How to recover when monitor goes blank.

[Ram Ramesh]

Hi,

  I have my monitor, keyboard and mouse shared through a KVM switch.
One host is Linux Debian bookworm 12.5 and another is laptop running
Windows 11. When I leave KVM on the laptop side for extended period I
have issues switching back to Debian side. When I switch, the screen is
blank and KB does not respond as if Debian is running headless. I had to
remote login and reboot Debian side with KVM locked on this side to get
back the monitor/KB. This happens regardless of whether I am in Xorg or
VT.  I do not know how to force Debian/Linux to check for monitor/KB
again after extended period of disconnect when it has assumed it is
running headless. Any solutions?

Regards
Ramesh

[David Christensen]

Post by Ram Ramesh
Hi,
  I have my monitor, keyboard and mouse shared through a KVM switch.
One host is Linux Debian bookworm 12.5 and another is laptop running
Windows 11. When I leave KVM on the laptop side for extended period I
have issues switching back to Debian side. When I switch, the screen is
blank and KB does not respond as if Debian is running headless. I had to
remote login and reboot Debian side with KVM locked on this side to get
back the monitor/KB. This happens regardless of whether I am in Xorg or
VT.  I do not know how to force Debian/Linux to check for monitor/KB
again after extended period of disconnect when it has assumed it is
running headless. Any solutions?
Regards
Ramesh

My Debian machines have Xfce. I configure Applications Menu -> Settings
-> Power Manager -> Display -> Display power management -> Off.


David

[Ram Ramesh]

My Debian machines have Xfce. I configure Applications Menu ->
Settings-> Power Manager -> Display -> Display power management -> Off.
David

This is not a dpms issue. This is the OS thinking that it is not
attached to a monitor/KB.  I can remote login and remove dpms any time.
Besides this happens in a VT also where there is no xfce. I just do not
know how to tell Linux/OS that a monitor and kb is attached and it
should look for enabling the respective drivers. I thought xrandr is the
way to go, but that is only when Xorg is running and not when we are at
a VT login prompt.

Regards
Ramesh

[Felix Miata]

Post by Ram Ramesh
  I have my monitor, keyboard and mouse shared through a KVM switch.
One host is Linux Debian bookworm 12.5 and another is laptop running
Windows 11. When I leave KVM on the laptop side for extended period I
have issues switching back to Debian side. When I switch, the screen is
blank and KB does not respond as if Debian is running headless. I had to
remote login and reboot Debian side with KVM locked on this side to get
back the monitor/KB. This happens regardless of whether I am in Xorg or
VT.  I do not know how to force Debian/Linux to check for monitor/KB
again after extended period of disconnect when it has assumed it is
running headless. Any solutions?

Not to recover, but to perhaps prevent, via kernel cmdline, one can direct the
kernel which framebuffer mode to force-enable with video=, e.g.:

video=***@60e

https://www.kernel.org/doc/Documentation/fb/modedb.txt

[Ram Ramesh]

Post by Felix Miata
Not to recover, but to perhaps prevent, via kernel cmdline, one can direct the
https://www.kernel.org/doc/Documentation/fb/modedb.txt
--
Evolution as taught in public schools is, like religion,
based on faith, not based on science.
Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!
Felix Miata

I think kernel thinks that no monitor is attached or KB is present. I
want to remote login and tell it to look again and find them.
It appears like there is no magic incantation exists for that. I was
hoping a write to /sys or /proc file will do the trick, but no such
thing seem to exists.

Regards
Ramesh

[Felix Miata]

Post by Ram Ramesh

Post by Felix Miata
Not to recover, but to perhaps prevent, via kernel cmdline, one can direct the
https://www.kernel.org/doc/Documentation/fb/modedb.txt

I think kernel thinks that no monitor is attached or KB is present. I
want to remote login and tell it to look again and find them.
It appears like there is no magic incantation exists for that. I was
hoping a write to /sys or /proc file will do the trick, but no such
thing seem to exists.

Did you try 'e' as I suggested, or read that page? From there:

[quote]
'e' will force the display to be enabled, i.e. it will override the detection
if a display is connected.
[/quote]

The way I read it, if it doesn't work, it's yet another kernel bug, because it
should - prevent - not fix.

[Ram Ramesh]

Post by Felix Miata
[quote]
'e' will force the display to be enabled, i.e. it will override the detection
if a display is connected.
[/quote]

Ok, I will try it, but that is a reboot. I guess if I booted with that
switch, it will always be on and I would not reach a point of blank
screen. Make sense.

I just do not like forcing resolution, but better than broken display.

Regards
Ramesh

[Felix Miata]

Post by Ram Ramesh

Post by Felix Miata
[quote]
'e' will force the display to be enabled, i.e. it will override the detection
if a display is connected.
[/quote]

Ok, I will try it, but that is a reboot. I guess if I booted with that
switch, it will always be on and I would not reach a point of blank
screen. Make sense.
I just do not like forcing resolution, but better than broken display.

The foibles of life complicated by a KVM switch. :p I hope it does what it claims.

If not, I suppose you could connect both Debian PC and laptop to display at the
same time, and use the display's input switch instead of the KVM for video out.