Chris Cant - Writing Windows WDM Device Drivers

dx->UsageCount = 1;

KeInitializeEvent(&dx->StoppingEvent, NotificationEvent, FALSE);

dx->Stopping = false;

dx->GotPortOrMemory = false;

dx->GotInterrupt = false;

dx->ConnectedToInterrupt = false;

dx->SetTimeout = 10;

dx->Timeout = –1;

dx->StopTimer = false;

dx->WriteCmds = NULL;

dx->ReadCmds = NULL;

dx->StartReadCmds = NULL;

// Initialise timer for this device (but do not start)

status = IoInitializeTimer(phddo, (PIO_TIMER_ROUTINE)Timeout1s, dx);

if (!NT_SUCCESS(status)) {

DebugPrintMsg("Could not initialise timer");

IoDeleteDevice(phddo);

return status;

}

// Create a symbolic link so our device is visible to Win32…

DebugPrint("Creating symbolic link %T", &linkName);

status = IoCreateSymbolicLink(&linkName, &deviceName);

if (!NT_SUCCESS(status)) {

DebugPrintMsg("Could not create symbolic link");

IoDeleteDevice(phddo);

return status;

}

// Initialise our DPC for IRQ completion processing

IoInitializeDpcRequest(phddo, PHDIoDpcForIsr);

return status;

}

When PHDIo is unloaded, its PHDIoUnload routine is called. This runs PHDIoDeleteDevice to stop the PHDIo device, remove its symbolic link, and delete its device object.

If a driver makes a variable number of devices, the unload routine could use the following technique to find all the devices to remove. The kernel sets the driver object DeviceObject field to point to the first device that belongs to the driver. The NextDevice field in each device object points to the next device. It is, therefore, a simple task to traverse this chain of device objects and delete them. The only catch is that you still have to generate the correct symbolic link name for each device so the symbolic link can be removed.

This section looks at how to allocate resources. The crucial kernel function is IoReportResourceUsage . This checks to see if any other devices are using the resources. If the resources are free, they are reserved so no other device can use them.

The PHDIo driver only finds out which resources are needed when the user calls CreateFile , passing the resource description in the filename string. The Create IRP arrives in the PHDIoCreate routine. All the previous Create IRP handlers have not done very much. However, PHDIoCreate has these jobs to do.

1. Get the resource details from the filename.

2. Check for resource conflicts and reserve the resources.

3. Translate and map the resources.

Getting the resource details is handled by the GetResourcesFromFilename routine. I will not go into the details of this code here, apart from saying that it uses three support routines that work with the UNICODE_STRING structure: usStrCmpN, usGetHex, and usGetDec. In the end, the GotPortOrMemory device extension field is true if an I/O port specifier has been found, GotInterrupt is true if an interrupt has been found and ResourceOverride is true if the \override specifier was used.

PHDIoCreate checks that an I/O port was specified. It then calls ClaimResources to check for resource conflicts and reserve the resources. Finally, TranslateAndMapResources is used to translate resource information and map memory. PHDIoCreate carefully ensures that all the resource bool fields are reset to false at the fail label if any error occurs.

Listing 18.2 PHDIoCreate routine

NTSTATUS PHDIoCreate(IN PDEVICE_OBJECT phddo, IN PIRP Irp) {

PPHDIO_DEVICE_EXTENSION dx = (PPHDIO_DEVICE_EXTENSION)phddo->DeviceExtension;

PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);

DebugPrint("Create File is %T", &(IrpStack->FileObject->FileName));

dx->GotPortOrMemory = false;

dx->GotInterrupt = false;

dx->PortNeedsMapping = false;

dx->ConnectedToInterrupt = false;

dx->ResourceOverride = FALSE;

// Get resources from filename string

PUNICODE_STRING usfilename = &(IrpStack->FileObject->FileName);

NTSTATUS status = *(usfilename,dx);

if (!NT_SUCCESS(status)) goto fail;

// We must have IO port resource

if (!dx->GotPortOrMemory) {

DebugPrintMsg("No IO Port resource in filename");

status = STATUS_INVALID_PARAMETER:

goto fail;

}

// Claim resources

status = ClaimResources(phddo);

if (!NT_SUCCESS(status)) {

DebugPrintMsg("Could not ClaimResources");

goto fail;

}

// Translate and map resources

status = TranslateAndMapResources(phddo);

if (!NT_SUCCESS(status)) {

UnclaimResources(phddo);

goto fail;

}

// Complete

return CompleteIrp(Irp,status);

// On error, make sure everything's off fail:

dx->GotPortOrMemory = false;

dx->GotInterrupt = false;

dx->PortNeedsMapping = false;

dx->ConnectedToInterrupt = false;

return CompleteIrp(Irp,status);

}

Claiming resources means working with Full and Partial Resource Descriptors. Chapter 9 showed that a device's resource assignments are given in these structures when the Plug and Play Start Device is received. The WdmIo driver obtains its resource assignments in this way.

NT style drivers have to build a resource list of these descriptors to pass to the IoReportResourceUsage routine. Note carefully that the raw resource details must be passed to IoReportResourceUsage , not the translated values. The kernel resource list structures are sufficiently intricate that it is worth showing them in Listing 1 8.3.

A resource list consists of one Full Resource Descriptor for each bus instance. Note carefully that this is an expandable structure. Although it is declared with only one Full Resource Descriptor, it may in fact contain one or more such descriptors.

A Full Resource Descriptor specifies the bus type and instance number. It also contains a Partial Resource List structure.

A Partial Resource List primarily contains an array of Partial Resource Descriptors. Again, note that a Partial Resource List is a structure that expands as more Partial Resource Descriptors are used.

A Partial Resource Descriptor finally contains the details of an individual resource. Table 9.2 in Chapter 9 gives full details of Partial Resource Descriptors.

Listing 18.3 Kernel resource list structures

typedef struct _CM_RESOURCE_LIST {

ULONG Count;

CM_FULL_RESOURCE_DESCRIPTOR List[1];

} CM_RESOURCE_LIST, *PCM_RESOURCE_LIST;

typedef struct _CM_FULL_RESOURCE_DESCRIPTOR {