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ezdma

Simple, zero-copy DMA to/from userspace.

Usage

  1. Specify which dmaengine-compatible DMA channels you'd like to create userspace-accessible device files for in your device tree:

    ezdma0 {
    compatible = "ezdma";
    dmas = <&loopback_dma 0 &loopback_dma 1>;
    dma-names = "loop_tx", "loop_rx";
    ezdma,dirs = <2 1>;     // direction of DMA channel: 
                            //   1 = RX (dev->cpu), 2 = TX (cpu->dev)
};

  2. After inserting the ezdma module, two devices, as named in your dma-names above, will become available:

        /dev/loop_tx
    /dev/loop_rx

  3. Sending data is as simple as:

        int tx_fd = open("/dev/loop_tx", O_WRONLY);
    int rx_fd = open("/dev/loop_rx", O_RDONLY);

    write(tx_fd, tx_buf, xfer_size);    // send a DMA transaction
    read (rx_fd, rx_buf, xfer_size);

See Documentation/devicetree/bindings/dma/ezdma.txt for additional example info.

Compiling

A Makefile for out-of-tree building is supplied. You just need to point it to the top-level directory of a kernel tree that you've already compiled.

make KERNELDIR=/path/to/your/kernel

Currently the Makefile assumes you want to cross-compile for ARM by default, but you're free to override the ARCH and/or CROSS_COMPILE variables on the command line or on in your environment. (I'd be interested to hear how it works on non-ARM platforms, as well!)

Other info

Tested with:

  • Xilinx AXI DMA on Zynq-7000 SoC.

Future enhancements:

  • Allow a forced transaction size to be specified in sysfs (such that DMA transfers are always performed in increments of the given size).
    • This would be useful when sending AXI stream packets of a particular size and would allow simple usages like cat packet_file > /dev/my_tx or cat /dev/my_rx > packet_file.
  • Add user-space scatter-gather readv()/writev() support.

In the future, I hope to refine and contribute ezdma to the mainline kernel.

Resources

  1. Linux Device Tree Background
  2. Xilinx AXI DMA Driver

License (GPL)

Copyright (C) 2015 Jeremy Trimble

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, see http://www.gnu.org/licenses/.