In cases where the DMA is of bulk uninterpreted or standard data, a driver wouldn't be necessary in userspace. It should work for userspace to ask the kernel a DMA handle that an MPEG-2 stream can be sent to to have the video card decode it with the hardware decoder, and then userspace plays video by passing the video data unmodified to the handle. Obviously, there's more going on, but the kernelspace driver would take care of that.
In the case of networking, the data is going to be the application's data, to be sent over the network, and it wouldn't matter what the device is to the userspace side.
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