IOMemoryCursor

Inherits from	`OSObject`
Availability	Available in OS X v10.0 and later.
Declared in	IODMACommand.h IOMemoryCursor.h

Overview

A mechanism to convert memory references to physical addresses.

The IOMemoryCursor declares the super class that all specific memory cursors must inherit from, but a memory cursor can be created without a specific format subclass by just providing a segment function to the initializers. This class does the difficult stuff of dividing a memory descriptor into a physical scatter/gather list appropriate for the target hardware.

A driver is expected to create a memory cursor and configure it to the limitations of its DMA hardware; for instance the memory cursor used by the FireWire SBP-2 protocol has a maximum physical segment size of 2^16 - 1 but the actual transfer size is unlimited. Thus it would create a cursor with a maxSegmentSize of 65535 and a maxTransfer size of UINT_MAX. It would also provide a SegmentFunction that can output a pagelist entry.

Below is the simplest example of a SegmentFunction:

void IONaturalMemoryCursor::outputSegment(PhysicalSegment segment,

void * outSegments,

UInt32 outSegmentIndex)

{

((PhysicalSegment *) outSegments)[outSegmentIndex] = segment;

}

Tasks

Miscellaneous

Instance Methods

genPhysicalSegments

Generates a physical scatter/gather list given a memory descriptor.

virtual UInt32 genPhysicalSegments( IOMemoryDescriptor *descriptor, IOByteCount fromPosition, void *segments, UInt32 maxSegments, UInt32 maxTransferSize = 0, IOByteCount *transferSize = 0);

Parameters

descriptor: IOMemoryDescriptor that describes the data associated with an I/O request.
fromPosition: Starting location of the I/O within a memory descriptor.
segments: Void pointer to base of output physical scatter/gather list. Always passed directly onto the SegmentFunction without interpretation by the cursor.
maxSegments: Maximum number of segments that can be written to segments array.
maxTransferSize: Maximum transfer size is limited to that many bytes, otherwise it defaults to the maximum transfer size specified when the memory cursor was initialized.
transferSize: Pointer to an IOByteCount variable that can contain the total size of the transfer being described. Defaults to 0 indicating that no transfer size need be returned.

Return Value

If the descriptor is exhausted of memory, a zero is returned, otherwise the number of segments that were filled in is returned.

Discussion

Generates a list of physical segments from the given memory descriptor, relative to the current position of the descriptor.

initWithSpecification

Primary initializer for the IOMemoryCursor class.

virtual bool initWithSpecification( SegmentFunction outSegFunc, IOPhysicalLength maxSegmentSize = 0, IOPhysicalLength maxTransferSize = 0, IOPhysicalLength alignment = 1);

Parameters

outSegFunc: SegmentFunction to call to output one physical segment.
maxSegmentSize: Maximum allowable size for one segment. Defaults to 0.
maxTransferSize: Maximum size of an entire transfer. Defaults to 0 indicating no maximum.
alignment: Alignment restrictions on output physical addresses. Not currently implemented. Defaults to single byte alignment.

Return Value

Returns true if the inherited classes and this instance initialize successfully.

withSpecification

Creates and initializes an IOMemoryCursor in one operation.

static IOMemoryCursor * withSpecification( SegmentFunction outSegFunc, IOPhysicalLength maxSegmentSize = 0, IOPhysicalLength maxTransferSize = 0, IOPhysicalLength alignment = 1);

Parameters

outSegFunc: SegmentFunction to call to output one physical segment.
maxSegmentSize: Maximum allowable size for one segment. Defaults to 0.
maxTransferSize: Maximum size of an entire transfer. Defaults to 0 indicating no maximum.
alignment: Alignment restrictions on output physical addresses. Not currently implemented. Defaults to single byte alignment.

Return Value

Returns a new memory cursor if successfully created and initialized, 0 otherwise.

Discussion

Factory function to create and initialize an IOMemoryCursor in one operation. For more information, see IOMemoryCursor::initWithSpecification.

Callbacks

SegmentFunction

typedef void ( *SegmentFunction)(
   PhysicalSegment segment,
   void *segments,
   UInt32 segmentIndex);

Parameters

segment: The physical address and length that is next to be output.
segments: Base of the output vector of DMA address length pairs.
segmentIndex: Index to output 'segment' in the 'segments' array.

Discussion

Pointer to a C function that outputs a single physical segment to an element in the array as defined by the segments and segmentIndex parameters.

Availability

Available in OS X v10.0 and later.

Declared In

IODMACommand.h

Constants

Defines

   
#define IOPhysicalSegment IOMemoryCursor::PhysicalSegment
#define OutputSegmentFunc IOMemoryCursor::SegmentFunction

Constants

IOPhysicalSegment

Backward compatibility define for the old non-class scoped type definition. See IOMemoryCursor::PhysicalSegment

Available in OS X v10.0 and later.

Declared in IOMemoryCursor.h.

OutputSegmentFunc

Backward compatibility define for the old non-class scoped type definition. See IOMemoryCursor::SegmentFunction

Available in OS X v10.0 and later.

Declared in IOMemoryCursor.h.

PhysicalSegment

struct PhysicalSegment {
   IOPhysicalAddress location;
   IOPhysicalLength length;
};

Discussion

A physical address/length pair.

Instance Variables

alignMask

IOPhysicalLength alignMask;

Currently unused. Reserved for automated aligment restriction code.

maxSegmentSize

IOPhysicalLength maxSegmentSize;

Maximum size of one segment in a scatter/gather list

maxTransferSize

IOPhysicalLength maxTransferSize;

Maximum size of a transfer that this memory cursor is allowed to generate

outSeg

SegmentFunction outSeg;

The action method called when an event has been delivered

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