Does sysrq work over USB?

Marc SCHAEFER schaefer at
Tue Nov 16 09:04:56 EST 2021


I made the following setup work, that is I can send break and '?'
(to get the magic sysrq help) or 's' to do an Emergency sync, and the
kernel logs it:

 laptop                                        apu2
    USB serial port ------- null modem ------- ttyS0 internal 16550A

(an apu2 is an embedded amd64 computer [4])

As it works, of course MAGIC_SYSRQ is enabled, including for serial
ports, and the correct value is in the /proc pseudo-file.

It works with the getty enabled or disabled.

However, the following does not work to support magic sysrq, although
bidirectionnal communication also works with cu [5], with the correct
speed set:

 laptop                                        apu2
    USB serial port ------- null modem ------- USB serial port

First, reading documentation, I thought that this would not be possible with
USB and only with the internal serial port [1], but then, reading kernel
source, it looks it should work with my adapter:

   Oct 11 14:30:56 apu2-init7 kernel: [    9.915105] usb 2-2: pl2303 converter now attached to ttyUSB0

since the driver [2] contains code for magic sysrq, see line 993 for
sysrq mode and line 892 for break handling, with implementation in [3]
(lines 589-597).

I am running Debian buster kernel 4.19.0-18-amd64 on the apu2.

Should I abandon all hope to make it work with USB, or should it work?

Thank your for any pointers.

    "On the serial console (PC style standard serial ports only)"




    from the days before 2003 where I was doing UUCP
       cu -l ttyUSB0 -s 9600

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