Hiding DMA HardwareVariations with Adapter Objects

Thepurpose of using DMA is to minimize the CPU's involvement in data transferoperations. To do this, DMA devices use an auxiliary processor, called a DMAcontroller, to move data between memory and a peripheral device. Thisallows the CPU to continue doing other useful work in parallel with the I/Ooperation.

Intelr Serial Io Dma Controller Driver for Windows 7 32 bit, Windows 7 64 bit, Windows 10, 8, XP. Uploaded on 4/20/2019, downloaded 7284 times, receiving a 88/100 rating by 4955 users. Supported OS: Windows 10, Windows 8/8.1, Windows 7, Windows Vista. Drivers Update Tool Information This utility contains the only official version for Intel(R) Platform Controller Hub EG20T DMA Controller #1 - 8810 Driver for Windows XP/7/Vista/8/8.1/10 32-bit and 64-bit versions.

Althoughthe exact details vary, most DMA controllers have a very similar architecture.In its simplest form, this consists of an address register for the startingaddress of the DMA buffer and a count register for the number of bytes or wordsto transfer. When these registers are properly programmed and the devicestarted, the DMA controller begins moving data on its own. With each transfer,it increments the memory address register and decrements the count register.When the count register reaches zero, the DMA controller generates aninterrupt, and the device is ready for another transfer.

TheScatter/Gather Problem

Althoughvirtual memory simplifies the lives of application developers, it introducestwo major complications for DMA-based drivers. The first problem is that thebuffer address passed to the I/O Manager is a virtual address. Since theDMA controller works with physical addresses, DMA drivers need some wayto determine the physical pages making up a virtual buffer.

The other problem is that a process doesn'tnecessarily occupy consecutive pages of physical memory, and what appears to bea contiguous buffer in virtual space is probably scattered throughout physicalmemory.

The Windows 2000 Virtual Memory Manager uses the platform's addresstranslation hardware togive the process the illusion of a single, unbroken virtual address space.Unfortunately, the DMA controller doesn't participate in this illusion.

Figure 12.1. Addressspaces involved in DMA operations.

Memory DescriptorLists

Asdescribed, loading physical addresses into mapping registers is an importantpart of setting up a DMA transfer. To make this process easier, the I/O Manageruses a structure called a Memory Descriptor List (MDL).An MDL keeps track of physical pages associated with a virtual buffer. Thebuffer described by an MDL can be in either user- or system-address space.

DirectI/O operations require the use of MDLs. If a Device object has the DO_DIRECT_IObit set in its Flags field, the I/O Manager automatically builds an MDLdescribing the caller's buffer each time an I/O request is sent to the device.It stores the address of this MDL in the IRP'sMdlAddress field, and adriver uses it to prepare the DMA hardware for a transfer.

Asseen in Figure 12.2,the MDL consists of a header describing the virtual buffer followed by an arraythat lists the physical pages associated with the buffer. Given a virtualaddress within the buffer, the MDL data describes the corresponding physicalpage. Some of the fields in the header help clarify the use of an MDL.

Figure12.2. Structure of a Memory Descriptor List (MDL).

StartVa andByteOffset.

The StartVa field contains theaddress of the buffer described by the MDL, rounded down to the nearest virtualpage boundary. Since the buffer doesn't necessarily start on a page boundary,the ByteOffset field specifies the distance from this page boundary tothe actual beginning of the buffer. Keep in mind that if the buffer is in userspace, a driver can use theStartVa field tocalculate indexes into the MDL, butnotas an actualaddress pointer.

MappedSystemVa - If the buffer described by the MDL is in user spaceand its contents must be accessed, the buffer must first be mapped into systemspace with MmGetSystemAddressForMdl. This field of the MDL is used tohold the system-space address where the user-space buffer has been mapped.

ByteCount andSize - Thesefields contain the number of bytes in the buffer described by the MDL and thesize of the MDL itself, respectively.

Process - Ifthe buffer lives in user space, the Process field points to the processobject that owns the buffer. The I/O Manager uses this information when itcleans up the I/O operation.

Table 12.1. FunctionsThat Operate on Memory Descriptor Lists

MDL Access Functions
Function	Description
IoAllocateMdl	Allocates an empty MDL
IoFreeMdl	Releases MDL allocation by IoAllocateMdl
MmBuildMdlForNonPagedPool	Builds MDL for an existing nonpaged pool buffer
MmGetSystemAddressForMdl	Returns a nonpaged system space address for the buffer described by an
	MDL
IoBuildPartialMdl	Builds an MDL describing part of a buffer
MmGetMdlByteCount	Returns count of bytes in buffer described by MDL
MmGetMdlByteOffset	Returns page-offset of buffer described by MDL
MmGetMdlVirtualAddress	Returns starting VA of buffer described by MDL

Maintaining CacheCoherency

Figure 12.3. Cachesinvolved in DMA processing.

CPUDATA CACHE

Modern CPUs support both on-chip andexternal caches for holding copies of recently used data. When the CPU wantssomething from physical memory, it first looks for the data in the cache. Ifthe CPU finds what it wants, it doesn't have to make the long, slow trip downthe system memory bus. For write operations, data moves from the CPU to thecache, where (depending on the cache and policy) it may stay for a while beforemaking its way out to main memory.

The problem is that on some architectures, the CPU'scache controller and the DMA hardware are unaware of each other. This lack ofawareness can lead to incoherent views of memory. For instance, if the CPUcache is holding part of the buffer, and that buffer is overwritten in physicalmemory by a DMA input, the CPU cache will contain stale data. Similarly, ifmodified data hasn't been flushed from the CPU cache when a DMA output begins,the DMA controller will be sending stale data from physical memory out to thedevice.

Oneway of handling this problem is to make sure that any portion of the DMA bufferresiding in the CPU's data cache is flushed before a DMA operation begins. Adriver can do this by calling KeFlushIoBuffers and giving it the MDLdescribing the DMA buffer. This function flushes any pages in the MDL from thedata cache of every processor on the system.

ADAPTEROBJECT CACHE

TheAdapter object is another place where data may be cached during a DMA transfer.Unlike the CPU cache, which is always a real piece of hardware, the Adapterobject cache is an abstraction representing platform-dependent hardware orsoftware. It might be an actual cache in a system DMA controller or a softwarebuffer maintained by the I/O Manager. In fact, for some combinations ofhardware, there might not even be a cache, but a driver still needs to use theAdapter object in order to guarantee portability.

Anotherbenefit of using the Adapter object is that problems presented by certain busesare transparently handled for the driver. For example, the DMA controller foran ISA bus can access only the first 16 megabytes of physical memory. If anypages of a user buffer are outside this range, the I/O Manager allocatesanother buffer in low memory when the driver sets up the DMA mapping registersof the Adapter object. For output operation, the I/O Manager also copies thecontents of the user buffer pages into this Adapter object buffer.

TheAdapter object cache must be explicitly flushed after an input operation, or tonotify the I/O Manager that it can release the memory in the adapter buffer.The function that performs the flush and release is FlushAdapterBuffers, a method of theAdapter object.

Packet-Basedand Common Buffer DMA

TheWindows 2000 DMA model divides drivers into two categories, based on thelocation of the DMA buffer itself: packet-based DMA and common bufferDMA.

Inpacket-based DMA, data moves directly between the device and the locked-downpages of a user-space buffer. This is the type of DMA associated with directI/O operations. The significant point is that each new I/O request willprobably use a different set of physical pages for its buffer. This impacts thekind of setup and cleanup steps the driver has to take for each I/O.

Incommon buffer DMA, the device uses a single nonpaged buffer from system spaceand all DMA transfers occur through this buffer.

Limitationsof the Windows 2000 DMA Architecture

While the use of the Windows 2000 DMA abstractionsimplifies driver construction, it does impose some restrictions. For one, themodel is somewhat biased toward slave DMA devices. A driver is burdened with additional work to forcethe Adapter object model to fit a master DMA device.

Moresignificantly, the Windows 2000 DMA model does not support device-to-devicedata transfers. Since modern buses such as PCI promote the concept ofpeer-to-peer relationships between devices, it is unfortunate that the Adaptermodel does not extend to nonsystem-hosted DMA.

Workingwith Adapter Objects

Table 12.2. FunctionPrototype for IoGetDmaAdapter

PDMA_ADAPTER	IoGetDmaAdapter IRQL PASSIVE_LEVEL
Parameter	Description
IN PDEVICE_OBJECT pdo	Points to the physical device object for the device
IN PDEVICE_DESCRIPTION	Points to a DEVICE_DESCRIPTION structure
pDeviceDescription
	IN OUT PULONG	Input: number of map registers requested.
NumberOfMapRegisters	Output: maximum number of map registers that the driver
	can allocate for any DMA transfer operation.
Return value		Pointer to DMA_ADAPTER

ScatterGather.

For bus master devices, this field signifies that thehardware supports transfer of data to and from noncontiguous ranges of physicalmemory. For slave devices, this field indicates that the device can be pausedbetween page transfers, allowing the I/O Manager to repoint the DMA channel'saddress register to a new page of physical memory.

DemandMode.

DMA demand mode is a transfer protocolthat allows a device to hold off the (slave) DMA controller. In normal mode(DemandModeFALSE), the DMA controller does not allow the device to delay itsrequest to transfer another block of data.

Autoinitialization.

DMAautoinitialization mode allows system DMA channels to restart themselves aftera completed transfer. Specified address and count values are automaticallyreset into the DMA hardware and another operation is 'good to go.'

IgnoreCount.

Some DMA hardware maintains an impropercount of bytes transferred. This can occur because the hardware counts wordsinstead of bytes. If this field is set to TRUE, the HAL manually maintains thetransfer count on behalf of the device.

Table 12.3. TheDEVICE_DESCRIPTION Structure for IoGetDmaAdapter

DEVICE_DESCRIPTION,
*PDEVICE_DESCRIPTION
Field	Contents
ULONG Version	• DEVICE_DESCRIPTION_VERSION
• DEVICE_DESCRIPTION_VERSION1
BOOLEAN Master	TRUE - Bus master device
FALSE - Slave device
BOOLEAN ScatterGather	TRUE - Device supports scatter/gather
BOOLEAN DemandMode	Slave device uses demand mode
BOOLEAN AutoInitialize	Slave device uses autoinitialize mode
BOOLEAN Dma32BitAddresses	DMA addressing uses 32 bits
BOOLEAN IgnoreCount	TRUE - Device's transfer count not maintained
	accurately
BOOLEAN Reserved1		Must be FALSE
BOOLEAN Dma64BitAddresses	DMA addressing uses 64 bits
ULONG BusNumber	System-assigned bus number (unused by WDM
	drivers)
ULONG DmaChannel		Slave device DMA channel number
INTERFACE_TYPE InterfaceType	• Internal
• Isa
• Eisa
• MicroChannel
• PCIBus
DMA_WIDTH DmaWidth	• Width8Bits
• Width16Bits
• Width32Bits
DMA_SPEED DmaSpeed	• Compatible
• TypeA
• TypeB
• TypeC
ULONG MaximumLength	Largest transfer size (in bytes) device can perform
ULONG DmaPort	Microchannel-type bus port number (obsolete)

Acquiringand Releasing the Adapter Object

Thereis no guarantee that the DMA resources needed for a device transfer will befree when a driver's Start I/O routine runs. For example, a slave device DMAchannel may already be in use by another device, or there may not be enoughmapping registers to handle the request. Consequently, all packet-based DMAdrivers and drivers for common buffer slave devices have to request ownershipof the Adapter object before starting a data transfer.

Sincethe Start I/O routine runs at DISPATCH_LEVEL IRQL, there is no way it can stopand wait for the Adapter object. Instead, it calls the Allocate-AdapterChannelmethod of the Adapter object (see Table 12.4)and then returns control to the I/O Manager.

When the requested DMA resources become available,the I/O Manager notifies the driver by calling its Adapter Control routine.It's important to keep in mind that this is an asynchronous callback. It mayhappen as soon as Start I/O calls AllocateAdapterChannel, or it may notoccur until some other driver releases the Adapter resources.

Notice that the caller of AllocateAdapterChannelmust be at DISPATCH_LEVEL IRQL. Since the function is normally called from theStart I/O routine, this poses no problem. However, if it is called from anotherdriver routine from PASSIVE_LEVEL, make sure to use KeRaiseIrql andKeLower-Irql before and afterthe call to AllocateAdapterChannel.

Table 12.4. FunctionPrototype for AllocateAdapterChannel

NTSTATUS	IRQL DISPATCH_LEVEL
AllocateAdapterChannel
	Parameter	Description
IN PDMA_ADAPTER	Points to the DMA_ADAPTER structure returned by
pDmaAdapter	IoGetDmaAdapter
IN PDEVICE_OBJECT	Points to the target DMA device object
pDeviceObject
	IN ULONG	Specifies the number of map registers to be used in the transfer
NumberOfMapRegisters
	IN PDRIVER_CONTROL	Points to a driver-supplied AdapterControl routine to be called as
ExecutionRoutine	soon the system DMA controller or bus master adapter is available
IN PVOID pContext	Points to the driver-determined context to be passed to the
	AdapterControl routine
Return value		STATUS_SUCCESS or STATUS_INSUFFICIENT_RESOURCES

The Adapter Control routine in a DMA driver isresponsible for calling MapTransfer to set up the DMA hardware andstarting the actual device operation. Table 12.5contains a prototype of the Adapter Control callback.

Table 12.5. FunctionPrototype for an Adapter Control Routine

IO_ALLOCATION_ACTION AdapterControl	IRQL DISPATCH_LEVEL
Parameter	Description
IN_PDEVICE_OBJECT pDeviceObject	Target device for DMA operation
IN PIRP pIrp	IRP describing this operation
IN PVOID MapRegisterBase	Handle to a group of mapping registers
IN PVOID pContext	Driver-determined context
l DeallocateObjectKeepRegisters
Return value
l KeepObject

TheMapRegisterBase argument is an opaque value that identifies the mappingregisters assigned to the I/O request. It is really a kind of handle to aspecific group of registers. This handle is used to set up the DMA hardware forthe transfer. Normally, this handle value is saved in the Device or Controllerextension because it is needed in later parts of the DMA operation.

The pIrp argumentpassed to the Adapter Control callback is valid only when

AllocateAdapterChanneliscalled from the Start I/O routine. If it is called from some other context,thepIrp pointer will be NULL. In such a case, another mechanism must be used topass the IRP (and its associated MDL address) to the Adapter Control routine.The context pointer argument can possibly be used for this purpose.

Afterit programs the DMA controller and starts the data transfer, the AdapterControl routine gives control back to the I/O Manager. Drivers of slave devicesshould return a value of KeepObject from this function so that the Adapterobject remains the exclusive property of this request. Bus master driversreturn DeallocateObjectKeepRegisters.

Whenthe DpcForIsr routine in a DMA driver completes an I/O request, it needs torelease any Adapter resources it owns. Drivers of DMA devices do this bycalling FreeAdapterChannel.

SettingUp the DMA Hardware - Allpacket-based drivers, as well as common buffer drivers for slave devices, haveto program the DMA hardware at the beginning of each data transfer. Using theabstract DMA model of Windows 2000, this means loading the Adapter object'smapping registers with physical page addresses taken from the MDL. This setupwork is done by the MapTransfer method of the Adapter object.

MapTransfer uses theCurrentVa and Length arguments to figure out what physical page addressesto put into the mapping registers. These values must fall somewhere within therange of addresses described by the MDL.

Keepin mind that MapTransfer may actually move the contents of the DMAoutput buffer from one place to another in memory.

For example, on an ISAmachine, if the pages in the MDL are outside the 16-megabyte DMA limit, callingthis function results in data being copied to a buffer in low physical memory. Similarly, if the DMA input buffer is out of range, MapTransferallocates a buffer in low memory for the transfer. On buses that support 32-bitDMA addresses, no copying or duplicate buffers are required.

Drivers of bus master devices also need to call MapTransfer.In this case, however, the function behaves differently, since it doesn't knowhow to program the bus master's control registers. Instead, MapTransfersimply returns address and length values that the driver again loads into thedevice's registers. For bus masters with built-in scatter/gather support, thissame mechanism allows the driver to create a scatter/gather list for thedevice. Later sections of this chapter explain how this works.

Flushingthe Adapter Object Cache

Atthe end of a data transfer, all packet-based DMA drivers and drivers for commonbuffer slave devices have to call FlushAdapterBuffers, a method of theAdapter object. Fordevices using the system DMA controller, this function flushes any hardwarecaches associated with the Adapter object.

Table 12.6. FunctionPrototype for MapTransfer

PHYSICAL_ADDRESS
MapTransfer	IRQL <= DISPATCH_LEVEL
Parameter	Description
IN PDMA_ADAPTER pDmaAdapter	Points to the DMA adapter object returned by
	IoGetDmaAdapter
IN PMDL pMdl		Memory Descriptor List for DMA buffer
IN PVOID MapRegisterBase	Handle to a group of mapping registers
IN PVOID CurrentVA	Virtual address of buffer within the MDL
IN OUT PULONG Length	• IN - count of bytes to be mapped
• OUT - actual count of bytes mapped
IN BOOLEAN bWriteToDevice	• TRUE - send data to device
• FALSE - read data from device
Return value	DMA logical address of the mapped region

Table 12.7. FunctionPrototype for FlushAdapterBuffers

BOOLEAN FlushAdapterBuffers	IRQL <= DISPATCH_LEVEL
Parameter	Description
IN PDMA_ADAPTER	Points to the DMA adapter object returned by
pDmaAdapter	IoGetDmaAdapter
IN PMDL pMdl	MDL describing the buffer
IN PVOID MapRegisterBase	Handle passed to AdapterControl
IN PVOID CurrentVA	Starting virtual address of buffer
IN ULONG Length	Length of the buffer
IN BOOLEAN WriteToDevice	• TRUE - operation was an output
• FALSE - operation was an input
Return value	• TRUE - Adapter buffers flushed
• FALSE - an error occurred

Inthe case of ISA devices doing packet-based DMA, this call releases any lowmemory used for auxiliary buffers. For input operations, it also copies databack to the physical pages of the caller's input buffer. Refer back to thesection on cache coherency for a discussion of this process.

Writinga Packet-Based Slave DMA Driver - Inpacket-based slave DMA, the device transfers data to or from the locked downpages of the caller's buffer using a shared DMA controller on the mainboard.The system is also responsible for providing scatter/gather support.

HowPacket-Based Slave DMA Works - Althoughthe specifics depend on the nature of the device, most packet-based slave DMAdrivers conform to a very similar pattern. The following subsections describethe routines of these drivers.

IRP_MN_START_DEVICEHandler

Along with itsusual duties, this PnP handler performs the following DMA preparation tasks:

1.Locatesthe DMA channel used by the device. The DMA resources would normally be sentwith the requesting IRP in the stack's Parameters.StartDevice.AllocatedResourcesTranslated field.

2.The DEVICE_DESCRIPTION structure isbuilt. IoGetDmaAdapter is invoked to identify the Adapter objectassociated with the device.

3.TheDMA_OBJECT pointer returned from IoGetDmaAdapter is saved in the DeviceExtension.

4.TheDO_DIRECT_IO bit in the Flags field of the Device object is set, causingthe I/O Manager to lock user buffers in memory and create MDLs for them.

STARTI/O ROUTINE

Unlikeits counterpart in a programmed I/O driver, the DMA Start I/O routine doesn'tactually start the device. Instead, it requests ownership of the Adapter objectand leaves the remainder of the work to the Adapter Control callback routine.Specifically, the Start I/O routine does the following:

1.Itcalls KeFlushIoBuffers to flush data from the CPU cache out to mainmemory (RAM).

2.StartI/O decides how many mapping registers to request. Initially, it calculates thenumber of registers needed to cover the entire user buffer. If this numberturns out to be more mapping registers than the Adapter object has available,it will ask for the maximum available.

3. Basedon the number of mapping registers and the size of the user buffer, Start I/Ocalculates the number of bytes to transfer in the first device operation. Thismay be the entire buffer or it may be onlythe first portion of a split transfer.

4.Next,Start I/O calls MmGetMdlVirtualAddress to recover the virtual address ofthe user buffer from the MDL. It stores this address in the Device Extension.Subsequent parts of the driver use this address as an offset in the MDL to setup the actual DMA transfer.

5.StartI/O then calls AllocateAdapterChannel to request ownership of theAdapter object. If this function succeeds, the rest of the setup work isperformed by the Adapter Control routine, so Start I/O simply returns controlto the I/O Manager.

6.IfAllocateAdapterChannel returns an error, Start I/O fails the requestingIRP, calls IoCompleteRequest, and starts processing the next IRP.

ADAPTERCONTROL ROUTINE - TheI/O Manager calls back the Adapter Control routine when the necessary Adapterresources become available. Its job is to initialize the DMA controller for thetransfer and start the device itself. In essence, it is the second half ofStart I/O that occurs with the Adapter object in hand. This routine does thefollowing:

1.It stores the value of theMapRegisterBase argument it receives in the Device Extension for subsequentuse.

2.TheAdapter Control routine then calls MapTransfer to load the Adapterobject's mapping registers. To make this call, it uses the buffer's virtualaddress and the transfer size calculated by the Start I/O routine.

3.Next,it sends appropriate commands to the device to begin the transfer operation.

4.Finally, the Adapter Control routinereturns the value KeepObject to retain ownership of the Adapter object.

Atthis point, the transfer is in progress, and other code is executing inparallel until an interrupt arrives from the device.

INTERRUPTSERVICE ROUTINE

Compared to a programmed I/O driver, the ISR in apacket-based DMA driver is not very complicated. Unless hardware limitationsforce the driver to split a large transfer request across several deviceoperations, there is only a single interrupt service when the whole transfercompletes. When this interrupt arrives, the ISR does the following:

1.It issues commands as necessary toacknowledge the device and prevent it from generating additional interrupts.

2.TheISR then stores device status (and any relevant error information) in theDevice Extension.

3.Itcalls IoRequestDpc to continue processing the request in the driver'sDpcForIsr routine.

4.TheISR returns the value of TRUE to indicate that it serviced the interrupt.

DpcForIsrROUTINE

TheDpcForIsr routine is triggered by the ISR at the end of each partial datatransfer operation. Its job is to start the next partial transfer (if there isone) or to complete the current request. Specifically, the DpcForIsr routine ina packet-based DMA driver does the following:

1.It calls FlushAdapterBuffers, amethod of the Adapter object, to force any remaining data from the Adapterobject's cache.

2.TheDpcForIsr routine checks the Device Extension to see if there were any errorsduring the operation. If so, it completes the request with an appropriatestatus code and length, and starts the next request.

3.Otherwise, it decrements the count ofbytes remaining by the size of the last transfer. If the whole buffer has beenprocessed, it completes the current request and starts the next.

4.Ifmore data remains, the DpcForIsr routine increments the user-buffer addresspointer (stored in the Device Extension) by the size of the last operation. Itthen calculates the number of bytes transferred in the next device operation,calls MapTransfer to reset the mapping registers, and starts the device.

Ifthe DpcForIsr routine started another partial transfer, the I/O Manager willreturn control to the driver when the device generates an interrupt.

SplittingDMA Transfers

Whena packet-based DMA driver receives a buffer, it may not be able to transfer allthe data in a single device operation. It could be that the Adapter objectdoesn't have enough mapping registers to handle the whole request at once, orthere could be limitations on the device itself. In any event, the driver hasto be prepared to split the request across multiple data transfer operations.

Thereare two solutions to this problem. One is to have the driver reject anyrequests that it can't handle in a single I/O. With this approach, any user ofthe driver is responsible for breaking the request into chunks small enough toprocess. Of course, the driver will have to provide some mechanism for lettingits clients know the maximum allowable buffer size (an IOCTL, for example). Forthis approach, it might make sense to write a higher-level driver that sits ontop of the DMA device driver and splits the requests. This has the advantage ofshielding application programs from the details of splitting the request.

This second method requires maintenance of a pointerthat tracks position within the user buffer as successive chunks of data aretransferred. There may also be a need to maintain a count of the number ofbytes left to process. The following sections explain how to initialize andupdate these data items during an I/O request.

FIRSTTRANSFER

The Start I/O routine normally sets things up for thefirst transfer. Initially, it tries to grab enough mapping registers to doeverything in one I/O. If the Adapter object doesn't have enough mappingregisters for this to work, Start I/O asks for as many as it can get and setsup the current transfer accordingly. The following code fragment shows how thisis done:

pDevExt->transferVA = (PUCHAR)MmGetMdlVirtualAddress( pIrp->MdlAddress );

pDevExt->bytesRemaining =MmGetMdlByteCount( pIrp->MdlAddress );

pDevExt->transferSize =pDevExt->bytesRemaining;

mapRegsNeeded = ADDRESS_AND_SIZE_TO_SPAN_PAGES(pDevExt->transferVA, pDevExt->transferSize);

if ( mapRegsNeeded >pDevExt->mapRegsAvailable ) { mapRegsNeeded = pDevExt->mapRegsAvailable;pDevExt->transferSize =

mapRegsNeeded * PAGE_SIZE -MmGetMdlByteOffset( pIrp->MdlAddress );

}

//Note the use of the Adapter object - the DmaAdapter pointer returned by IoGetDmaAdapter contains function pointers for Adapter operations.

pDevExt->pDmaAdapter->DmaOperations->AllocateAdapterChannel(pDevExt->pDmaAdapter, pDevObj, mapRegsNeeded, AdapterControl, pDevExt );

ADDITIONAL TRANSFERS

Aftereach interrupt, the DpcForIsr checks to see if there is any data left toprocess. If there is, it calculates the number of mapping registers needed totransfer all the remaining bytes in a single I/O operation. If there are notenough mapping registers available, it sets up another partial transfer. Thefollowing code fragment illustrates the procedure:

pDevExt->bytesRemaining -= pDevExt->transferSize; if(pDevExt->bytesRemaining > 0)

{

pDevExt->transferVA += pDevExt->transferSize;pDevExt->transferSize = pDevExt->bytesRemaining; mapRegsNeeded = ADDRESS_AND_SIZE_TO_SPAN_PAGES(pDevExt->transferVA, pDevExt->transferSize );

if (mapRegsNeeded >pDevExt->mapRegsAvailable ) {

mapRegsNeeded = pDevExt->mapRegsAvailable;pDevExt->transferSize = mapRegsNeeded * PAGE_SIZE - BYTE_OFFSET( pDevExt->transferVA);

}

pDevExt->pDmaAdapter->DmaOperations->MapTransfer( pDevExt->pDmaAdapter, pMdl, pDevExt>mapRegisterBase,

pDevExt->transferVA,transferSize, pDevExt->bWriting );

}

CodeExample: A Packet-Based Slave DMA Driver - This example is a skeleton of a packet-based driverfor a generic slave DMA device. Although it doesn't actually manage a specifickind of hard-ware, it may help in understanding how these drivers work. Thecompletecode for this example is included on the CD that accompanies this bookand on the companion website www.W2KDriverBook.com.

DRIVER.H - Thisexcerpt from the driver-specific header file shows the changes that need to bemade to the Device Extension structure.

typedefstruct _DEVICE_EXTENSION {

PDMA_ADAPTERpDmaAdapter;

ULONGmapRegisterCount;

ULONGdmaChannel;

//This is the 'handle' assigned to the map registerswhen the AdapterControl routine is called back

PVOIDmapRegisterBase;

ULONGbytesRequested;

ULONGbytesRemaining;

ULONGtransferSize;

PUCHARtransferVA;

// This flag is TRUE if writing, FALSE if reading

BOOLEANbWriting;

}DEVICE_EXTENSION, *PDEVICE_EXTENSION;

#defineMAX_DMA_LENGTH 4096

GetDmaInfoRoutine

Intel Serial Io Drivers Windows 10

The GetDmaInfohelper routine is responsible for making the call to IoGetDmaAdapter.Primarily, this is anexample of setting up the DEVICE_ DESCRIPTION structure.

NTSTATUS GetDmaInfo( IN INTERFACE_TYPEbusType, IN PDEVICE_OBJECT pDevObj )

{

PDEVICE_EXTENSIONpDevExt = (PDEVICE_EXTENSION)pDevObj->DeviceExtension;

DEVICE_DESCRIPTIONdd;

// Zero out the entire structure RtlZeroMemory( &dd,sizeof(dd) );

dd.Version = DEVICE_DESCRIPTION_VERSION1; dd.Master = FALSE; //this is a slave device dd.ScatterGather = FALSE;

dd.DemandMode = FALSE; dd.AutoInitialize = FALSE;dd.Dma32BitAddresses = FALSE;

dd.InterfaceType= busType;// as passed in

dd.DmaChannel = pDevExt->dmaChannel; dd.MaximumLength =MAX_DMA_LENGTH; dd.DmaWidth = Width16Bits;

dd.DmaSpeed =Compatible;

//Compute the maximum number of mapping regs this device could possibly need. Since thetransfer may not be paged aligned, add one to allow the max xfer size to span a page.

pDevExt->mapRegisterCount = (MAX_DMA_LENGTH / PAGE_SIZE) + 1;

pDevExt->pDmaAdapter =IoGetDmaAdapter( pDevObj, &dd, &pDevExt->mapRegisterCount);

// If the Adapter object can't be assigned, fail

if(pDevExt->pDmaAdapter NULL)

return STATUS_INSUFFICIENT_RESOURCES;

else

returnSTATUS_SUCCESS;

}

StartI/O Changes

Start I/O no longer starts the device. Instead, itsets up the DMA operation and defers to the Adapter Control routine, calledback when the Adapter Channel can be allocated. Nevertheless, the DMA setup issignificant work.

VOIDStartIo( IN PDEVICE_OBJECT pDevObj, IN PIRP pIrp ) {

PIO_STACK_LOCATIONpStack = IoGetCurrentIrpStackLocation(pIrp );

PDEVICE_EXTENSIONpDevExt = (PDEVICE_EXTENSION) pDevObj->DeviceExtension;

//The IRP holds the MDL structure, already set up by the I/O Manager because DO_DIRECT_IO flag is set

PMDL pMdl =pIrp->MdlAddress;

ULONGmapRegsNeeded;

NTSTATUSstatus;

pDevExt->bWriting= FALSE; // assume read operation

switch ( pStack->MajorFunction )

{

case IRP_MJ_WRITE:

pDevExt->bWriting = TRUE; // badassumption

case IRP_MJ_READ:

pDevExt->bytesRequested =MmGetMdlByteCount( pMdl );

pDevExt->transferVA = (PUCHAR)MmGetMdlVirtualAddress( pMdl );

pDevExt->bytesRemaining = pDevExt->transferSize = pDevExt->bytesRequested;

mapRegsNeeded = ADDRESS_AND_SIZE_TO_SPAN_PAGES(pDevExt->transferVA, pDevExt->transferSize );

mapRegsNeeded = pDevExt->mapRegisterCount;

pDevExt->transferSize = mapRegsNeeded * PAGE_SIZE - MmGetMdlByteOffset( pMdl );

}

status =pDevExt->pDmaAdapter->DmaOperations-> AllocateAdapterChannel( pDevExt->pDmaAdapter, pDevObj, mapRegsNeeded, AdapterControl,pDevExt );

if (!NT_SUCCESS(status )) {

//fail the IRP & don't continue with it

pIrp->IoStatus.Status = status;

//Show no bytes transferred

pIrp->IoStatus.Information = 0;

IoCompleteRequest( pIrp,IO_NO_INCREMENT );

IoStartNextPacket( pDevObj, FALSE);

}

break;// nice job -AdapterControlfrom here on

default:

// Shouldn't be here - ditch this strange

IRPpIrp->IoStatus.Status = STATUS_NOT_SUPPORTED;

pIrp->IoStatus.Information= 0;

IoCompleteRequest(pIrp, IO_NO_INCREMENT );

IoStartNextPacket(pDevObj, FALSE );

}

}

AdapterControlRoutine - This callback routine completes the work started withStart I/O. It programs the DMA hardware and starts the device itself. It iscalled by the I/O Manager after the Adapter object is assigned to the deviceand sufficient mapping registers are available to handle the request.

IO_ALLOCATION_ACTIONAdapterControl( IN PDEVICE_OBJECTpDevObj, IN PIRP pIrp, IN PVOIDMapRegisterBase, IN PVOID pContext )

{

PDEVICE_EXTENSIONpDevExt = (PDEVICE_EXTENSION) pContext;

//Save the handle to the mapping register set

pDevExt->mapRegisterBase = MapRegisterBase;

//Flush the CPU cache(s),if necessary on this platform..

KeFlushIoBuffers(pIrp->MdlAddress,

!pDevExt->bWriting,
TRUE );

pDevExt->pDmaAdapter->DmaOperations->MapTransfer( pDevExt->pDmaAdapter,

pIrp->MdlAddress, MapRegisterBase, pDevExt->transferVA,&pDevExt->transferSize, pDevExt->bWriting );

// Start the device

StartTransfer( pDevExt );

returnKeepObject;

}

DpcForIsrRoutine - The Interrupt Service Routine for a DMA device isusually straightforward. An interrupt is generated at the end of each partialtransfer, or when a transfer error occurs. As usual, the ISR schedules a DPC,using IoRequestDpc. The DPC fires the registered routine, DpcForIsr.

DpcForIsrsets up the next partial transfer. If the entire transfer has completed, itmarks the IRP for completion and starts the next.

VOIDDpcForIsr(IN PKDPC pDpc,

IN PDEVICE_OBJECT pDevObj,

IN PIRP pIrp,

IN PVOID pContext )

{

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ULONGmapRegsNeeded;

PMDLpMdl = pIrp->MdlAddress;

//Flush the Apapter buffer to system RAM or device.

pDevExt->pDmaAdapter->DmaOperations-> FlushAdapterBuffers(pDevExt->pDmaAdapter,

pMdl, pDevExt->mapRegisterBase, pDevExt->transferVA,pDevExt->transferSize, pDevExt->bWriting );

//If the device is reporting errors, fail the IRP

if(DEVICE_FAIL( pDevExt )) {

// An error occurred, the DMA channel is now free

pDevExt->pDmaAdapter->DmaOperations->FreeAdapterChannel( pDevExt->pDmaAdapter);

pIrp->IoStatus.Status = STATUS_DEVICE_DATA_ERROR;

pIrp->IoStatus.Information = pDevExt->bytesRequested - pDevExt->bytesRemaining;

IoCompleteRequest(pIrp, IO_NO_INCRMENT );

IoStartNextPacket(pDevObj, FALSE);

}

// Device had no errors, seeif another partial needed

pDevExt->bytesRemaining -=pDevExt->transferSize;

if (pDevExt->bytesRemaining > 0)

{

//Another partial transfer needed Update the transferVA and try to finish it

pDevExt->transferVA += pDevExt->transferSize;

pDevExt->transferSize =pDevExt->bytesRemaining; mapRegsNeeded =

ADDRESS_AND_SIZE_TO_SPAN_PAGES(pDevExt->transferVA, pDevExt->transferSize );

//If it still doesn't fit in one swipe cut back the expectation

if (mapRegsNeeded > pDevExt->mapRegisterCount)

{

mapRegsNeeded= pDevExt->mapRegisterCount;

pDevExt->transferSize = mapRegsNeeded * PAGE_SIZE - BYTE_OFFSET( pDevExt->transferVA);

}

//Now set up the mapping registers for another

pDevExt->pDmaAdapter->DmaOperations->

MapTransfer( pDevExt->pDmaAdapter,pMdl, pDevExt->mapRegisterBase, pDevExt->transferVA,&pDevExt->transferSize, pDevExt->bWriting );

//And start the device

StartTransfer(pDevExt );

} else {

//Entire transfer has now completed - Free the DMA channel for another device

pDevExt->pDmaAdapter->DmaOperations->FreeAdapterChannel( pDevExt->pDmaAdapter );

//And complete the IRP in glory

pIrp->IoStatus.Status = STATUS_SUCCESS;

pIrp->IoStatus.Information = pDevExt->bytesRequested;

// Choose a priority boost appropriate for device

IoCompleteRequest( pIrp, IO_DISK_INCREMENT );

IoStartNextPacket( pDevObj, FALSE);

}

}

Writing aPacket-Based Bus Master DMA Driver - Inpacket-based bus master DMA, the device transfers data to or from the lockeddown pages of the caller's buffer, using DMA hardware that is part of thedevice itself. Depending on the capabilities of the device, it might beproviding its own scatter/gather support as well.

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Thearchitecture of a packet-based bus master driver is almost identical to that ofa driver for a slave device. The only difference is the way the driver sets upthe bus master hardware. The following sections describe these differences.

Setting Up BusMaster Hardware - A bus master device is more complicatedto program because Windows 2000 doesn't know how to program the device'sonboard DMA controller. The most the I/O Manager can do is provide the driverwith two pieces of information.

• An address in DMA logicalspace, where a contiguous segment of the buffer begins
• A count indicating thenumber of bytes in that segment

Itthen becomes the driver's responsibility to load this information into theaddress and length registers of the device and start the transfer.

Thefunction that performs this work is familiar— MapTransfer. If theDEVICE_DESCRIPTION supplied in the call to IoGetDmaAdapter described abus mastering device, then MapTransfer returns a physical address thatcan be used to program the device's DMA address register. The physical addresscorresponds to the MDL and CurrentVa arguments of the call.

Thereis little chance that the user buffer, if it spans more than a page, resides incontiguous physical RAM. Therefore, MapTransfer returns the physical startingaddress and the length of the 'first' block that can be transferredcontiguously. In all likelihood, the length returned by MapTransfer isthe length of a page—n o part of the user buffer is likely to be contiguous.Should two or more pages of the user buffer happen to be contiguous,then MapTransfer correctly reports such a length.

Afterthe device transfers what it can from the contiguous buffer, it must bereprogrammed to transfer the next part of the buffer from another contiguousblock. Figure 12.4shows how this process needs to work, with the somewhat optimistic case of somepages of the user buffer actually residing in contiguous memory.

Figure 12.4. MapTransferpoints to contiguous buffer segments.

Supportingbus master devices requires some changes to the driver's AdapterControl andDpcForIsr routines. The following sections contain fragments of these routines.Compare them with the corresponding routines in the packet-based slave DMAdriver in the previous section of this chapter.

AdapterControlROUTINE

Being optimistic, theAdapterControl routine asks MapTransfer to map the entire buffer at thestart of the first transfer. MapTransfer reports to the driver how muchcontiguous memory is actually available in the first segment of the buffer.

PHYSICAL_ADDRESSDmaAddress;

PMDL pMdl =pIrp->MdlAddress;

pDevExt->transferVA = (PUCHAR)MmGetMdlVirtualAddress( pMdl );

PDevExt->transferSize = pDevExt->bytesRemaining = MmGetMdlByteCount(pMdl );

DmaAddress= pDevExt->pDmaAdapter->DmaOperations->MapTransfer(pDevExt->pDmaAdapter,

Lil wayne right above it mp3 download. pMdl, pDevExt->mapRegisterBase, pDevExt->transferVA,&pDevExt->transferSize, pDevExt->bWriting );

//transferSize has been reset to the maximumcontiguous length for thefirst segment WriteDmaAddress is a device-specific routine that

programs the device's DMAaddress register

WriteDmaAddress( pDevExt, DmaAddress.LowPart );

//Similarly, WriteDmaCount is device-specific

WriteDmaCount(pDevExt, pDevExt->transferSize );

//Now the device is started

StartDevice(pDevExt );

// must return indicating bus master at work returnDeallocateObjectKeepRegisters;

DpcForIsr ROUTINE

Aftereach partial transfer, the DpcForIsr routine increments the CurrentVa pointerby the previously returned Length value. It then calls MapTransfer againwith this updated pointer and asks to map all the bytes remaining in thebuffer. MapTransfer returns another logical address and a new Lengthvalue indicating the size of the next contiguous buffer segment. This processcontinues until the whole buffer has been processed.

PHYSICAL_ADDRESSDmaAddress;

PMDL pMdl =pIrp->MdlAddress;

// Clear out the adapter object buffer (if any);

pDevExt->pDmaAdapter->DmaOperations->FlushAdapterBuffers(pDevExt->pDmaAdapter, pMdl, pDevExt->mapRegisterBase,pDevExt->transferVA, pDevExt->transferSize, pDevExt->bWriting );

pDevExt->bytesRemaining -= pDevExt->transferSize;

if(pDevExt->bytesRemaining > 0) {

pDevExt->transferVA += pDevExt->transferSize;

pDevExt->transferSize = pDevExt->bytesRemaining;

DmaAddress= pDevExt->pDmaAdapter->DmaOperations-> MapTransfer( pDevExt->pDmaAdapter,pMdl, pDevExt->mapRegisterBase, pDevExt->transferVA, &pDevExt->transferSize,pDevExt->bWriting );

WriteDmaAddress( pDevExt, DmaAddress.LowPart );

WriteDmaCount(pDevExt, pDevExt->transferSize ); // Now the device is re-started

StartDevice(pDevExt );

}

Hardware withScatter/Gather Support - Somebus master devices contain multiple pairs of address and length registers, eachone describing a single contiguous buffer segment. This allows the device toperform I/O using buffers that are scattered throughout DMA address space.These multiple address and count registers are often referred to as a scatter/gatherlist, but they can be considered devices with their own built-in mappingregisters. Yet anotherway to consider scatter/gather is as a devicewith its own private page table hardware. Figure12.5shows a bus master device with scatter/gather hardware.

Figure 12.5. Busmaster device with scatter/gather hardware.

BuildingScatter/Gather Lists with MapTransfer

Onceagain, MapTransfer is used to find contiguous segments of the DMAbuffer. In this case, however, the driver calls it several times before eachdata transfer operation—once for each entry in the hardwar e scatter/gatherlist. These fragments of an AdapterControl and DpcForIsr routine demonstratethe process.

AdapterControlRoutine

Beforethe first transfer operation, the AdapterControl routine loads the hardwarescatter/gather list and starts the device. The remainder of the buffer ishandled by the ISR and DpcForIsr routines.

Forscatter/gather devices, the state of each address/counter pair may need to bepersisted somewhere, perhaps in the device extension. An array or linked listwould be appropriate structures.

PHYSICAL_ADDRESSDmaAddress;

ULONGbytesLeftInBuffer;

ULONGsegmentSize;

ULONGmapRegisterIndex;

PUCHAR segmentVA;

PMDL pMdl = pIrp->MdlAddress;

pDevExt->transferVA =MmGetMdlVirtualAddress( pMdl );

pDevExt->bytesRemaining =MmGetMdlByteCount( pMdl );

pDevExt->transferSize = 0;

bytesLeftInBuffer = pDevExt->bytesRemaining;

segmentVA =pDevExt->transferVA;

mapRegisterIndex = 0;

while (mapRegisterIndex >pDevExt->mapRegisterCount && bytesLeftInBuffer > 0)

{

//Try for the whole enchilada

segmentSize = bytesLeftInBuffer;DmaAddress =

pDevExt->pDmaAdapter->DmaOperations-> MapTransfer(pDevExt->pDmaAdapter,

pMdl, pDevExt->mapRegisterBase, segmentVA, &segmentSize,pDevExt->bWriting );

//WriteMapRegister is a device-specific method.

//It programs one pair of scatter/gather regs. WriteMapRegister(pDevExt, mapRegisterIndex, DmaAddress.LowPart,

segmentSize ); mapRegisterIndex++;

//Move on the next scatter/gather pair

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pDevExt->transferSize += segmentSize; segmentVA +=segmentSize; bytesLeftInBuffer -= segmentSize;

}

//Now start the device StartDevice( pDevExt );

//And indicate that scatter/gather regs are in use returnDeallocateObjectKeepRegisters;

DpcForIsrROUTINE

After each transfer is finished, the ISR issues a DPCrequest. The DpcForIsr routine flushes the previous request, and if there aremore bytes left to transfer, it rebuilds the scatter/gather list.

ULONG bytesLeftInBuffer;

ULONG segmentSize;

ULONG mapRegisterIndex;

PUCHAR segmentVA;

PMDL pMdl = pIrp->MdlAddress;

// Clear out the adapter object buffer (if any)

pDevExt->pDmaAdapter->DmaOperations->FlushAdapterBuffers( pDevExt->pDmaAdapter,pMdl, pDevExt->mapRegisterBase, pDevExt->transferVA,pDevExt->transferSize, pDevExt->bWriting );

pDevExt->bytesRemaining -= pDevExt->transferSize;

if(pDevExt->bytesRemaining > 0) {

pDevExt->transferVA += pDevExt->transferSize; pDevExt->transferSize= 0;

bytesLeftInBuffer = pDevExt->bytesRemaining; segmentVA =pDevExt->transferVA;

mapRegisterIndex= 0;

while (mapRegisterIndex >pDevExt->mapRegisterCount && bytesLeftInBuffer > 0) {

segmentSize = bytesLeftInBuffer; DmaAddress =

pDevExt->pDmaAdapter->DmaOperations->MapTransfer(pDevExt->pDmaAdapter,

pMdl, pDevExt->mapRegisterBase, segmentVA,

&segmentSize, pDevExt->bWriting );

WriteMapRegister( pDevExt,mapRegisterIndex, DmaAddress.LowPart, segmentSize );

mapRegisterIndex++;

// Move on the next scatter/gather pair

pDevExt->transferSize+= segmentSize;

segmentVA += segmentSize;

bytesLeftInBuffer -= segmentSize;

}

// Then re-start the device

StartDevice( pDevExt );

} else {

//Last device operation completed transfer

//Free up the mapping registers:

pDevExt->pDmaAdapter->DmaOperations->FreeMapRegisters(..);

// Andcomplete the IRP IoCompleteRequest(..); IoStartNextPacket(..);

}

Writing a CommonBuffer Slave DMA Driver - Incommon buffer slave DMA, the device transfers data to or from a contiguousbuffer in nonpaged pool, using a system DMA channel. Although originallyintended for devices that use the system DMA controller's autoinitialize mode,common buffers can also improve throughput for some types of ISA-based slavedevices.

Allocating a CommonBuffer

Memoryfor a common buffer must be physically contiguous and visible in the DMAlogical space of a specific device. To guarantee that both these conditions aremet, the function AllocateCommonBuffer, a method of the Adapter object,is used. It is described in Table 12.8.

TheCacheEnabled argument for this function is normally set to FALSE. Usingnoncached memory for the common buffer eliminates the need to call KeFlushIoBuffers.On some platforms, this can significantly improve the performance of both thedriver and the system.

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Table 12.8. FunctionPrototype for AllocateCommonBuffer

UsingCommon Buffer Slave DMA to Maintain Throughput

Commonbuffer slave DMA is useful if the driver can't afford to set up mapping registersfor each data transfer that it performs. The overhead of setting up mappingregisters using MapTransfer can be significant, especially for ISAbuses. There is always the possibility that MapTransfer will be forcedto copy the transfer buffer into the lowest 16 MB of RAM—clearly an expensiveproposition. Since common buffers areguaranteed to be accessible by their associated DMA devices, there is never aneed to move data from one place to another.

Forexample, drivers of some ISA-based tape drives need to maintain very highthroughput if they want to keep the tape streaming. They may not be able to dothis if the buffer copy must occur during a call to MapTransfer.To prevent this, the driver can use a ring of common buffers for the actual DMAoperation.Other, less time-critical portions of the driver move databetween these common buffers and the actual user buffers.

Considerthe operation of the driver for a hypothetical ISA output device. To maintain ahigh DMA data rate, it uses a series of common buffers that are shared betweenthe driver's Dispatch and DpcForIsr routines. The Dispatch routine copies user-outputdata into an available common buffer and attaches the buffer to a queue ofpending DMA requests. Once a DMA is in progress, the DpcForIsr removes buffersfrom the queue and processes them as fast as it can. Figure 12.6 shows the organization of thisdriver, and the various driver routines are described in the sections thatfollow.

Figure 12.6. Usingcommon buffers to improve I/O throughput.

AddDevice ROUTINE

Besides creating the device object, this routineshould set the DO_ BUFFERED_IO bit in the Flags field. Even though DMAis used for the actual device transfer, the user buffers are initially copiedinto system space.

IRP_MN_START_DEVICEHANDLER

Besidesthe usual responsibilities of initializing the physical device, the handlermust now perform the following:

1.Twoqueues should be initialized in the device extension. One holds a list of freecommon buffers. The other is for work requests in progress.

2.Two spin locks should be created to guardeach queue. The spin lock for the work list also protects a flag in the deviceextension called DmaInProgress.

3.IoGetDmaAdapter isused to find the adapter object associated with the device. Using the count ofmapping registers returned by this function is helpful to determine thesize of the common buffers.

4.Individualcommon buffers should be allocated, using AllocateCommonBuffer once foreach buffer. Initially they should be placed in the free list of the deviceextension.

5.Finally,the Start handler initializes a semaphore object and sets its initial count tothe number of common buffers it has just created.

DISPATCHROUTINE

TheDispatch routine of this driver is somewhat uncommon. The Dispatch routine isresponsible for queuing and starting each request. This is what the Dispatchroutine does to process an output request:

1.Itcalls KeWaitForSingleObject to wait for the Semaphore object associatedwith the driver's list of free buffers. The thread issuing the call willsuspend until there is at least one buffer in the queue.

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2.TheDispatch routine removes an available common buffer from the free list and usesRtlMoveMemory to fill it with data from the user's buffer.

3.Itprevents the I/O Manager from completing the request by calling IoMarkIrpPending.

4.Next,it acquires the spin lock associated with the queue of active requests. As aside-effect, acquiring the spin lock raises IRQL up to DISPATCH_LEVEL. After itowns the spin lock, the Dispatch routine adds the new request to the list ofbuffers to be output.

5.Stillholding the spin lock, the Dispatch routine checks an internal DmaInProgressflag to see if other parts of the driver are already performing an outputoperation. If the flag is TRUE, it simply releases the spin lock. If the flagis FALSE, the Dispatch routine sets it to TRUE and starts the device. It thenreleases the spin lock.

6.Finally, it returns a value ofSTATUS_PENDING.

Atthis point, the work request for this buffer has been either started or queued.The next phase of the transfer occurs within the Start I/O routine.

STARTI/O ROUTINE

If the device is idle,the Start I/O function is called to start it. It performs the following tasks:

1.Itremoves the first request from the work queue and saves its address in theDevice Extension as the current request.

2.It programs the device's DMA registers topoint to the dequeued buffer.

3.The device is started and DMA transferbegins.

INTERRUPT SERVICEROUTINE

Aswith packet-based DMA, the ISR in a common buffer driver for a slave devicemerely saves hardware status in the Device Extension. It then calls IoRequestDpcto request the DpcForIsr routine.

DpcForIsr ROUTINE

In this driver, theDpcForIsr routine sets up each additional work request after the first.

1.Itcalls FlushAdapterBuffers to flush any data from the system DMAcontroller's hardware cache.

2.Itattempts to dequeue the next I/O request from the work queue. If there isanother request, the driver makes it the new current request and restarts thedevice. Otherwise, it clears the DmaInProgress flag in the Device Extension.

3.Next,it moves the just-completed work buffer back in the free queue. KeReleaseSemaphoreis used to signal an increase in the number of available free buffers.

4.Finally, the IRP is marked as complete.

Eachcompleted DMA operation causes another interrupt that brings the driver backthrough the DpcForIsr routine. This loop continues until all the requests inthe work queue have been processed.

IRP_MN_STOP_DEVICEHANDLER

Whenthe device is stopped, the driver should ensure that the device will no longerattempt use of the common buffer. Once the device is quiescent, the Stophandlercalls FreeCommonBuffer to release the memory associated with the ring ofbuffers.

Writinga Common Buffer Bus Master DMA Driver

Incommon-buffer bus master DMA, the device transfers data to or from a contiguousnonpaged pool buffer, using a DMA controller that is part of the device itself.Frequently, this kind of hardware treats the common buffer as a mailbox forexchanging control and status messages with the driver.

HowCommon-Buffer Bus Master DMA Works

Theexact operation of a common-buffer bus master driver will depend on thephysical device. The description that follows is based on the typicalarchitecture. It assumes the device uses one mailbox for commands and anotherto return status information. Figure12.7illustrates this arrangement.

Figure 12.7. Commonbuffer implementation of message exchange

IRP_MN_START_DEVICEHANDLER

Thishandler must now perform the following tasks:

1. InvokeIoGetDmaAdapterto locate theAdapter object associated with the device.

2. UseAllocateCommonBuffertoget a block of contiguous, nonpaged memory that both the driverandthe device can access.

3. Store the virtual addressof the common buffer in the Device Extension for subsequent use.

START I/O ROUTINE

To send a command to thedevice, the Start I/O routine performs the following:

1. Builds a command structure in thecommon buffer, using the virtual address stored in the DeviceExtension.

2. If needed (CacheEnabled parameterset to TRUE),KeFlushIoBuffersisinvoked to force data fromthe CPU's cache out tophysical memory.

3. Finally, Start I/O sets a bit in thedevice control register to notify the device that there is a commandwaitingfor it. In essence, this is equivalent to starting the device.

Inresponse to the notification bit being set, the device begins processing thecommand in the common buffer.

INTERRUPT SERVICEROUTINE

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Whenthe device has finished processing the command in the common buffer, it puts amessage in the status mailbox and generates an interrupt. In response to thisinterrupt, the driver's Interrupt Service Routine does the following:

1. Copiesthe contents of the status mailbox into various fields of the Device Extension.

2. If necessary, the ISR sets anotherbit in the device control register to acknowledge that it has readthestatus message.

3. CallsIoRequestDpcto continueprocessing the request at a lower IRQL.

IRP_MN_STOP_DEVICEHANDLER

Whenthe device is stopped, a driver should quiet the device. The Stop handler callsFreeCommonBuffer to release the memory associated with the buffer.

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