DMA Technology Guide

Purpose

The DMA framework provides a generic simple interface for the +clients to access DMA resources in a device. The primary clients are +device drivers that will use the DMA framework to set up DMA transfers. +A device can have more than one DMA controller (DMAC).

Architectural +concepts

The key concepts related to DMA framework are:

+DMA Framework + +

DMA Client: A device driver or a kernel object that needs to use the resources +of the DMA framework. On the Symbian platform, the Physical Device +Drivers are the primary clients of the DMA framework.
DMA Platform Service API: The generic interface to provide DMA services to the clients.
DMA Import Library: The DMA clients link against the dma.lib file.
DMA Platform-Independent Layer (PIL): The platform-independent layer contains the generic framework +independent from the hardware.
DMA Platform-Specific Layer (PSL): The platform-specific layer is specific to the baseport and +the hardware used.
DMA Controller (DMAC): The DMAC is the DMA hardware implementation. The number of +DMAC in the implementation of the DMA Framework depends on the hardware. +The DMAC is a peripheral to the CPU and is programmed to control data +transfers without using the CPU.

DMA +functionality

The key concepts related to functionality +provided by the DMA framework are:

Scatter/Gather Mode: Some DMA controllers transfer data using scatter/gather mode. +In this mode, the operating system creates a DMA descriptor with source +and destination memory addresses and the configuration. The source +and destination data can be a list of scattered memory blocks. The +DMA controller uses this configuration to perform the data transfer.
DMA Descriptors: The data structure used by the DMA framework to store the configuration +of the data transfer. These will store details such as source address, +destination address, number of bytes and pointer to the next descriptor, +when the DMA is used to transfer disjoint blocks of data. This structure +is defined in the platform specific layer. The descriptors are always +used to store the configuration even if the DMA controller does not +support scatter/gather mode. When the scatter/gather mode is not supported +by the DMAC, the descriptors used to store the configuration are called +as pseudo-descriptors.
Descriptor Headers: The descriptor headers are used to store additional information +about the descriptors. The descriptors are accessed using the descriptor +headers which contain the pointer to the descriptor.
Transfer Request: The device drivers configure and initialize a data transfer +using the class DDmaRequest. The data can be transferred +between:
+
memory to memory
+
memory to peripheral
+
peripheral to memory
+
The transfer request stores the callback function of the clients. +The callback function is used to notify the success or failure of +a data transfer.
DMA Channel: The object which represents a single hardware, logical or a +virtual channel. In the DMA platform service API, each DMA channel +is referred to by a 32-bit value called a cookie. Each DMA transfer +is in the form of a description header and its associated DMA descriptor. +The queue of transfer requests (both being transferred and pending) +consists of a linked list of the DMA headers.
DMA Channel Allocator: The channel manager controls the opening and closing of DMA +channels. The channel manager functions are defined in the platform +independent layer and implemented in the platform specific layer.

Typical +Uses

The typical use cases of a DMA framework are:

programmable data transfer between hardware peripherals and +memory buffers
programmable data transfer of block of data between different +regions of memory