Intel Extensible Firmware Interface - Manual (Page 769)

Protocols — Device I/O Protocol
Version 1.10 12/01/02 18-9
Related Definitions
//*******************************************************
// EFI_IO_OPERATION_TYPE
//*******************************************************
typedef enum {
EfiBusMasterRead,
EfiBusMasterWrite,
EfiBusMasterCommonBuffer
} EFI_IO_OPERATION_TYPE;
EfiBusMasterRead A read operation from system memory by a bus master.
EfiBusMasterWrite A write operation to system memory by a bus master.
EfiBusMasterCommonBuffer Provides both read and write access to system memory
by both the processor and a bus master. The buffer is
coherent from both the processor’s and the bus master’s
point of view.
Description
The DEVICE_IO.Map() function provides the device specific addresses needed to access system
memory. This function is used to map system memory for bus master DMA accesses.
All bus master accesses must be performed through their mapped addresses and such mappings
must be freed with
Unmap() when complete. If the bus master access is a single read or write data
transfer, then EfiBusMasterRead or EfiBusMasterWrite is used and the range is
unmapped to complete the operation. If performing an EfiBusMasterRead operation, all the
data must be present in system memory before the Map() is performed. Similarly, if performing
an EfiBusMasterWrite , the data cannot be properly accessed in system memory until the
Unmap() is performed.
Bus master operations that require both read and write access or require multiple host device
interactions within the same mapped region must use
EfiBusMasterCommonBuffer .
However, only memory allocated via the DEVICE_IO.AllocateBuffer() interface is
guaranteed to be able to be mapped for this operation type.
In all mapping requests the resulting NumberOfBytes actually mapped may be less than
requested.
Status Codes Returned
EFI_SUCCESS The range was mapped for the returned NumberOfBytes .
EFI_INVALID_PARAMETER The Operation or HostAddress is undefined.
EFI_UNSUPPORTED The HostAddress cannot be mapped as a common buffer.
EFI_DEVICE_ERROR The system hardware could not map the requested address.
EFI_OUT_OF_RESOURCES The request could not be completed due to a lack of resources.