TDmaChannel Derived Class Implementation

diff -r 578be2adaf3e -r 307f4279f433 Adaptation/GUID-877EEB52-40C8-4880-87A0-9736A625F85F.dita --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Adaptation/GUID-877EEB52-40C8-4880-87A0-9736A625F85F.dita Fri Oct 15 14:32:18 2010 +0100 @@ -0,0 +1,76 @@ + + + + + +TDmaChannel Derived Class ImplementationDescribes how to write a class describing a DMA channel. +

The TDmaChannel class is part of the PSL implementation. +It is an abstract class with pure virtual functions which you must +derive in order to implement it. The implementation of a DMA channel +involves hardware-specific decisions such as the choice of DMA mode

The purpose of DMA is to transfer data such as streaming audio +produced by a client application. A typical transfer involves passing +the data from user side to kernel side via a shared chunk and then +a transfer from one memory location to another by DMA over a buffer. +If the amount of data to be transferred exceeds the maximum transfer +size supported by the controller or involves non-contiguous physical +memory, the data is fragmented and transmitted in a sequence of transfers +in one of three modes.

In single buffer mode the hardware controller provides a single +set of transfer registers used alternately to perform an active transfer +and to set up a pending one: however this mode incurs overhead in +the form of significant periods of time when the driver is idle during +the pending phase.
In double buffer mode the hardware controller provides two +sets of transfer registers, one set for the active transfer and one +for the pending transfer..
In scatter-gather mode, a single procedure call sequentially +writes data from multiple buffers to a single data stream or from +a data stream to multiple buffers.

The PIL supplies three derived classes corresponding to the DMA +modes:

TDmaSbChannel
for a single-buffer channel,
TDmaDbChannel
for a double-buffer channel, +and
TDmaSgChannel
for a scatter-gather channel.

These three classes are defined in dma_v1.h.

You may simply use one of these classes or derive further classes +from them and implement those. For example, the template scatter-gather +implementation defines a TTemplateSgChannel class +derived from TDmaSgChannel for storing data when +appending new descriptors to a running channel:

+class TTemplateSgChannel : public TDmaSgChannel + { +public: + TDmaDesc* iTmpDes; + TPhysAddr iTmpDesPhysAddr; + }; + +

In general, design decisions are made at the level of implementation +in hardware and are subsequently reflected in the structure of the +derived classes.

There are two ways of extending the DMA Framework:

to provide platform-specific functionality on a per-channel +basis, and
to provide platform-specific functionality on a channel independent +basis

In the first case, TDmaChannel::Extension() calls TDmac::Extension() and the PSL provides an implementation +of the virtual function TDmac::Extension(). The +default implementation just returns KErrNotSupported. In the second case, TDmaChannel::StaticExtension() calls DmaChannelMgr::StaticExtension() and the +PSL provides an implementation of the static function DmaChannelMgr::StaticExtension(). The template PSL implementation just returns KErrNotSupported.

+DMA +Channels +

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