6.7. Adding a DMA Device

DMA devices are Tilelink widgets which act as masters. In other words, DMA devices can send their own read and write requests to the chip’s memory system.

For IO devices or accelerators (like a disk or network driver), instead of having the CPU poll data from the device, we may want to have the device write directly to the coherent memory system instead. For example, here is a device that writes zeros to the memory at a configured address.

class Top(implicit p: Parameters) extends System
  with testchipip.CanHaveTraceIO // Enables optionally adding trace IO
  with testchipip.CanHaveBackingScratchpad // Enables optionally adding a backing scratchpad
  with testchipip.CanHavePeripheryBlockDevice // Enables optionally adding the block device
  with testchipip.CanHavePeripherySerial // Enables optionally adding the TSI serial-adapter and port
  with sifive.blocks.devices.uart.HasPeripheryUART // Enables optionally adding the sifive UART
  with sifive.blocks.devices.gpio.HasPeripheryGPIO // Enables optionally adding the sifive GPIOs
  with icenet.CanHavePeripheryIceNIC // Enables optionally adding the IceNIC for FireSim
  with chipyard.example.CanHavePeripheryInitZero // Enables optionally adding the initzero example widget
  with chipyard.example.CanHavePeripheryGCD // Enables optionally adding the GCD example widget
{
  override lazy val module = new TopModule(this)
}

class TopModule[+L <: Top](l: L) extends SystemModule(l)
  with testchipip.CanHaveTraceIOModuleImp
  with testchipip.CanHavePeripheryBlockDeviceModuleImp
  with testchipip.CanHavePeripherySerialModuleImp
  with sifive.blocks.devices.uart.HasPeripheryUARTModuleImp
  with sifive.blocks.devices.gpio.HasPeripheryGPIOModuleImp
  with icenet.CanHavePeripheryIceNICModuleImp
  with chipyard.example.CanHavePeripheryGCDModuleImp
  with freechips.rocketchip.util.DontTouch

We use TLHelper.makeClientNode to create a TileLink client node for us. We then connect the client node to the memory system through the front bus (fbus). For more info on creating TileLink client nodes, take a look at Client Node.

Once we’ve created our top-level module including the DMA widget, we can create a configuration for it as we did before.