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Scatter-gather list
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Source Address Dest. Address Transfer length Next descriptor System Memory Source Address Dest. Address Transfer length Descriptor pool End of list mark
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Descriptor address register Peripheral DMA Controller
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Block A
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Block B
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Scatter-gather DMA transfer
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The scatter-gather list contains two linked descriptors each providing information on one of the blocks. This list is supplied by the control software, which also loads a descriptor address register in the DMA controller with the address of the rst descriptor. The actual descriptor format is generally hardware dependent and often more complex than that shown in the diagram. For example, it may contain information on the addressing mode for the source and destination addresses. For each channel that involves peripheral-related DMA transfer, the controller normally has to be programmed with information on the burst size and port width of the peripheral. Burst size is the amount of data that
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PERIPHERAL SUPPORT
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has to be transferred to service each individual DMA request from the peripheral device. Many peripherals employ a FIFO buffer and for these, the burst size depends on the size of the FIFO and the threshold level within the FIFO that triggers a DMA request from it (for example, FIFO half-empty or quarter-empty). The port width speci es the granularity of data transfer (for example, byte, half-word and so on).
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13.1.2 DMA software framework
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Symbian OS provides kernel-side DMA services through its DMA framework. We leave the choice of whether or not a particular device will use DMA for data transfer to the developers who are creating a new phone platform and because of this, the consumers of this service are generally components such as physical device drivers (PDDs) and the platform-speci c layers of peripheral bus controllers (see Section 13.4). The framework itself is divided into a platform-independent layer (PIL) and a platform-speci c layer (PSL), with both parts being combined into the kernel-side DLL, DMA.DLL. As with most platform-speci c components, the PSL interfaces with the controller hardware via functions exported from the variant or ASSP DLL. Figure 13.2 shows this arrangement.
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Physical Device Driver (PDD) Physical Device Driver (PDD)
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Platform Independent Layer Platform Specific Layer
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DMA framework (DMA.DLL)
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ASSP / Variant
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kernel hardware DMA Controller
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DMA.DLL
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DMA support may be required by certain peripheral services that are started at system boot time. Because of this, the DMA framework is implemented as a kernel extension and is normally one of the rst of these to be initialized by the kernel. Drivers may request a transfer involving a memory buffer that is speci ed in terms of its linear address. As I explained in 7, Memory Models, this contiguous linear memory block is often made up of non-contiguous physical areas. Even when a transfer request involves a physically contiguous region of memory, the total length of the transfer may exceed the maximum transfer size that the underlying DMA controller supports. You don t need to worry about this though the Symbian OS DMA framework performs any fragmentation necessary, due either to requests exceeding the maximum transfer size, or to buffers not being physically contiguous. The framework speci es each fragment with a separate descriptor. If the controller doesn t support scatter-gather then each fragment has to be handled by the framework as a separate DMA transfer but the framework insulates the device driver from this detail, by only signaling completion back to the driver at the end of the entire transfer. Figure 13.3 shows a diagram of the classes that make up the DMA framework.
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TDmac 1 iController n TDmaChannel 1 iReqQ iChannel n DDmaRequest
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iFirstHdr
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SDmaDesHdr
iCurHdr
iNext SDmaDesHdr iLastHdr
TDmaSbChannel
TDmaDbChannel
TDmaSgChannel
SDmaDesHdr
PSL TPlatDmac 1 iChannels n TPlatSgChannel SPlatDmaDesc
Figure 13.3 Class diagram for the DMA framework
The framework includes the singleton TDmac, which is the abstraction of the DMA controller. This object manages all the DMA channels and de nes the main interface between the PIL and PSL. It is an abstract class with each phone platform deriving a controller object from it. This is shown as TPlatDmac on the diagram.