DMA Buffer write operation not persisted

[Rafael Machado <rafaelrodrigues.machado@gmail.com>]

Hi everyone.

I'm currently work on a task, and I need to write some data at a DMA buffer.
At the UEFI Driver Writer's guide, at page 359 there is a sample code of
how to do that.

Considering that code and adapting to my scenario I got the following
function (some debug prints are present for clarification):

EFI_STATUS
EFIAPI
DoBusMasterWrite (
  IN EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL *PciIo,
  IN UINT8 *HostAddress,
  IN UINTN *Length,
  IN UINT32 Value
)
{
  EFI_STATUS Status;
  UINTN NumberOfBytes;
  EFI_PHYSICAL_ADDRESS DeviceAddress;
  VOID *Mapping;
  UINT64 ReadValue;

  //
  // Call Map() to retrieve the DeviceAddress to use for the bus
  // master write operation. The Map() function may not support
  // performing a DMA operation for the entire length, so it may
  // be broken up into smaller DMA operations.
  //
  NumberOfBytes = *Length;
  Status = PciIo->Map (PciIo, // This

 EfiPciIoOperationBusMasterCommonBuffer, // Operation
                                     (VOID *)HostAddress, // HostAddress
                                     &NumberOfBytes, // NumberOfBytes
                                     &DeviceAddress, //DeviceAddress
                                     &Mapping //Mapping);

  if (EFI_ERROR (Status)) {
    return Status;
  }

  //
  // Write the data to the desired address
  // This write operation also starts the DMA transaction
  //
  Status = PciIo->Mem.Write (PciIo, // This
                                               EfiPciIoWidthUint32, // Width
                                               *HostAddress,
                                               1, // Count
                                               &Value // Buffer
                                               );

  Print(L"NumberOfBytes: %d\r\n", NumberOfBytes);
  Print(L"address: 0x%x\r\n", HostAddress);
  Print(L"Value: 0x%x\r\n", Value);
  Print(L"Status: %r\r\n", Status);

  if (EFI_ERROR (Status)) {
return Status;
  }

  //
  // The operations performed by PollMem() also flush all posted
  // writes from the PCI bus master and through PCI-to-PCI bridges.
  //
  Status = PciIo->PollMem (PciIo, // This
                                               EfiPciIoWidthUint32, // Width
                                               *HostAddress, // Offset
                                               0xFFFFFFFF,// Mask
                                               Value,// Value
                                               EFI_TIMER_PERIOD_SECONDS
(1), // Timeout
                                               &ReadValue // Result
                                               );
  Print(L"Status2: %r\r\n", Status);

  if (EFI_ERROR (Status)) {
return Status;
  }

  //
  // Call Flush() to flush all write transactions to system memory
  //
  Status = PciIo->Flush (PciIo);
  Print(L"Status3: %r\r\n", Status);

  if (EFI_ERROR (Status)) {
return Status;
  }

  //
  // Call Unmap() to complete the bus master write operation
  //
  Status = PciIo->Unmap (PciIo, Mapping);
  Print(L"Status4: %r\r\n", Status);

  if (EFI_ERROR (Status)) {
return Status;
  }
  return Status;
}

The output of this function is this:
  NumberOfBytes: 4
  address: 0xCCCAC000
  Value: 0xAAAAA
  Status: Success
  Status2: Success
  Status3: Success
  Status4: Success

The problem is that when I try to read this memory content using the dmem
command at the efiShell the value 0xAAAAA cannot be found. Seems something
is locking the DMA trasaction.
Can someone give me some light?

Thanks and Regard
Rafael R. Machado