What is the use of the DMA controller in a processor?

Question

DMA controllers are present on disks, networking devices. So they can transfer data to main memory directly. Then what is use of the dma controller inside processor chip ?Also i would like to know, if there are different buses (i2c, pci, spi) outside of processor chip and only one bus (AXI) inside processor. how does this work?(shouldn’t it result in some bottleneck)

Answer 1

The on-chip DMA can take the task of copying data from devices to memory and viceversa for simple devices that cannot implement a DMA of their own. I can think that such devices can be a mouse, a keyboard, a soundcard, a bluetooth device, etc. These devices have simple logic and their requests are multiplexed and sent to a single general purpose DMA on the chip.

Peripherals with high bandwidths like GPU cards, Network Adapters, Hard Disks implement their own DMA that communicates with the chip's bus in order to initiate uploads and downloads to the system's memory.

if there are different buses (i2c, pci, spi) outside of processor chip and only one bus (AXI) inside processor. how does this work?(shouldn’t it result in some bottleneck)

That's actually simple. The on-chip internal AXI bus is much faster - running at a much higher frequency (equal or in the same range to the CPU's frequency) (has a much higher bandwidth) than all the aggregated bandwidths of i2c+pci+spi. Of course multiple hardware elements compete on the AXI bus but usually you have priorities implemented and different optimization techniques.

Answer 2

From Wikipedia:

Direct memory access (DMA) is a feature of computerized systems that allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). [...] A DMA controller can generate memory addresses and initiate memory read or write cycles. It contains several processor registers that can be written and read by the CPU. These include a memory address register, a byte count register, and one or more control registers.