Linux host can't connect to Raspberry Pi Zero serial gadget console

Question

I followed this guide and enabled a serial console over USB. I can connect and log in from Windows but my Ubuntu laptop can't access the serial console.

The Raspberry shows up ad a tty device in /dev/ttyACM0 but when I connect to it I get nothing. dmesg shows a "failed to set dtr/rts" message every time I try to access the tty:

[ 3851.710852] usb 1-8: USB disconnect, device number 7
[ 3875.185832] usb 1-3: new high-speed USB device number 8 using xhci_hcd
[ 3880.921852] usb 1-3: device descriptor read/64, error -110
[ 3881.178907] usb 1-3: New USB device found, idVendor=0525, idProduct=a4a7
[ 3881.178913] usb 1-3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 3881.178917] usb 1-3: Product: Gadget Serial v2.4
[ 3881.178921] usb 1-3: Manufacturer: Linux 4.14.79+ with 20980000.usb
[ 3881.179990] cdc_acm 1-3:2.0: ttyACM0: USB ACM device
[ 3897.226350] cdc_acm 1-3:2.0: failed to set dtr/rts
[ 3902.346458] cdc_acm 1-3:2.0: failed to set dtr/rts
[ 3919.242818] cdc_acm 1-3:2.0: failed to set dtr/rts

The command udevadm info -a -n /dev/ttyACM0 gives this information:

  looking at device '/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3:2.0/tty/ttyACM0':
    KERNEL=="ttyACM0"
    SUBSYSTEM=="tty"
    DRIVER==""

  looking at parent device '/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3:2.0':
    KERNELS=="1-3:2.0"
    SUBSYSTEMS=="usb"
    DRIVERS=="cdc_acm"
    ATTRS{authorized}=="1"
    ATTRS{bAlternateSetting}==" 0"
    ATTRS{bInterfaceClass}=="02"
    ATTRS{bInterfaceNumber}=="00"
    ATTRS{bInterfaceProtocol}=="01"
    ATTRS{bInterfaceSubClass}=="02"
    ATTRS{bNumEndpoints}=="01"
    ATTRS{bmCapabilities}=="2"
    ATTRS{iad_bFirstInterface}=="00"
    ATTRS{iad_bFunctionClass}=="02"
    ATTRS{iad_bFunctionProtocol}=="01"
    ATTRS{iad_bFunctionSubClass}=="02"
    ATTRS{iad_bInterfaceCount}=="02"
    ATTRS{interface}=="CDC Abstract Control Model (ACM)"
    ATTRS{supports_autosuspend}=="1"

  looking at parent device '/devices/pci0000:00/0000:00:14.0/usb1/1-3':
    KERNELS=="1-3"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ATTRS{avoid_reset_quirk}=="0"
    ATTRS{bConfigurationValue}=="2"
    ATTRS{bDeviceClass}=="02"
    ATTRS{bDeviceProtocol}=="00"
    ATTRS{bDeviceSubClass}=="00"
    ATTRS{bMaxPacketSize0}=="64"
    ATTRS{bMaxPower}=="2mA"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bNumInterfaces}==" 2"
    ATTRS{bcdDevice}=="0414"
    ATTRS{bmAttributes}=="c0"
    ATTRS{busnum}=="1"
    ATTRS{configuration}=="CDC ACM config"
    ATTRS{devnum}=="8"
    ATTRS{devpath}=="3"
    ATTRS{idProduct}=="a4a7"
    ATTRS{idVendor}=="0525"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="Linux 4.14.79+ with 20980000.usb"
    ATTRS{maxchild}=="0"
    ATTRS{product}=="Gadget Serial v2.4"
    ATTRS{quirks}=="0x0"
    ATTRS{removable}=="removable"
    ATTRS{speed}=="480"
    ATTRS{urbnum}=="140"
    ATTRS{version}==" 2.00"

Any idea on how I can troubleshoot this further?

Answer 1

Had the same issue. It appeared that, in the context of Ubuntu, ModemManager was concurrently messing around with the device. Solved the issue by deactivating ModemManager (systemctl disable ModemManager.service).