Enable kernel options CONFIG_EARLY_PRINTK_USB_XDBC and USB_XHCI_DBGCAP for serial Superspeed(+) connections with kernels >= 4.16

[Renne]

Since kernel 4.13 Linux supports an USB debug console with passive USB 3.x host-to-host cables aka "USB 3.0 super-speed A-to-A debugging cable".
Beginning with kernel 4.16 a serial port (/dev/ttyDbC) has been added (https://www.kernel.org/doc/html/v4.16/driver-api/usb/usb3-debug-port.html#serial-tty).

I suggest to enable USB DbC by enabling the kernel options "CONFIG_EARLY_PRINTK_USB_XDBC=Y" and "USB_XHCI_DBGCAP=Y" in Ubuntu kernels >= 4.16 for Bionic HWE and newer ubuntu versions.

It would also be great to get feedback about line speed from others as USB_XHCI_DBGCAP serial ports could be used for network connections via tun/tap devices or PPP/SLIP.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1821537

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Renne (renne)]

@ubuntu-kernel-bot: Missing things can't be logged. ;-)

@all: Don't use simple A-to-A USB cables for USB 3.x slave devices as they can fry your hardware. See cable specs in section 5.5.2 of the USB 3.0 specification (https://i.stack.imgur.com/YwsX3.png).

[Po-Hsu Lin (cypressyew)]

Series tracked for Cosmic, as per Seth's comment:
USB_XHCI_DBGCAP has been enabled since cosmic. I've now enabled
CONFIG_EARLY_PRINTK_USB_XDBC as well in disco.

[Renne (renne)]

I suggest to enable CONFIG_EARLY_PRINTK_USB_XDBC and USB_XHCI_DBGCAP in Bionic HWE kernels as this will allow to debug via serial console with a simple USB 3 crossover-cable (aka "USB 3.0 super-speed A-to-A debugging cable") without the need of a physical RS-232 interface.