SysRq Over Usb Serial Adapters

Home » CentOS » SysRq Over Usb Serial Adapters
CentOS 3 Comments

I am having trouble sending sysrq commands over USB serial, and I’m not sure what else to check.

I have two identical machines, each with usb-to-serial adapters, joined by a null modem adapter. One runs an agetty, the other minicom. I can log in over the agetty as a user and it works as expected, until I try to send a sysrq.

To send a break in minicom, I type [Ctrl]+[a] then [f]. Then I type the character [m] to do a test. I expect to see memory information print to screen, or at least show up in dmesg. Instead I only see the letter m on my shell.

I’m using 115200, 8N1, VT102. I can echo m > /proc/sysctl-trigger, and dmesg shows that the sysrq was received when initiated via the procfs interface. But not over serial, and that’s what I need.

3 thoughts on - SysRq Over Usb Serial Adapters

  • I could be wrong .. but you may need to edit your grub config to convince your system that the serial terminal is a/the console .. I don’t know that just sending it through a getty would work that way.


  • I’m familiar with that howto, and have set up serial console many times. The only thing new here is the usb serial adapter.

    I am not particularly interested in having serial access during the early stages of booting, which is what you configure in grub. I want it to come up post-boot, preferably after insertion of a specific USB
    serial dongle. And I cannot break (if you pardon the pun) sysrq (or any other) capability at the physical ps2/vga console.

    To be clear, serial console WORKS currently for me with usb serial adapters. What DOESN’T work is SysRq via that serial console.

    My understanding is that the ‘console=’ kernel argument controls pre-init serial console, so that you can see the output of various kernel modules as they load. Once an agetty starts, it takes over the serial console functionality from the kernel.

    Because I’m only interested in adding this serial console post-boot, I
    don’t think I need to configure console=. I’d like to know why this is wrong if it is.