Xilinx AXI DMA Driver and Library (Quick Start Guide)

Overview

A zero-copy, high-bandwidth Linux driver and userspace interface library for Xilinx's AXI DMA and VDMA IP blocks. The purpose of this software stack is to allow userspace Linux applications to interact with hardware on the FPGA fabric. The driver and userspace library act as a generic layer between the procesor and FPGA, and abstracts away the details of setting up DMA transactions. The pupose of AXI DMA and VDMA IP blocks is to serve as bridges for communication between the processing system and the FPGA, through one of the DMA ports on the Zynq processing system.

The driver enables userspace application to allocate zero-copy, physically contiguous DMA buffers for transfers, allowing for high bandwidth communication between the FPGA and ARM core. The driver exposes its functionality via a character device, which the library interacts with.

This driver supports 4.x version Xilinx kernels. It has been tested with the mainline Xilinx kernel, and the Analog Devices' kernel on the Zedboard. The driver should work with any 4.x kernel and any board that uses a Zynq-7000 series processing system.

Features

1. Zero-copy transmit (processor to FPGA), receive (FPGA to processor), and two-way combined DMA transfers.
2. Support for transfers with Xilinx's AXI DMA and AXI VDMA (video DMA) IP blocks.
3. Allocation of DMA buffers that are contiguous in physical memory, allowing for high-bandwidth DMA transfers, through the kernel's contiguous memory allocator (CMA).
4. Allocation of memory that is coherent between the FPGA and processor, by disabling caching for those pages in the DMA buffer.
5. Synchronous and asynchronous modes for transfers.
6. Registration of callback functions that are called when an asynchronous transfer completes.
7. Delivery of a POSIX real-time signal upon completion of an asynchronous transfer.
8. Support for DMA buffer sharing, or external DMA buffers. Currently the driver can only import a DMA buffer from another driver. This is useful, for example, when transfers need to be done with a frame buffer allocated by a DRM driver.

Setting Up the Driver

Linux Kernel

The driver depends on the contiguous memory allocator (CMA), Xilinx's DMA and VDMA driver, and DMA buffer sharing. These must be enabled in the kernel the driver is complied against. These should be enabled by default in any Xilinx Linux kenrel fork. To be sure, make sure to double check the kernel configuration by running make menuconfig or by opening the .config file at the top level of your kernel source tree. The following options should be enabled:

CONFIG_CMA=y
CONFIG_DMA_CMA=y
CONFIG_XILINX_DMAENGINES=y
CONFIG_XILINX_AXIDMA=y
CONFIG_XILINX_AXIVDMA=y
CONFIG_DMA_SHARED_BUFFER=y

Device Tree

The driver requires a node in the device tree. This node describes the DMA channels that the driver has exclusive access. It is also used to probe the driver, so the driver is activited only when this node is present. The node has the following properties:

• compatible - This must be the string "xlnx,axidma-chrdev". This is used to match the driver with the device tree node.
• dmas - A list of phandles (references to other device tree nodes) of Xilinx AXI DMA or VDMA device tree nodes, followed by either 0 or 1. This refers to the child node inside of the Xilinx AXI DMA/VDMA device tree node, 0 of course being the first child node.
• dma-names - A list of names for the DMA channels. The names can be completely arbitrary, but they must be unique. This is required by the DMA interface function dma_request_slave_channel(), but is otherwise unused by the driver. In the future, the driver will use the names in printed messages.

For the Xilinx AXI DMA/VDMA device tree nodes, the only requirement is that the device-id property is unique, but they can be completely arbitrary. This is how the channels are referred to in both the driver and from userspace. For more information on creating AXI DMA/VDMA device tree nodes, consult the kernel documentation for them.

Here is a simple example of the device tree nodes for a system with a single AXI DMA IP, with both a transmit and receive channel. Note you will need to adjust this for your kernel tree and setup:

axidma_chrdev: axidma_chrdev@0 {
    compatible = "xlnx,axidma-chrdev";
    dmas = <&axi_dma_0 0 &axi_dma_0 1>;
    dma-names = "tx_channel", "rx_channel";
};

axi_dma_0: axidma0@40400000 {
    #dma-cells = <1>;
    compatible = "xlnx,axi-dma", "xlnx,axi-dma-6.03.a", "xlnx,axi-dma-1.00.a";
    reg = <0x40400000 0x10000>;
    clocks = <&clkc 15>, <&clkc 15>, <&clkc 15>, <&clkc 15>;
    clock-names = "s_axi_lite_aclk", "m_axi_sg_aclk", "m_axi_mm2s_aclk", "m_axi_s2mm_aclk";    
    xlnx,include-sg;    
    xlnx,addrwidth = <32>;

    dma-mm2s-channel@40400000 {
        compatible = "xlnx,axi-dma-mm2s-channel";
        dma-channels = <1>;
        xlnx,datawidth = <64>;
        xlnx,device-id = <0>;
        interrupt-parent = <&intc>;
        interrupts = <0 29 4>;
    };
    
    dma-s2mm-channel@40400000 {
        compatible = "xlnx,axi-dma-s2mm-channel";
        dma-channels = <1>;
        xlnx,datawidth = <64>;
        xlnx,device-id = <1>;
        interrupt-parent = <&intc>;
        interrupts = <0 30 4>;
    };
};

Kernel Command Line

The contiguous memory allocator works by reserving a pool of memory for contiguous memory allocations that it uses when requested. By default this size is too small for typical uses with this driver. The size can be changed by appending cma=<size>M to the kernel command line arguments. This sets the pool's size to size MBs.

The kernel command line can be updated by changing the device tree or from the U-Boot console. For example, to set the CMA's pool size to 25 MB from the U-Boot console:

setenv bootargs "${bootargs} cma=25M"

NOTE: In the future, specifying the CMA region size will be moved into the device tree, so this will not be necessary.

Compilation

Makefile Variables

The Makefile supports both native and cross compilation, and is repsonsible for compiling the driver and a few example programs. It has the following variables:

• CROSS_COMPILE - The prefix for the compiler to use for cross-compilation.
• ARCH - The architecture being compiled for. Required for compiling the driver when cross-compiling.
• KBUILD_DIR - The path to the kernel source tree to compile the driver against. The kernel must already be built. Required for compiling the driver.
• OUTPUT_DIR - The path to the output directory to place the generated files in. Defaults to outputs in the top-level directory.
• XILINX_DMA_INCLUDE_PATH_FIXUP - This specifies to fixup the issue with the location of the xilinx_dma.h header file in the kernel being compiled against. Specify this if you see an include error when compiling the driver.

For a complete list of targets, and a more complete description of the options, run:

make help

The Makefile also supports specifying these variables in a configuration file. The repository distirbutes a template for this file at config_template.mk. To use this file, copy it to config.mk, and uncomment and fill in the varaibles as needed.

Compiling the Driver

The Makefile supports out-of-tree compilation for the driver. To compile the driver, you must specify the path to a kernel source tree that is already built. Also, you can specify any cross-compilation options as necessary. For example, to compile the driver for ARM:

make CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm KBUILD_DIR=</path/to/kernel/tree> driver

This will generate a kernel object for the driver at outputs/axidma.ko.

Compiling the Examples

The driver and library come with several example programs that show how to use the API. There's a program that benchmarks a two-way transfer, one that transmits a file over a channel, and one that displays an image (assuming the proper hardware is there). Consult the command line help for each program on usage. To cross-compile the examples for ARM:

make CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm examples

This will generate executables for the examples under outputs.

Compiling and Using the Library

The userspace library is compiled the typical shared object file. To compile the library for ARM:

make CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm library

This will generate a shared object file for the AXI DMA library at outputs/libaxidma.so.

To use the library with your application, you need to compile your program against the library, by specifying the flags -I </path/to/repo>/include -L </path/to/repo>/outputs/ -l axidma when compiling. Then, so the executable can find the library at runtime, you need to copy the library file to the board's filesytem. You can either copy to one of the default system library directories, such as /usr/lib/, or you can add the outputs directory to the LD_LIBRARY_PATH environment variable. See how the examples programs are compiled in their Makefile.

Generating the Library Documentation

The userspace library has Doxygen documentation for its interface. To generate and view this documentation, run:

make library_docs

This will generate both HTML and Latex documentation for the AXI DMA library under docs. This will also open up the HTML documentation in Firefox.

Running an Application

Before you run an application that uses the AXI DMA software stack, you must make sure that the driver has been inserted into the kernel. This can be accomplished with:

insmod axidma.ko

This should create a character device for the driver at /dev/axidma. To verify that the insertion was successful, run dmesg to view the kernel log messages. You should see a message like the following:

axidma: axidma_dma.c: 672: DMA: Found 1 transmit channels and 1 receive channels.
axidma: axidma_dma.c: 674: VDMA: Found 0 transmit channels and 0 receive channels.

Naturally, the numbers will vary based on your specific configuration.

Debugging Issues with the Software Stack

The driver prints out a detailed message every time that it encounters an error to the kernel log message buffer. If the library says that an error occured, run dmesg to see the kernel log. The driver will print out a detailed message, along with the file, function, and line number that the error occured on.

There is an issue with the location of the Xilinx DMA header file, xilinx_dma.h in the Xilinx kernels. In between the 3.x and 4.x version, the location of this file changed, but certain 4.x kernels still use the old location from the 3.x kernel. If you see an error message like the following when compiling the driver, then specify the XILINX_DMA_INCLUDE_PATH_FIXUP Makefile variable when compiling:

In file included from /home/bmperez/projects/xilinx_axidma/driver/axidma_dma.c:24:0:
/home/bmperez/projects/xilinx_axidma/driver/version_portability.h:25:68: fatal error: linux/dma/xilinx_dma.h: No such file or directory
 #include <linux/dma/xilinx_dma.h>   // Xilinx DMA config structures
                                                                    ^
compilation terminated.

Using the Driver with a PetaLinux Kernel

For how to add the driver to a PetaLinux project and build it against a PetaLinux kernel, see issue #24.

Limitations/To-Do's

1. There is currently no explicit support for concurrency, so only one thread should access the driver at a time.
2. The character device is opened in exclusive mode, so only one process can access the driver at a time.
3. The driver cannot export DMA buffers for sharing, it only supports importing at the moment.
4. There is no support for multi-channel mode.

Additional Information

For more detailed information, including how to boot Linux on the Zedboard, and how to run a simple DMA loopback example to test the software stack's functionality, see the wiki.

References

Xilinx's AXI VDMA and DMA Driver - Source Code

Xilinx's AXI DMA Test Driver (Shows How to Write a Driver to Use Xilinx's AXI DMA Driver) - Source Code

Documentation on Xilinx AXI DMA Device Tree Node Bindings - Documentation

Documentation for DMA Device Tree Bindings - Documentation

Documentation for CMA (reserved memory regions) Device Tree Bindings - Documentation

Documentation on DMA Buffer Sharing - Documentation

Documentation for Xilinx's AXI DMA IP Core (Vivado 2015.2, Version 7.1) - Documentation

Documentation for Xilinx's AXI VDMA IP Core (Vivado 2015.2, Version 6.2) - Documentation

License (MIT)

The MIT License (MIT)

Copyright (c) 2015-2016 Brandon Perez bmperez@andrew.cmu.edu
Copyright (c) 2015-2016 Jared Choi

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.