Comment 28 for bug 1810239

From a pdf file by Intel with title "Intel® Virtualization Technology for Directed I/O
Architecture Specification":
--snip--
3.6.1.4 PCI Express Devices Using Phantom Functions
To increase the maximum possible number of outstanding requests requiring completion, PCI Express allows a device to use function numbers not assigned to implemented functions to logically extend the Tag identifier. Unclaimed function numbers are referred to as Phantom Function Numbers (PhFN). A device reports its support for phantom functions through the Device Capability configuration register, and requires software to explicitly enable use of phantom functions through the Device Control configuration register.

Since the function number is part of the requester-id used to locate the context-entry for processing a DMA request, when assigning PCI Express devices with phantom functions enabled, software must program multiple context entries, each corresponding to the PhFN enabled for use by the device function. Each of these context-entries must be programmed identically to ensure the DMA requests with any of these requester-ids are processed identically.
--snip--

grep -ri phant says pci_regs.h knows about the capability, but it doesn't appear anywhere else in the kernel as far as I can see. Look for PCI_EXP_DEVCAP_PHANTOM and PCI_EXP_DEVCTL_PHANTOM.

Unfortunately, lspci indicates that the Marvell chip is not using phantom functions (lspci upload to follow), so at this point I can't tell if I'm on the right trail.

Caveat lector: I don't have any previous experience with low-level PCI stuff.