Re: sysrq over *USB*

[Marc SCHAEFER <schaefer@alphanet.ch>]

On Fri, Oct 15, 2021 at 09:02:50PM +0200, Marc SCHAEFER wrote:
> Should I abandon all hope to make it work with USB, or should it work?

Yes, sysrq can work with USB, but not with stock Debian kernels,
because of [1].

Here is the work-around:

1) recompile kernel (see [2]) with the following options:

      CONFIG_USB=y
      CONFIG_USB_SERIAL=y
      CONFIG_USB_SERIAL_CONSOLE=y
      CONFIG_U_SERIAL_CONSOLE=y

2) configure /etc/default/grub with console=ttyUSB0,9600 console=tty0
   and run upgrade-grub

3) I then get:

      # cat /proc/consoles
      tty0                 -WU (EC p  )    4:1
      ttyUSB0              -W- (E  p  )  188:0

and I can then do sysrq from USB:

   schaefer@acer-1:~$ cu -l ttyUSB0 -s 9600
   Connected.
   ~%break
   [ 1633.701624] sysrq: HELP : loglevel(0-9) reboot(b) crash(c)
   terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i)
   thaw-filesystems(j) sak(k) show-backtrace-all-active-cpus(l)
   show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p)
   show-all-timers(q) unraw(r) sync(s) show-task-states(t) unmount(u)
   force-fb(V) show-blocked-tasks(w) dump-ftrace-buffer(z)

Also, the HDMI console still works (logs & sysrq).

The problem and solution was found with help from the kernel-newbies
mailing-list.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=868352
[2] https://wiki.debian.org/BuildADebianKernelPackage