What's the deal with #Linux (specifically, #Debian) and #multimonitor setups (specifically, with #ThinkPad and #DisplayPort)?

I have an #X230Tablet with a monitor (VGA) and an e-ink screen (HDMI over DisplayPort) in addition to its in-built screen, but I can only use two of them at a time. The third one just remains blank, and whichever it is, only starts working when I disable one of the other two 🫣️

Is this a general thing or just a "me and my laptop" thing?

#BoostsAppreciated #HelpWanted

@badrihippo Display cards have a limited number of hardware (Cathode Ray Tube Controller, CRTC) that can scan out buffers to display ports.
On Haswell-era Intel hardware it was 3, so max displays was 3.
I never used more than 2 displays on my X230, so it's at least 2 but maybe not more :)
You could probably look it up how many there are.

@badrihippo I suggest you to ask in Debian support groups (on IRC) as well.

@badrihippo hi,

Maybe a limitation on the graphic card, in displayable size ?

@dcz hmm, could be! I have no mindspace this weekend but I'll look into it more next week. Seems odd to me though that they'd make a dock with multiple outputs if the ThinkPad doesn't support multiple outputs, but maybe other ThinkPads are greater than mine... 🤔

@dcz okay, quick lookup and my processor is Haswell. Looks like I have more digging to do! (Maybe the touchscreen counts as a display? Sounds unlikely though 😛)

@badrihippo Haswell on X230t sounds rather unexpected, but I don't have one :P

Either way, the touch layer doesn't count as a display, it's just a USB device.
The display under that touch layer definitely does.

@dcz @badrihippo X220 is limited to two crtcs. X230 should lift that limit, IIRC, so you should be able to use digital output + VGA + internal LCD at the same time.

@pavel @dcz well I can't but also my laptop was modded to use a more hi-res screen so maybe that changes something (ie. uses up more resources that would otherwise be available for a third screen)?

@mostlyharmless what was the mod exactly?

(Yeah I did see results somewhere that X220 can only handle two screens at most)

@badrihippo @pavel @mostlyharmless sounds like that might be it.
The screen mod disconnects the internal LVDS connector and uses the DP interface that's normally routed to the docking station.
If one CRTC is hard-assigned to LVDS, then you ony have 2 free ones left.

Hey, my X230's display is dying, where did you get the mod board and display?

@dcz I bought the final thing from @mostlyharmless so they'd know the details!

https://mostlyharmless.io/

@pavel

@dcz okay so I just checked and it looks like I'm actually running on the original resolution (which checks out because my output is LVDS-1)

I have heard of people doing it, but didn't look into the details 🤐

@mostlyharmless @pavel