Integrating AXI DMA in Embedded Linux: A Comprehensive Guide

Introduction to AXI DMA in Embedded Linux
Direct Memory Access (DMA) plays a crucial role in embedded development as it enables the system to access the main memory, typically DDR, without occupying the CPU. This leaves the CPU free to perform other tasks during memory read/write operations, enhancing the overall performance and efficiency of the system. DMA operates by initiating a transfer, followed by an interrupt that signals the completion of the transfer. Therefore, the CPU can focus on other tasks until the task is completed.

Challenges in Implementing DMA

Though the concept of DMA is simple, implementing it on an FPGA with an embedded processor, such as the ARM-core processor in the Xilinx Zynq System-on-a-Chip (SoC), presents a significant challenge. Adding to this complexity is the process of accessing the DMA from the userspace of an operating system like Linux. Achieving this requires a deep understanding of both hardware and software aspects.

In this blog, we will guide you through the setup and implementation of AXI DMA in an embedded Linux environment, focusing on:

Setting up the hardware design in Vivado Configuring the PetaLinux project settings for kernel drivers and Linux applications Understanding and using the AXI DMA functionality in the programmable logic of the FPGA

Add DMA to the Hardware Design in Vivado

The Zynq processing system comes with native DMA channels tailored for memory-to-memory transfers. However, for peripherals that require stream-to-memory or memory-to-stream transfers, additional DMA-capable interfaces are necessary. Memory-to-memory transfers involve reading or writing data between different memory locations, whereas stream-to-memory or memory-to-stream transfers involve converting serialized data to a memory format or extracting data from memory.

In Vivado, you can add DMA functionality to your hardware design by following these steps:

Create a new project: Start by creating a new Vivado project and selecting the appropriate hardware design. Configure IP Integrator: Use the IP Integrator to add DMA cores to your design. Select the AXI DMA core, which includes the necessary registers and logic to manage data transfers. Design Constraints: Define the constraints for your design, such as frequency, clock rates, and signal timings. This is crucial for optimizing the performance of the DMA transfer. Generate Bitstream: Compile and synthesize the design to generate the bitstream. This step ensures that the design is correctly mapped to the FPGA.

Configuring PetaLinux for DMA Use

After setting up the hardware design, you need to configure the PetaLinux project for kernel drivers and Linux applications that will interact with the DMA.

Create PetaLinux Project: Create a new PetaLinux project based on the Zynq image. Configure Kernel Drivers: Add the necessary kernel modules to support AXI DMA. This includes configuring DMA controllers, interrupt handling, and memory management drivers. Develop Linux Applications: Write userspace applications that interface with the DMA. These applications should utilize standard Linux APIs to request and manage DMA transfers.

Controlling AXI DMA in Programmable Logic

The programmable logic in the FPGA handles the DMA transfers according to the configuration set in the hardware design. Key functions of AXI DMA in the programmable logic include:

Data Transfer Initiators: These handle the DMA request, initiate the transfer, and manage the data flow. Interconnects: These routes data between the initiator and the target memory locations. Bus Controllers: These manage the communication and synchronization between the AXI interface and the rest of the FPGA fabric.

Conclusion
Integrating AXI DMA into an embedded Linux environment can significantly enhance the performance and efficiency of your system. By following the steps outlined in this guide, you can successfully set up and use DMA to perform various data transfers in your FPGA-based system. Ensure that you have a solid understanding of both hardware and software aspects to achieve optimal results.