StartIo Routines in Lowest-Level Drivers

Article
12/15/2021

The I/O manager's call to a driver's dispatch routine is the first stage in satisfying a device I/O request. The StartIo routine is the second stage. Every device driver with a StartIo routine is likely to call IoStartPacket from its DispatchRead and DispatchWrite routines, and usually for a subset of the I/O control codes it supports in its DispatchDeviceControl routine. The IoStartPacket routine adds the IRP to the device's system-supplied device queue or, if the queue is empty, immediately calls the driver's StartIo routine to process the IRP.

You can assume that when a driver's StartIo routine is called, the target device is not busy. This is because the I/O manager calls StartIo under two circumstances; either one of the driver's dispatch routines has just called IoStartPacket and the device queue was empty, or the driver's DpcForIsr routine is completing another request and has just called IoStartNextPacket to dequeue the next IRP.

Before the StartIo routine in a highest-level device driver is called, that driver's dispatch routine should have probed and locked down the user buffer, if necessary, to set up valid mapped buffer addresses in the IRP queued to its StartIo routine. If a highest-level device driver sets up its device objects for direct I/O (or for neither buffered nor direct I/O), the driver cannot defer locking down a user buffer to its StartIo routine; every StartIo routine is called in an arbitrary thread context at IRQL = DISPATCH_LEVEL.

Note

Any buffer memory to be accessed by a driver's StartIo routine must be locked down or allocated from resident, system-space memory and must be accessible in an arbitrary thread context.

In general, any lower-level device driver's StartIo routine is responsible for calling IoGetCurrentIrpStackLocation with the input IRP and then doing whatever request-specific processing is necessary to start the I/O operation on its device. Request-specific processing can include the following:

Setting up or updating any state information about the current request that the driver maintains. The state information might be stored in the device extension of the target device object or elsewhere in nonpaged pool allocated by the driver.

For example, if a device driver maintains an InterruptExpected Boolean for the current transfer operation, its StartIo routine might set this variable to TRUE. If the driver maintains a time-out counter for the current operation, its StartIo routine might set up this value, or the StartIo routine might queue the driver's CustomTimerDpc routine.

If the StartIo routine shares access to state information or hardware resources with other driver routines, the state information or resource must be protected by a spin lock. (See Spin Locks.)

If the StartIo routine shares access to state information or resources with the driver's InterruptService routine, StartIo must use KeSynchronizeExecution to call a SynchCritSection routine that accesses the state or resource information. (See Using Critical Sections.)
Assigning a sequence number to the IRP in case the driver must log a device I/O error while processing the IRP.

See Logging Errors for more information.
If necessary, translating the parameters in the driver's I/O stack location into device-specific values.

For example, a disk driver might need to calculate the starting sector or byte offset to the physical disk address for a transfer operation, and whether the requested length of the transfer will cross a particular sector boundary or exceed the transfer capacity of its physical device.
If the driver controls a removable-media device, checking for media changes before programming the device for I/O and notifying its overlying file system if the media has changed.

For more information, see Supporting Removable Media.
If the device uses DMA, checking whether the requested Length (number of bytes to be transferred, found in the driver's I/O stack location of the IRP) should be split into partial-transfer operations, as explained in Input/Output Techniques, assuming a closely coupled higher-level driver does not presplit large transfers for the device driver.

The StartIo routine of such a device driver also can be responsible for calling KeFlushIoBuffers and, if the driver uses packet-based DMA, for calling AllocateAdapterChannel with the driver's AdapterControl routine.

See Adapter Objects and DMA, and Maintaining Cache Coherency, for additional details.
If the device uses PIO, mapping the base virtual address of the buffer, described in the IRP at Irp->MdlAddress, to a system-space address with MmGetSystemAddressForMdlSafe.

For read requests, the device driver's StartIo routine can be responsible for calling KeFlushIoBuffers before PIO operations begin. See Maintaining Cache Coherency for more information.
If a non-WDM driver uses a controller object, calling IoAllocateController to register its ControllerControl routine.
If the driver handles cancelable IRPs, checking whether the input IRP has already been canceled.
If an input IRP can be canceled before it is processed to completion, the StartIo routine must call IoSetCancelRoutine with the IRP and the entry point of the driver's Cancel routine. The StartIo routine must acquire the cancel spin lock for its call to IoSetCancelRoutine. Alternatively, a driver can use IoSetStartIoAttributes to set the NonCancelable attribute for the StartIo routine to TRUE. This prevents the system from trying to cancel an IRP that has been passed to StartIo by a call to IoStartPacket.

As a general rule, a driver that uses buffered I/O has a simpler StartIo routine than one that uses direct I/O. Drivers that use buffered I/O transfer small amounts of data for each transfer request, while those that use direct I/O (whether DMA or PIO) transfer large amounts of data to or from locked-down buffers that can span physical page boundaries in system memory.

Higher-level drivers layered above physical device drivers usually set up their device objects to match those of their respective device drivers. However, a highest-level driver, particularly a file system driver, can set up device objects for neither direct nor buffered I/O.

Drivers that set up their device objects for buffered I/O can rely on the I/O manager to pass valid buffers in all IRPs it sends to the driver. Lower-level drivers that set up device objects for direct I/O can rely on the highest-level driver in their chain to pass valid buffers in all IRPs sent through any intermediate drivers to the underlying lower-level device driver.

Using Buffered I/O in StartIo Routines

If a driver's DispatchRead, DispatchWrite, or DispatchDeviceControl routine determines that a request is valid and calls IoStartPacket, the I/O manager calls the driver's StartIo routine to process the IRP immediately if the device queue is empty. If the queue is not empty, IoStartPacket queues the IRP. Eventually, a call to IoStartNextPacket from the driver's DpcForIsr or CustomDpc routine causes the I/O manager to dequeue the IRP and call the driver's StartIo routine.

If access to the physical device (or the device extension) must be synchronized with an InterruptService routine, the StartIo routine must call a SynchCritSection routine to perform the necessary device programming. For more information, see Using Critical Sections.

A physical device driver that uses buffered I/O transfers data either to or from a system-space buffer, allocated by the I/O manager, that the driver finds in each IRP at Irp->AssociatedIrp.SystemBuffer.

Using Direct I/O in StartIo Routines

The StartIo routine calls IoGetCurrentIrpStackLocation and determines which operation must be performed to satisfy the request. It preprocesses the IRP in any way necessary, such as splitting up a large DMA transfer request into partial-transfer ranges and saving state about the Length of an incoming transfer request that must be split. Then it programs the physical device to carry out the I/O request.

If access to the physical device (or the device extension) must be synchronized with the driver's ISR, the StartIo routine must use a driver-supplied SynchCritSection routine to perform the necessary programming. For more information, see Using Critical Sections.

Any driver that uses direct I/O either reads data into or writes data from a locked-down buffer, described by a memory descriptor list (MDL), that the driver finds in the IRP at Irp->MdlAddress. Such a driver commonly uses buffered I/O for device control requests. For more information, see Handling I/O Control Requests in StartIo Routines.

The MDL type is an opaque type that drivers do not access directly. Instead, drivers that use PIO remap user-space buffers by calling MmGetSystemAddressForMdlSafe with Irp->MdlAddress as a parameter. Drivers that use DMA also pass Irp->MdlAddress to support routines during their transfer operations to have the buffer addresses remapped to logical ranges for their devices.

Unless a closely coupled higher-level driver splits up large DMA transfer requests for the underlying device driver, a lowest-level device driver's StartIo routine must split up each transfer request that is larger than its device can manage in a single transfer operation. Drivers that use system DMA are required to split transfer requests that are too large for the system DMA controller or for their devices to handle in a single transfer operation.

If the device is a subordinate DMA device, its driver must synchronize transfers through a system DMA controller with a driver-allocated adapter object, representing the DMA channel, and a driver-supplied AdapterControl routine. The driver of a bus-master DMA device also must use a driver-allocated adapter object to synchronize its transfers and must supply an AdapterControl routine if it uses the system's packet-based DMA support, or an AdapterListControl routine if it uses the system's scatter/gather support.

Depending on the driver's design, it might synchronize transfer and device control operations on a physical device with a controller object and supply a ControllerControl routine.

See Adapter Objects and DMA and Controller Objects for more information.

Handling I/O Control Requests in StartIo Routines

In general, only a subset of device I/O control requests are passed on from a driver's DispatchDeviceControl or DispatchInternalDeviceControl routine for further processing by the driver's StartIo routine. The driver's StartIo routine only has to handle valid device control requests that require device state changes or return volatile information about the current device state.

Each new driver must support the same set of public I/O control codes as all other drivers for the same kind of device. The system defines public, device-type-specific I/O control codes for IRP_MJ_DEVICE_CONTROL requests as buffered requests.

Consequently, physical device drivers make data transfers to or from a system-space buffer that each driver finds in the IRP at Irp->AssociatedIrp.SystemBuffer for device control requests. Even drivers that set up their device objects for direct I/O use buffered I/O to satisfy device control requests with public I/O control codes.

The definition of each I/O control code determines whether data transferred for that request is buffered. Any privately defined I/O control codes for driver-specific IRP_MJ_INTERNAL_DEVICE_CONTROL requests between paired drivers can define a code with method buffered, method direct, or method neither. As a general rule, any privately defined I/O control code should be defined with method neither if a closely coupled higher-level driver must allocate a buffer for that request.

Programming the Device for I/O Operations

Usually, the StartIo routine in a lowest-level device driver must synchronize access to any memory or device registers it shares with the driver's ISR by using KeSynchronizeExecution to call a driver-supplied SynchCritSection routine. The driver's StartIo routine uses the SynchCritSection routine to actually program the physical device for I/O at DIRQL. For more information, see Using Critical Sections.

Before calling KeSynchronizeExecution, the StartIo routine must do any preprocessing necessary for the request. Preprocessing might include calculating an initial partial-transfer range and saving any state information about the original request for other driver routines.

If a device driver uses DMA, its StartIo routine usually calls AllocateAdapterChannel with a driver-supplied AdapterControl routine. In these circumstances, the StartIo routine postpones the responsibility for programming the physical device to the AdapterControl routine. It, in turn, can call KeSynchronizeExecution to have a driver-supplied SynchCritSection routine program the device for a DMA transfer.