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I'm trying to disable USB3 entirely on my raspberry pi 4 running Ubuntu server but I can't find a way to do it. Some things that I've tried are:

  • Blacklisting xhci - doesn't work because it's a builtin module.
  • Using setpci as documented here to disable it - this command doesn't work because -H1 isn't an argument in the version of setpci that is available. Without -H1 it doesn't seem to do anything.
  • Disabling USB 3 at some lower level - normally you can do this in the bios but I think the pi doesn't have a bios, I couldn't find any other way to do this.

Is there anything else I can try?

More context: I'm hoping that this will allow me to use more than 32 USB devices as explained here.

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Try adding

initcall_blacklist=xhci_hcd_init

to your kernel parameters (/boot/firmware/cmdline.txt). Or maybe xhci_init_driver. You'll have to experiment here.

Also take a look at the "quirks" available in drivers/usb/host/xhci.h. Specifically, if this is an Intel-only limitation, you could try XHCI_NEC_HOST. There's also XHCI_EP_LIMIT_QUIRK, though I honestly have no idea what it does.

If the experiments are fruitless, you'll have to build a custom kernel.

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  • Oh, I actually tried this too but Ubuntu on the raspberry pi doesn't seem to have a /boot/cmdline.txt file. Do you know what the equivalent place is? – dshepherd Mar 10 at 15:12
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    @dshepherd Not really. Find out which bootloader you are using (grub?) and how to provide it with custom kernel parameters. – Dmitry Grigoryev Mar 10 at 15:14
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    Turns out that it uses a modified version of U-boot which provides a /boot/firmware/cmdline.txt which works like the normal raspberry pi /boot/cmdline.txt. – dshepherd Mar 10 at 15:56
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    That particular blacklist didn't do anything but I searched the kernel source code for __init and xhci and found xhci_pci_init. Blacklisting that definitely stopped it from using xhci for the USB ports but unfortunately it doesn't use ehci instead, so I guess this is a dead end. Thanks anyway! – dshepherd Mar 10 at 16:43
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    I also had a look at the quirks config that you mentioned but from the comments it sounds like that would give me a completely broken USB system rather than a working USB system with a larger number of devices. – dshepherd Mar 10 at 16:44

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