Common DMA Functions (Compact 7)
3/12/2014
The following table lists three CEDDK.dll functions that are useful for common buffer direct memory access (DMA) transfers. These functions handle bus and hardware platform-specific address translations for you. They also handle address translations between the system and the PCI bus for the DMA controller to use for copying data.
| Function | Description |
|---|---|
Allocates memory, locks it down, and maps it so that the microprocessor and the device can access it simultaneously. |
|
Frees a common buffer that is allocated by HalAllocateCommonBuffer, together with all the resources that the buffer uses. |
|
Translates a physical system address to a logical bus address, which you can pass to a bus controller for DMA operations. |
These functions translate a physical RAM address to the corresponding bus-relative physical address for the DMA controller. To set up a common buffer for bus master DMA by using the CEDDK.dll functions, a bus master DMA device driver can call HalAllocateCommonBuffer, passing in a DMA_ADAPTER_OBJECT structure that contains information about the DMA adapter.
The following code example shows a call to HalAllocateCommonBuffer to allocate a physical memory buffer address for DMA.
// Set up the DMA adapter descriptor structure.
DMA_ADAPTER_OBJECT AdapterObject;
AdapterObject.ObjectSize = sizeof(AdapterObject);
AdapterObject.InterfaceType = Internal;
AdapterObject.BusNumber = 0;
// Allocate a physical buffer.
m_vuaBuf = (PBYTE)HalAllocateCommonBuffer(&AdapterObject,
cbBufSize, &m_paBuf, FALSE);
// Unable to allocate a physical buffer.
if (m_vuaBuf == NULL)
{
RETAILMSG(1,(L"unable to allocate %d bytes\r\n", cbBufSize));
ret = ERROR_OUTOFMEMORY;
ASSERT(0);
HalFreeCommonBuffer(NULL, 0, m_paBuf, m_vuaBuf, FALSE);
goto cleanup;
}
// . . .
// Use the buffer
// . . .
// When done with the buffer:
HalFreeCommonBuffer(&AdapterObject, cbBufSize, m_paBuf,
m_vuaBuf, FALSE);
In this example, m_vuaBuf is the DMA buffer virtual uncached address, cbBufSize is the buffer length, in bytes, and m_paBuf is the buffer physical address. The last parameter is set to FALSE to disable caching. When HalAllocateCommonBuffer successfully returns, it allocates a shared buffer of locked, physically contiguous pages that the microprocessor and the device can access simultaneously for DMA operations.
For an example that uses these common DMA functions for scatter/gather DMA, see the sample ATAPI device driver source that is located in %_WINCEROOT%\public\COMMON\oak\drivers\block\atapi. Note that to set up scatter/gather DMA, you must simultaneously use multiple pairs of base addresses and lengths as shown in the sample driver.
For more information about common DMA functions, see Ceddk.dll DMA Functions.