Beaglebone PRU DMA support

This project aims to provide support for bidirectional DMA transfers between PRU and Cortex-A8 on AM335x SoC. It is developed on BeagleBone Black.

I am developing this project for GSoC 2017 for beagleboard.org.

Task description

Goal: Create a sample program to demonstrate using EDMA on the PRU to transfer data to and from the main (DDR) memory with a Linux host. Most existing code utilizes (wastes) the 2nd PRU on the PRUSS for data xfer. Using DMA can allow the PRU to be used for other purposes.

Software requirements

This application was developed and tested using following SW revisions:

• BBB:
  • Beagleboard.org Debian, version 8.8
  • linux: 4.9.30-ti-r38
• Host:
  • toolchain: arm-linux-gnueabihf gcc linaro 6.3.1-2017.05
  • dtc: 1.4.4
  • PRU CGT: 2.1.5
  • PRU Software Support Package: v5.1.0

To work properly, PRU remoteproc must be enabled in device tree.

Implementation overview

Implementation of the project is split in two main parts, kernel module and PRU firmware. PRU firmware and kernel module communicate via interrupts.

Following scenario is implemented:

• During loading procedure of PRU firmware, driver performs buffer allocation (using kzalloc call) to obtain contiguous buffer in memory based on requirements specified in PRU DMA entry in resource table,
• Buffer is then mapped using dma_map_single call to ensure the address is in DMA able region and caching is turned off for the buffer,
• Resource table is modified to include address of the mapped buffer,
• After boot, PRU configures EDMA based on parameters passed in resource table (buffer address and size, EDMA channel, PaRAM slot)
• PRU schedules the transfer by writing to EDMA registers,
• PRU polls periodically for the transfer completion event,
• Once the transfer is complete, interrupt is optionally sent to the driver (configurable via option in resource table entry)
• In interrupt routine driver signals finished transfer using Linux completion mechanism

Project structure

• firmware - code for PRU software. Receives transfer parameters via resource table, configures EDMA for transfer and INTC to trigger event on EDMA interrupt

• kernel-patch - kernel-side implementation of the project, contains patches to ti-linux-kernel-dev repository (based on 4.9.36-ti-r46 release)

• pru-swpkg-patch - contains patch that needs to be applied to pru software support package (based on v5.1.0 release)

• dts - basic device tree overlay needed to load the module. More concrete dt overlays exist for each example

• examples - every subdirectory is an example usage of pru-dma, demonstrating practical application, consisting of PRU firmware, kernel module and dt overlay

• Documentation - contains detailed description of the project, build instructions and description of API for PRU firmware and kernel modules

• wip - (work in progress) isolated examples created to test specific features (e.g. linux DMA API, PRU-ARM communication), not an essential part of the project.

Demonstration video