Bad serial connection on USB OTG Serial on pi Zero

Question

I am running Ubuntu 18.04 with Jakeday's surface linux kernel on a Surface Book 2.

I had a serial connection working yesterday. I had this problem on another pi zero. I researched the "failed to set dtr/rts" issue and it seems that I need to specify device platform settings or something like this. I think the pi is not reporting itself correctly to my laptop. I had used a different pi with this recently and it was having this problem. Then I switched the SD card into this pi and it was fine. Now this pi is acting up.

Also the pi behaves just great on Windows and I can use putty to get in.

Here is the output of dmesg:

[142367.770182] usb 1-1.1.3: USB disconnect, device number 11
[142368.089461] usb 1-1.1.3: new high-speed USB device number 12 using xhci_hcd
[142381.665842] usb 1-1.1.3: new high-speed USB device number 13 using xhci_hcd
[142381.790983] usb 1-1.1.3: New USB device found, idVendor=0525, idProduct=a4a7, bcdDevice= 4.19
[142381.790988] usb 1-1.1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[142381.790991] usb 1-1.1.3: Product: Gadget Serial v2.4
[142381.790994] usb 1-1.1.3: Manufacturer: Linux 4.19.66+ with 20980000.usb
[142381.796034] cdc_acm 1-1.1.3:2.0: ttyACM0: USB ACM device
[142397.562400] cdc_acm 1-1.1.3:2.0: failed to set dtr/rts
[142402.682581] cdc_acm 1-1.1.3:2.0: failed to set dtr/rts

Answer 1

I had exactly the same issue on my pi zero W, with exactly this config on it, using Ubuntu 18.04 as host. I found the answer here: https://raspberrypi.stackexchange.com/a/109482/125981

Turns out it was ModemManager poking at the tty at the same time. Solution as described in the linked answer is to stop and disable ModemManager (or only stop if you need it on your machine for other stuff):

$ sudo systemctl stop ModemManager.service

and

$ sudo systemctl disable ModemManager.service

Answer 2

On my RPi zero I've seen the same behaviour and also the dmesg output looked the same:

failed to set dtr/rts

Being desperate for a long time today I tried again step by step the list in following answer: https://raspberrypi.stackexchange.com/a/75551 and suddenly it worked. Now I'd underline number 2. and 4. from that list.

Apparently it seems that using the only the USB data cable to power and communicate with RPi causes the flow control failure. When I use USB power cord for powering the RPi and later on the data cable is attached, the driver in Ubuntu/Mint is not complaining any more and the serial communication works like a charm.