeResource (
   IN PCI_RESOURCE_NODE *PMem64Node
   )
 {
+#ifdef MDE_CPU_X64
   PCI_IO_DEVICE        *PciIoDevice;
   LIST_ENTRY           *ChildDeviceLink;
   LIST_ENTRY           *ChildNodeLink;
@@ -1101,6 +1102,7 @@ DegradeResource (
     }
     ChildDeviceLink = ChildDeviceLink->ForwardLink;
   }
+#endif
 
   //
   // If firmware is in 32-bit mode,
-- 
2.7.4