sysreq key equivalent

[Jim Lesurf]

I've (mostly) switched to using a Logitech k400r wireless keyboard for most
of the time with the linux box that feeds AV to our TV, etc. However it
doesn't have a key which is labelled as the sysreq. And when I tried the
'print screen' key to see if that could be used for this purpose it didn't
work.

So, anyone know which key on this model of keyboard will deliver the
sysreq? Or how I can check to find out?

I tried a websearch but couldn't find any specific answer to this.

Jim

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[Jim Lesurf]

[Spiros Bousbouras]

On Sat, 14 Aug 2021 10:16:00 +0100
Jim Lesurf <no...@audiomisc.co.uk> wrote:
> I've (mostly) switched to using a Logitech k400r wireless keyboard for most
> of the time with the linux box that feeds AV to our TV, etc. However it
> doesn't have a key which is labelled as the sysreq. And when I tried the
> 'print screen' key to see if that could be used for this purpose it didn't
> work.
>
> So, anyone know which key on this model of keyboard will deliver the
> sysreq? Or how I can check to find out?

I think keyboards deliver "scancodes" and an application uses a library to
translate from the scancode to a human readable symbol like a letter or a
number , etc. How an application does things also depends on whether it uses
the X Windows API or the Wayland API (or something else ?) and that of course
will also depend on what is running on the Linux box in question. I don't
know if different keyboards will necessarily deliver the same scancode for
the same letter on the keyboard or <X Windows / Wayland / library /
whatever> has some way of doing an appropriate translation depending on the
model of keyboard and possibly other factors.
www.win.tue.nl/~aeb/linux/kbd/scancodes-1.html has some information.

This is all a bit esoteric of course and perhaps will set you towards the
wrong path. A more productive approach may be the following : on the Linux
box in question what application is responsible for reading sysreq (when it
exists) and do the operation you want to achieve ? The documentation of that
application may provide a clue.

> I tried a websearch but couldn't find any specific answer to this.

Have you tried the old and trusted method of pressing random keys and see
if any do what you want ? Obviously this isn't something you should do if
the results can be catastrophic like for example if the same Linux box
controls the self-destruct mechanism of your house.

[Martin Gregorie]

On Sat, 14 Aug 2021 10:16:00 +0100, Jim Lesurf wrote:

> I've (mostly) switched to using a Logitech k400r wireless keyboard for
> most of the time with the linux box that feeds AV to our TV, etc.
> However it doesn't have a key which is labelled as the sysreq. And when
> I tried the 'print screen' key to see if that could be used for this
> purpose it didn't work.
>
> So, anyone know which key on this model of keyboard will deliver the
> sysreq? Or how I can check to find out?
>
> I tried a websearch but couldn't find any specific answer to this.
>

Does running 'showkey -a' help?


--
--
Martin | martin at
Gregorie | gregorie dot org

[Jim Lesurf]

In article <VkuwifgC...@bongo-ra.co>, Spiros Bousbouras

<spi...@gmail.com> wrote:

> This is all a bit esoteric of course and perhaps will set you towards
> the wrong path. A more productive approach may be the following : on the
> Linux box in question what application is responsible for reading
> sysreq (when it exists) and do the operation you want to achieve ? The
> documentation of that application may provide a clue.

AIUI the purpose of SysKey is that the 'OS' picks this up as a trigger that
what follows is to deal with something like a 'kernel panic'. i.e. it
bypasses (or should) the GUI and apps.

[Martin Gregorie]

[Spiros Bousbouras]

On Sun, 15 Aug 2021 14:57:17 +0100
Jim Lesurf <no...@audiomisc.co.uk> wrote:
> In article <VkuwifgC...@bongo-ra.co>, Spiros Bousbouras
> <spi...@gmail.com> wrote:
>
> > This is all a bit esoteric of course and perhaps will set you towards
> > the wrong path. A more productive approach may be the following : on the
> > Linux box in question what application is responsible for reading
> > sysreq (when it exists) and do the operation you want to achieve ? The
> > documentation of that application may provide a clue.
>
> AIUI the purpose of SysKey is that the 'OS' picks this up as a trigger that
> what follows is to deal with something like a 'kernel panic'. i.e. it
> bypasses (or should) the GUI and apps.

Is the information at
https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html of any use ?

It might also help if you explain what operation you want to achieve with
sysreq .

--
According to quantum mechanics there exists an alternative universe where there
are movies "Silence of the wolves" and "Dancing with lambs".

[Spiros Bousbouras]

[Jim Lesurf]

In article <18250...@f1.n221.z2.fidonet.fi>, Spiros Bousbouras

<Spiros.B...@f1.n221.z2.fidonet.fi> wrote:

> Is the information at
> https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html of any
> use ?

It is certainly useful as a handy guide, yes. :-)

> It might also help if you explain what operation you want to achieve
> with sysreq .

The machine in question sometimes seems to 'stiff' its GUI, etc following a
mouseclick. Most often this is when a click on a ROX-Filer 'app' icon calls
starting up VLC to play a file.

I *think* what happens is due to using a wireless mouse/keyboard where the
control isn't 100% reliable. It *may* be due to more than one command being
sent causing something like multiple instances of VLC to be started as they
both try to get going. But this is a guess on my part.

The result is partly displayed VLC windows and a totally unresponsive
(usually) system. i.e. the mouse and normal uses of the keyboard cease to
have any visible effect.

An old wireless keyboard however then will let me run though the standard
REISUB sequence to shot down and start up again.

I got the k400r as an alternative and it is, indeed, much more reliable in
communicating with the machine. So the problem happens much less often. But
when it did (for the first time) I found that the keyboard lacked a SysRq
key label, and 'print screen' seemed not to work for a REISUB sequence. Had
to get the 'old' keyboard and that did the trick.

So I can keep the old keyboard handy for when needed. But it would be
better if I can suss how to get the new one to let me send the REISUB.

The new one is much easier to use. The old one meant I had to keep moving
the keyboard about until I found a place where it worked. However I hadn't
got the problem with the new keyboard until recently. Seemed to have almost
totally cured the problem - until it cropped up again recently and I
realised the lack of the SysRq key!

[Spiros Bousbouras]

On Fri, 20 Aug 2021 10:13:38 +0100
Jim Lesurf <no...@audiomisc.co.uk> wrote:
> In article <18250...@f1.n221.z2.fidonet.fi>, Spiros Bousbouras
> <Spiros.B...@f1.n221.z2.fidonet.fi> wrote:
>
> > Is the information at
> > https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html of any
> > use ?
>
> It is certainly useful as a handy guide, yes. :-)
>
> > It might also help if you explain what operation you want to achieve
> > with sysreq .
>
> The machine in question sometimes seems to 'stiff' its GUI, etc following a
> mouseclick. Most often this is when a click on a ROX-Filer 'app' icon calls
> starting up VLC to play a file.
>
> I *think* what happens is due to using a wireless mouse/keyboard where the
> control isn't 100% reliable. It *may* be due to more than one command being
> sent causing something like multiple instances of VLC to be started as they
> both try to get going. But this is a guess on my part.
>
> The result is partly displayed VLC windows and a totally unresponsive
> (usually) system. i.e. the mouse and normal uses of the keyboard cease to
> have any visible effect.
>
> An old wireless keyboard however then will let me run though the standard
> REISUB sequence to shot down and start up again.

Does Ctrl+Alt+F1 work to take you to Linux console ? Note that the link
above says

Write a character to /proc/sysrq-trigger. e.g.:
echo t > /proc/sysrq-trigger

so , even if you cannot type the REISUB sequence , if you can type in Linux
console then something like the above may work. You can even make sure that
you always have such a command already typed in so that you would only need
to do Ctrl+Alt+F1 and then press <Enter> , assuming the system has not
become so unresponsive to handle even these.

In order to see the keycodes emitted by keys you can try showkey (mentioned
in a different post) or xev and see if some key in the new keyboard emits
the same code as sysreq in the old keyboard.

Or you can try to figure out why the system freezes to begin with. This would
be the ideal solution but , based on your description above , it may be hard
to narrow it down.

> I got the k400r as an alternative and it is, indeed, much more reliable in
> communicating with the machine. So the problem happens much less often. But
> when it did (for the first time) I found that the keyboard lacked a SysRq
> key label, and 'print screen' seemed not to work for a REISUB sequence. Had
> to get the 'old' keyboard and that did the trick.
>
> So I can keep the old keyboard handy for when needed. But it would be
> better if I can suss how to get the new one to let me send the REISUB.

--
vlaho.ninja/prog