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I am getting an "Access Denied" on a USB port '/dev/bus/usb/001/004' on a Raspberry Pi 3B/4/4B that is running Raspbian Lite.

  • When I issue the command:

    ls -l /dev/bus/001/004

    I see the following:

    crw-rw-r-- 1 root root 189, 3 Jan 6 13:45 /dev/bus/usb/001/004

  • The current user is a member of the following groups:

    pi adm dialout cdrom sudo audio video plugdev games users input netdev spi i2c gpio

  • While the current user is a member of the dialout group that is often cited as a requirement to access USB ports, the USB port itself is only a member of the root group.

  • When I look at the UDEV rules in /lib/udev/rules.d, there is a rules file named 40-usb_modeswitch.rules. This file, however, is empty.

My questions are:

  1. Am I on the right track in thinking that I need to enable the USB ports by adding entries to the 40-usb_modeswitch.rules?
  2. Do I need to add rules to the 60-serial.rules file?
  3. Will this add the USB ports to the dialin group?
  4. Are there references on how to enable USB ports on a Raspberry Pi 3B/4/4B?
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  • "how to enable USB ports" Enable in what sense? There's nothing here that implies the USB ports are disabled or need enabling. All you are talking about is permissions on a /dev node, and one that is pretty abstract in practice: When you plug in a USB device such as a keyboard or hard drive, this usually creates a new dev node with separate permissions and ownership -- but none of this stuff requires a user have permission on, eg., /dev/input/mouse0 in order to make normal use of the mouse. Make sure you don't have an XY problem.
    – goldilocks ♦
    Commented Jan 6, 2022 at 16:00
  • Eg., If this is about accessing a USB UART serial adapter, you are going about it the wrong way.
    – goldilocks ♦
    Commented Jan 6, 2022 at 16:05
  • I am not sure if this is USB UART. We were seeing a "Access denied" on a port whenever we tried to programmatically open a USB port on a Pi 4 running Raspbian Lite. It looks like there are no rules in the out-of-the-box Raspbian Lite related to USB ports. I am not a Linux expert, but from on-line documentation, (as far as I could tell), the absence of rules that place the USB device in a group other than root and the absence of a permanent name are problems that prevent USB ports from being opened programmatically. The open function will not accept "/dev/bus/001/004" as a port name.
    – Doug Kimzey
    Commented Jan 7, 2022 at 11:02
  • A USB UART serial adapter is something you plug into a USB port and the other end is 4+ wires for attaching to UART pins. Saying briefly that the context is to access the port in code clarifies that it isn't an XY problem; I was just concerned that someone was off on a wild goose chase trying to make a serial adapter (or some other USB device) work.
    – goldilocks ♦
    Commented Jan 7, 2022 at 15:53
  • Thanks very much for explaining the USB UART. XY problems are certainly expensive and something to avoid when possible. Any comments that keep our project on track are most welcome.
    – Doug Kimzey
    Commented Jan 7, 2022 at 17:59

1 Answer 1

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1&2 :In general, it is better to keep your own rules separate from the system rules. I would create a file /etc/udev/rules.d/50-localusb.rules with the content:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", GROUP="dialin", MODE="0666"

You may get idVendor and idProduct from an lsusb.

3: Depends on the rule that you place there, ofcourse.

4: There are many Linux references; you should be able to find them with google udev usb devices permissions or something like that.

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  • I have not yet been able to get this to work. It appears that the new rules are not being loaded at all. I have tried placing this in /etc/udev/rules and /usr/lib/udev/rules.d. I am working through documentation to see what needs to be done to have rules take effect.
    – Doug Kimzey
    Commented Jan 7, 2022 at 19:05
  • Changed "dialin" to "dialout". This worked.
    – Doug Kimzey
    Commented Jan 7, 2022 at 20:12

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