Enabling a ACPI eMMC device is causing kernel reboots with FreeBSD 11.2

[Rajesh Kumar]

Hi,

I have amd64 board, which has a eMMC device on the ACPI bus. When I try
booting FreeBSD 11.2 on this board, with eMMC device disabled in BIOS, it
boots without any issues.  But, when eMMC device is enabled in BIOS,
FreeBSD 11.2 continuously reboots.

When I have the following tunable set, FreeBSD 11.2 boots without any
issues with eMMC enabled.
hw.pci.mcfg=0

Observations :
a) We have verified the ACPI device addresses (from DSDT table) doesn't
conflict with the PCI address base (configured in MCFG table).
b) The PCI address base from MCFG table is read correctly by kernel
(Verbose boot prints that address). So, I don't think MCFG table pointer is
corrupted and getting read properly.

Questions :
a) What does hw.pci.mcfg=0 actually do to get the reboot issue resolved?
b) Why enabling a device on ACPI bus needs hw.pci.mcfg (PCI specific) to
boot?

Looks like the system reboots when the ACPI and PCI devices enumeration
starts during the boot. So, seems ACPI (or) PCI devices aren't getting
enough resources and so reboots.

Thanks,
Rajesh.