use of EFI_PCI_ATTRIBUTE_DUAL_ADDRESS_CYCLE

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

The UEFI spec describes the EFI_PCI_ATTRIBUTE_DUAL_ADDRESS_CYCLE
attribute as follows:

"""
EFI_PCI_ATTRIBUTE_DUAL_ADDRESS_CYCLE
This bit may only be used in the Attributes parameter to
AllocateBuffer(). If this bit is set, then the PCI controller
that is requesting a buffer through AllocateBuffer() is
capable of producing PCI Dual Address Cycles, so it is able to
access a 64-bit address space. If this bit is not set, then the PCI
controller that is requesting a buffer through
AllocateBuffer() is not capable of producing PCI Dual
Address Cycles, so it is only able to access a 32-bit address space.
"""

However, both the UEFI driver writer's guide, and the de facto use in
Tianocore violate this, i.e., it use the pattern

"""
//
// Enable Command register and Dual Address Cycle
//
Status = PciIo->Attributes (
    PciIo,
    EfiPciIoAttributeOperationEnable,
    (PciSupports & EFI_PCI_DEVICE_ENABLE) |
    EFI_PCI_IO_ATTRIBUTE_DUAL_ADDRESS_CYCLE,
    NULL
    );
"""

to enable it in the PCI I/O layer, and the Map/Unmap implementations
will switch to 64-bit flavors of the map operation, by the likes of

"""
  if ((PciIoDevice->Attributes &
EFI_PCI_IO_ATTRIBUTE_DUAL_ADDRESS_CYCLE) != 0) {
    Operation = (EFI_PCI_IO_PROTOCOL_OPERATION) (Operation +
EfiPciOperationBusMasterRead64);
  }
"""

So what should we be doing in new code that needs to support 64-bit DMA?

-- 
Ard.