Zynq DMA Linux Driver

This Linux driver has been developed to run on the Xilinx Zynq FPGA. It is a wrapper driver used to talk to the low level Xilinx driver (xilinx_axidma.c) that interfaces to a Xilinx DMA Engine implemented in the PL section of the Zynq FPGA. Userspace applications uses this wrapper driver to configure and control the DMA operations.

Compile

Kernel modules need to be built against the version of the kernel it will be inserted in. It is recommended to uses the Linux kernel maintained by Xilinx.

git clone https://github.com/Xilinx/linux-xlnx.git

It has been tested to work with the linux-xlnx master-next merge tag 'v3.15' (commit 40dde7e248951426abcba254e7e070f209005afb).

The driver module can be compiling outside of the Linux kernel source tree. A variable 'KDIR' in the Makefile is used to point to the kernel source directory. The default value has it pointing to the default Linux install location for kernel sources. However, if cross compiling or if the sources are in a non-default location the value can be overridden using an exported environmental variable or as an argument passes into the make command.

cd zynq-xdma/dev/
export KDIR=../../linux-xlnx
make

or

cd zynq-xdma/dev/
make KDIR=../../linux-xlnx

Cross Compiling

A cross compile tool chain is installed onto your system with the Xilinx SDK. Once Xilinx is installed, export the following environmental variables and afterwards invoking "make" will cross compile the library's and applications in this repo.

export PATH=/opt/Xilinx/SDK/2014.4/gnu/arm/lin/bin:$PATH
export CROSS_COMPILE=arm-xilinx-linux-gnueabi-
export ARCH=arm

Compile Order

Compile directories in that order First : /dev Second : /lib Third : /demo

Inserting Module

Use of the driver module requires it to be inserted into the running Linux kernel. Once inserted it will automatically create a character device file in '/dev' called '/dev/xdma'. However, the default permissions will not allow non-root users to read/write to the file. These permissions can be overridden by installing the udev rule file found in this projects 'util' directory into the systems '/etc/udev/rules.d/' directory. Alternatively, once the modules inserted the permissions can be changed manually using 'chmod'.

sudo cp util/80-xdma.rules /etc/udev/rules.d/
sudo insmod dev/xdma.ko

or

sudo insmod xdma.ko && sudo chmod 666 /dev/xdma

To remove the module.

sudo rmmod xdma

To install the module and have it loaded at boot, first install the udev rule as shown above and then follow the below instructions.

sudo mkdir -p /lib/modules/$(uname -r)/extra/
sudo cp xdma.ko /lib/modules/$(uname -r)/extra/
sudo depmod -a
sudo modprobe xdma
sudo sh -c 'echo "xdma" >> /etc/modules'

Compiling and Running Demo

The demo application assumes that you have the Zynq PL configured as a DMA loopback device and that it is being compiled on the Zedboard.

cd demo
make

Tips for getting working hardware

When defining the DMA engine for the hardware, set the width of the buffer length register to 23 bits. (Double click on the DMA core in Vivado IP integrator).

A simple PlanAhead project for a Zedboard hardware loopback system that can be used with the driver can be found: https://github.com/bmartini/zedboard-simple-loopback It includes a working devicetree source file.

If the error " Error: allocating dma memory failed" is generated, check when compiling the Linux kernel that Contiguous Memory Allocator (CMA) is built in. In 'make menuconfig' you can find it in 'Device Drivers -> Generic Driver Options' under 'Contiguous Memory Allocator'. Or in ".config" CONFIG_DMA_CMA=y.