To perform a DMA transfer that uses the routines in version 3 of the DMA operations interface, your driver should follow the steps described in the following list. These steps are common to both subordinate devices and bus-master devices. Version 3 of this interface is available starting with Windows 8. For more information about the routines in this interface, see DMA_OPERATIONS.
Step 1: Obtain a DMA adapter object
In preparation for a DMA transfer, the driver calls the IoGetDmaAdapter routine to obtain a DMA adapter object. A DMA adapter object is a software object that represents either a bus-master device, or a request line on a system DMA controller. This object contains the DMA operations interface for the bus that is used to transfer data to or from the device. Additionally, this object synchronizes the driver's access to the shared resources that are required to perform the transfer. For more information, see Introduction to Adapter Objects.
Step 2: Obtain a description of the required DMA resources
The driver calls the GetDmaTransferInfo routine to get a description of the DMA resources that it needs to perform the transfer.
The input parameters to this call describe the memory buffer to use for the transfer, and the direction (read or write) of the transfer.
The resource requirements obtained from this call include the number of map registers and the size of the scatter/gather list that is needed to describe the data buffer for the transfer. In the subsequent call to the AllocateAdapterChannelEx routine (see step 3), the driver supplies the map register count as an input parameter.
The driver calls the AllocateAdapterChannelEx routine to allocate resources to assign to the DMA adapter object. These resources include a DMA channel and map registers.
An AllocateAdapterChannelEx call can be asynchronous or synchronous.
If the DMA_SYNCHRONOUS_CALLBACK flag is not set, the call is asynchronous. In this case, the ExecutionRoutine parameter points to a caller-supplied execution routine that is called when the requested resources are available. If successful, an asynchronous AllocateAdapterChannelEx call returns STATUS_SUCCESS without waiting for the execution routine to run.
If the DMA_SYNCHRONOUS_CALLBACK flag is set, the AllocateAdapterChannelEx call is synchronous. In this case, the ExecutionRoutine parameter in the call is optional, and AllocateAdapterChannelEx behaves as follows:
If ExecutionRoutine is non-NULL, and the DMA resources can be allocated immediately, AllocateAdapterChannelEx calls the execution routine in the context of the calling thread. After the execution routine finishes running, AllocateAdapterChannelEx returns STATUS_SUCCESS. If the resources are not immediately available, AllocateAdapterChannelEx fails and returns error status code STATUS_INSUFFICIENT_RESOURCES.
If ExecutionRoutine is NULL, and AllocateAdapterChannelEx can immediately allocate the DMA resources, AllocateAdapterChannelEx returns STATUS_SUCCESS. If all resources are not immediately available, the call fails with error status code STATUS_INSUFFICIENT_RESOURCES.
For synchronous calls that return STATUS_SUCCESS, if the MapRegisterBase parameter to AllocateAdapterChannelEx is non-NULL, AllocateAdapterChannelEx writes the base address of the allocated map registers to the address pointed to by the MapRegisterBase parameter. If ExecutionRoutine is NULL, MapRegisterBase must be non-NULL. If ExecutionRoutine is non-NULL, the MapRegisterBase parameter to AllocateAdapterChannelEx is optional, and the execution routine receives the map register base address as an input parameter.
For asynchronous AllocateAdapterChannelEx calls, ExecutionRoutine must be non-NULL, and the execution routine receives the map register base address as an input parameter.
If ExecutionRoutine is non-NULL, the execution routine returns a status value to indicate the disposition of the allocated resources. For system DMA transfers, this return value must be KeepObject. This value informs the operating system that the adapter object (and all of its allocated resources) is in use and should not be freed. If no execution routine is supplied, the driver must instead call the FreeAdapterObject routine and supply KeepObject as the AllocationOption parameter.
Step 4: If necessary, cancel the pending resource request
After an AllocateAdapterChannelEx call queues a DMA adapter to wait for DMA resources, the driver can, if necessary, call the CancelAdapterChannel routine to cancel the pending resource request.
If CancelAdapterChannel returns TRUE, the resource request is successfully canceled. If an execution routine was supplied in the AllocateAdapterChannelEx call, this routine does not run.
If CancelAdapterChannel returns FALSE, the resource request cannot be canceled because it was already granted. If an execution routine was supplied in the AllocateAdapterChannelEx call, this routine will be called.
The driver calls MapTransferEx to initialize the DMA resources and to start the DMA transfer. This call might occur in the same driver thread that calls AllocateAdapterChannelEx, or it might occur in the execution routine that the driver supplies to AllocateAdapterChannelEx. If more than one MapTransferEx call is required to transfer the entire DMA data buffer, a later MapTransferEx call might occur in the completion routine for the previous MapTransferEx call.
MapTransferEx supports chained MDLs as input parameters. Each MDL describes a region of the DMA buffer that is contiguous in virtual memory. When MapTransferEx builds the scatter/gather list, it automatically handles transitions from one virtually contiguous buffer region to the next without driver intervention. For more information, see Using the MapTransferEx Routine.
For a system DMA transfer, a pointer to a DMA completion routine can be passed to MapTransferEx in the optional DmaCompletionRoutine parameter. This routine is scheduled to run at dispatch level in response to an interrupt from the system DMA controller that indicates that the DMA transfer is complete.
If MapTransferEx is unable to map the entire requested transfer size, it will set the *Length output parameter to the length that was mapped, and return STATUS_SUCCESS.
Step 6: If necessary, perform hardware-specific operations
MapTransferEx returns STATUS_SUCCESS to indicate that the DMA transfer is successfully initiated. On some platforms, the driver might have to take some additional action, outside of the MapTransferEx call, to start the transfer, but this type of delayed start is not required for all platforms. Drivers must not depend on such delays for decisions about using and freeing allocated resources.
The routines in the DMA operations interface maintain cache coherency for DMA transfers in a way that is transparent to the drivers that use these routines. On platforms that do not enforce cache coherency in hardware, MapTransferEx ensures that processor data caches are flushed before write (memory-to-device) transfers. For read (device-to-memory) transfers, the caches are invalidated during the call to the FlushAdapterBuffersEx routine (see step 8) that follows every MapTransferEx call.
Step 7: Receive notification when the DMA transfer finishes
When a DMA transfer completes, the driver is notified in one of these two ways:
- An interrupt to the device driver, for a bus-master device
- Execution of the driver-supplied completion routine, for a subordinate device that uses a system DMA controller
For a system DMA transfer, a driver can supply a completion routine to MapTransferEx as an input parameter.
After the DMA transfer completes, the driver must call the FlushAdapterBuffersEx routine to flush any data that remains in the cache. The driver must call FlushAdapterBuffersEx after every MapTransferEx call.
If a MapTransferEx call maps only a part of the DMA data buffer, the driver must call MapTransferEx again to map the remaining data. A complex transfer might require several MapTransferEx calls. For each additional MapTransferEx call, repeat steps 5 through 8.
Step 9: Free the DMA channel and map registers
After the entire DMA data buffer is successfully mapped and the final transfer completes, the driver must call the FreeAdapterChannel routine to free the DMA channel and any previously allocated map registers.
Step 10: Release the DMA adapter object
After all DMA transfers are complete and any previously allocated map registers are freed, the driver calls the PutDmaAdapter routine to release the adapter object.