Hello all,
I've written a kernel module that allows data to be sent to or received from
any DMA device that supports the dmaengine API, using simple read()/write()
calls from userspace.
Motivation:
While doing some work for my master's thesis recently using the Xilinx Zynq
SoC, I needed a way to stream test data into and out of an FPGA soft-core I was
developing. I was using the Xilinx AXI DMA soft-core (and its
dmaengine-compatible driver that I believe is related to Kedareswara rao
Appana's patch submitted recently).
Although dmaengine provides a versatile in-kernel API, to my knowledge there's
no direct userspace interface for the case where one wants to simply
send/receive data through a slave dma channel -- writing a separate
userspace-facing module would be required. (Or some hack using /dev/kmem.)
Usage:
The ezdma ("easy DMA") module implements a read()/write() interface for
receiving/sending data through any slave DMA channel.
It is configured using the device tree:
1. Include a node like below into the 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 easy 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);
Currently, each read()/write() call causes a single transfer to occur in the
underlying slave DMA channel, but support could be added for readv()/writev()
as well.
Notes:
Although in my case I was using it with the AXI DMA to send and receive
AXI4-Stream packets, there is no dependency on any particular dmaengine driver
or hardware; it *should* work with any device that has a dmaengine driver.
I think it might be useful to others as an engineering/research tool.
I'm cleaning up the code and intend to submit it as a patch to this list in the
coming days. Though I've been writing kernel code for a few years now, this
will be my first patch going upstream. I appreciate any feedback.
Currently, I've posted my original code (before cleanup) here:
https://github.com/jeremytrimble/ezdma
Also, I'm open to changing the name if "ezdma" isn't seen as descriptive.
Cheers.
-Jeremy Trimble
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