Is it possible to reset HDMI connection or power it down and back on or something like this without restarting the PC? The problem is: I have a kubuntu 22.04 HTPC connected to Samsung AU9070 TV which has an annoying quirk: if I turn on the TV first all is fine, but if turn the TV on while PC is already on TV doesn't detect it - says the device isn't turned on, and that's that, and I have to ssh to PC and reboot it. Cost me quite a bit of time initially while I tried to figure out why the perfectly good PC doesn't show anything. Weirdly it didn't have this issue with my old 1920*1080 HTPC, it started only after I upgraded it to 4K-capable hardware with built-in intel graphics (12100 i3 CPU), and had to turn on some advanced HDMI mode in TV settings because otherwise 4K was limited to 30fps - another quirk, again time wasted to figure it out.

What you describe is not a fault. On startup, X configures every monitor about. If your hdmi doesn't have a live tv on the hdmi, X thinks: "OK, nothing on the hdmi."

Try editing /etc/inittab and change the default runlevel to 3, instead of 4 or 5. Then when you want the tv, connect it to hdmi. Log out of X; and type startx again. Usually that's just up-arrow & return. Then X will find the monitor. The + is that you only have to log out, not reboot to have the monitor recognised; The - is that you have to log into a console before you can start X.

That certainly isn't my experience. If I power up a PC with the monitor or TV connected by hdmi powered down and then power up the TV/monitor everything works fine. In fact, I just did it twice, once with my monitor and once with an hdmi connected TV, just to make sure I was remembering correctly. I don't have 4K however, so that may create issues I'm unaware of. I think something is wrong, either in your TV settings or your HTPC settings. First place I would look is in System Settings > Display Configuration. Make sure your TV is properly identified and configured as "Enabled" and "Primary".

Starting a pc without the primary monitor sounds fairly weird to me. In the absence of information from the OP, I won't debate this. I presumed it was a secondary monitor.

I do that continuously with my RPi boards. And hdmi is only plugged in when network is down. I have never experienced an issue like this, probably it depends on the TV too.

HDMI is the ONLY video out on the RazPi. No laptop will spot hdmi plugging after X has started. X configures the primary monitor, and any others it finds. But it's likely to puke with a "No screens found" error if it's booted headless.

Anyhow this debate is pointless and doesn't help the OP. Solve his problem, instead of hijacking his thread to take swipes at me.

I agree with the assessment that the HDMI device needs to be powered on before booting, as I've seen this exact behavior before. The caveat, though, is if the device itself is actually powered off or what I call 'software off'; meaning the SCREEN is powered off but the logic board is powered, so HDMI will be recognized as 'on'. I believe that's something the audio/video folks call HDMI-CEC. Turning that on for the device means other things can 'power it on'.
https://www.howtogeek.com/207186/how...hy-you-should/

Most actual HDMI monitors won't have it, but televisions will. The device mentioned by the OP does:
https://www.samsung.com/in/support/t...sung-smart-tv/

I am rather surprised the discussion went this way, but at least the conversation is flowing :) Linux (or windows for that matter) supports on-the-fly monitor configuration changes - that's what randr is for, so the idea that computer won't be able to use the monitor if it wasn't connected at startup is ridiculous, but in my particular case this is not even a factor. PC does see the TV even when the TV is off - not unplugged, just off, HDMI device is connected and display is properly configured:

but this blasted buggy TV at the very same time reports that PC is off and shows no picture.

I'd check the CEC settings.

This can be a quirk of some combinations of TV firmware, motherboard firmware, GPU firmware, distro (X version; Xorg vs Wayland), and driver versions. I have lots of each. Some combinations just refuse to play nice together. At least for those with a new (enough) TV, one has the option to request a firmware upgrade, and if the request is denied, either return it, or exchange it, hoping you just got a bad one.

In this case, I'd look for a HTPC firmware update. I'd also try live media of some non-Debian-based distro to rule out Kubuntu. For that matter, you appear have broken a Linux wisdom, which is make sure your OS is months (somewhere between 3 and 12) newer than your hardware. You have an Alder Lake CPU/GPU released only about a month or so prior to Kubuntu 22.04. Likely that's functionally backwards the way prerelease version freezes work. You might solve the problem with a backport kernel and/or Xorg, or some newer distro release.

All that said, sometimes an expensive HDMI cable is no better than WalMart's cheapest, and a proven high quality cable is all that's needed.

Not necroposting, but successful conclusion: after a firmware update TV works fine and detects PC regardless of power on order. Not that I did anything.

I had similar issue with one of my boxes, running 'xrandr --auto' fixed it every time. Must set the DISPLAY variable when running over SSH, that's all.