MdeModulePkg/PciHostBridgeDxe: Ignore unsupported I/O BARs.

[michaelescue]

Description

Bootup fails when an endpoint requests I/O space BARs and the Root Bridge does not support I/O space causing a resource conflict during bus enumeration. While I/O space is traditionally supported in x86_64, platforms using an Arm architecture may or may not support I/O space.

This change adds logic allowing the platform to detect I/O space support for each Root Bridge and prevent submission of the required I/O resources for allocation when unsupported. Endpoints will not receive I/O resources, but they will continue to receive Memory BAR resources.

• Breaking change?
• Impacts security?
• Includes tests?

How This Was Tested

• Boot on ARM64 platform with a mix of PCIe endpoints that request and do not request I/O Space BARs.

Integration Instructions

• N/A

[ardbiesheuvel]

This looks like a reasonable workaround to me - it is a bit of a hack, but it gets rid of the system-wide PCD, and takes the properties of the individual root bridges into account.

The only thing missing here is a DEBUG() that explains that I/O resources are being disregarded because the I/O aperture is missing entirely.

[josephoresko]

Looks good to me.

[michaelescue]

@ardbiesheuvel, @mdkinney,
Thanks for the feedback!

The only thing missing here is a DEBUG() that explains that I/O resources are being disregarded because the I/O aperture is missing entirely.

I've added the debug print statement as requested.

Could also keep the line unconditionally sets Status to ResSubmitted, and then set to ResNone if the Root Bridge does not support the requested Type.

I agree with this and have made the necessary change.

[michaelescue]

Corrected spelling errors and capitalization.

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