My laptop has a 'HDA Intel PCH' soundcard with a 'Realtek ALC255' chip. There are no separate audio out, mic or line in ports; only a single headphone/microphone combo port.

In Windows 10, when I plug my guitar cable into the combo port, a popup appears and I can select which method to use ('Line in', 'Headphone', 'Headset', 'Mic' and 'Speaker out').

But in Linux, I can't seem to find a way to use the port as line in. When the guitar cable is plugged in, Gnome only shows two options: 'Headphone' and 'Headset'. No line in. What should I do to make the combo port act as line in?

I'm not sure if 'hda-jack-restask' may be needed here, but review the following to start with...

https://www.omgubuntu.co.uk/2013/12/...ne-jack-ubuntu
https://unix.stackexchange.com/quest...hones-for-outp

Thanks for the links. I tried using hda-jack-retask but apparently it didn't work. Pavucontrol still displayed Headphone as output and Headphone Mic as input. No mention of Line-in anywhere. There should be right?
Nonetheless, I couldn't capture my guitar's signal in any programme. Is hda-jack-retask the only way to do this?

As far as I know it is a jack restasking issue, and will ultimately also come down to the support offered by the Linux driver for your hardware. The 'alsa-info.sh' script can be used to gain indepth information about your audio hardware and configuration, so that is a good place to start...

https://www.alsa-project.org/wiki/Help_To_Debug
https://wiki.ubuntu.com/Audio/AlsaInfo

Most distros include it as part of alsa-utils or similar.

It may be that a bug report is required to help progress this.

I had a similar issue and ended up getting a USB sound card (external) for not much money - works a treat.

I'm not sure, but jackaudio might be helpful, esp. combined with a graphical connection manager like qjackctl.

It is also unclear - do you want to
a) plug the line in into that special combo port
b) separate out the mic and use it as line in, while still being able to use the headphones?

If a), you are using a plug that is not necessarily compatible with that jack, no?

Thanks for the valuable information. I'll try submitting a bug report as soon as I get more free time (It's finals week ).

Actually I'm going to buy one tomorrow. Seems to be the best solution. I just hope it'll have low latency.

I have both jack and qjackctl installed. But I don't know how to use them for this.
I want to do a)
I don't think so. It works perfectly in Windows. I just have to select 'Line-in' from the popup that the Realtek software display when I plug my guitar in. I think that port is meant to be used that way. Why would it even have a 'Line-in' option if it's not compatible?

jackaudio is more than just an application and you'll need to set it up properly, and probably remove or disable pulseaudio.

I got jack to work side-by-side with pulseaudio and now it's working fine with reaper and some other audio applications. But I don't think jack can do anything about the combo port. Now, buying an external sound card could solve my problem, but the original issue here is not solved. And like ferrari said, I think it's best to figure this out with alsa devs. I was wondering if I should mark the thread as SOLVED or not...

Yes, if hda-Jack-retask can’t do the job it’s time to file a bug report I think. The alsa-info.sh script will provide verbose output, including the pin mapping for the combo port exposed by the driver. If ‘line in’ is not present then that would indicate a possible issue that requires addressing with the driver (or firmware) itself IMHO.