"To understand how the DMA works, you really need to carefully study the reference manual about the USB hardware, particularly the device-mode QH and qTD structures. Reading the USB 2.0 spec and EHCI spec also helps, but to be realistic, those are very long and EHCI is quite complicated. It's the sort of learning curve you could expect to measure in years, not days or weeks."
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https://forum.pjrc.com/index.php?threads/teensy-4-1-usb-serial-with-dma.66740/#post-274794