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How do you put a processor's USB peripheral into test mode on an embedded Linux system?

I have tried writing the setup sequence defined in the reference manual to registers directly using devmem. Although this appears to be accepted, there are no test packets emitted from USB so I'm wondering if the kernel is interfering with USB during this process.

Is there a better way to do this in Linux?

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    Since every processor is different, we're going to need to know exactly which one you are using. We'll also need to see an minimal reproducible example and a description of its expected behavior compared to its actual behavior. Commented May 21, 2024 at 18:27
  • Qhat exactly do you mean under "processor's USB preipheral"? What do you mean under "test mode"? Commented May 21, 2024 at 19:35
  • "so I'm wondering if the kernel is interfering with USB during this process" - If you happen to be referring to a USB controller (integrated with a processor in an SoC) that has a kernel driver interacting with it, then yes, you have created a resource conflict. Commented May 21, 2024 at 23:12
  • "Is there a better way to do this in Linux?" - Create an ioctl in the kernel driver to suspend normal USB operations, and switch to a test mode? Or use U-Boot to perform such tests. Commented May 21, 2024 at 23:16
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