18.5.3 Bus Master Read and Write Operations

When a DMA transaction starts or stops, the ownership of the DMA buffer transitions from the processor to the DMA bus master and back to the processor. The PCI I/O Protocol provides the Map() and Unmap() services used to set up and complete a DMA transaction.
The implementation of the PCI Root Bridge I/O Protocol uses the EDK II library BaseLib function MemoryFence() to guarantee all system memory transactions from the processor are completed before the DMA transaction is started. This prevents a DMA bus master reading from a location in the DMA buffer before a write transaction is flushed from the processor. Because this functionality is built into the PCI Root Bridge I/O Protocol itself, the PCI driver writer need not worry about bus master read/ write operations.
A PCI driver is responsible for flushing all posted write data from a PCI controller when a bus master write operation is completed. First, the PCI driver should read from a register on the PCI controller to guarantee that all posted write operations are flushed from the PCI controller and through any PCI-to-PCI bridges between the PCI controller and the PCI root bridge.
Because PCI drivers are polled, they typically read from a status register on the PCI controller to determine when the bus master write transaction is completed. This read operation is usually sufficient to flush the posted write buffers. The PCI driver must also call the PciIo->Flush() service at the end of a bus master write operation. This service flushes all the posted write buffers in the system chipset and guarantees their commitment to system memory. The combination of the read operation and the PciIo>Flush() call guarantee that the bus master's view of system memory and the processor's view of system memory are consistent.
An example of how a bus master write transaction should be completed to guarantee the bus master's view of system memory is consistent with that of the processor follows.

Example 177-Completing a bus master write operation

#include <Uefi.h>
#include <Protocol/PciIo.h>
#include <Library/UefiLib.h>
UINT64 Result64
// Call PollMem() to poll for Bit #0 in MMIO register 0x24 of Bar #1 to be set.
// This example shows polling a status register to wait for a bus master write
// transaction to complete.
Status = PciIo->PollMem (
PciIo, // This
EfiPciIoWidthUint32, // Width
1, // BarIndex
0x24, // Offset
BIT0, // Mask
BIT0, // Value
&Result64 // Result
if (EFI_ERROR (Status)) {
return Status;
// Call Flush() to flush all write transactions to system memory
Status = PciIo->Flush (PciIo);
if (EFI_ERROR (Status)) {
return Status;