[Question] More CRTCs than supported displays; questions about intel-virtual-output and VIRTUAL1

[Adam Nielsen]

> While investigating this, I came across something I didn't quite
> understand. Officially, the Intel UHD 620 supports three displays. However,
> `xrandr` shows that there are four CTRCs available on Provider 0 (Intel).

You'll probably have to avoid using grep and check the full output.  I
suspect CRTCs 0, 1 and 2 are for the physical outputs on your video
card, and CRTC 3 is attached to the virtual interface, and is not
actually present on your video card.

As far as I am aware most onboard Intel cards are limited to only three
active outputs at a time, which is a pain for those of us who wish to
use four or more monitors.  I have connected more than three to Intel
boards before via a DisplayPort hub and it can drive any of them, but
you can only pick three at a time and the others are blank.

> It seems that the fourth CRTC ("third" when zero-indexed) is used by
> VIRTUAL1 output.

This would make sense - it's a virtual CRTC for a virtual display.

> Out of curiosity, I thought to move CRTC 3 to a connected but disabled
> monitor (DP2). However, this doesn't seem to work.

If CRTC #3 doesn't actually exist on your card then this makes sense.

> After some Googling, I've come to believe that the VIRTUAL1 output has been
> created by the intel-virtual-output utility. To be honest, I find the man
> page for this package vague. "The tool connects local VirtualHeads to a
> remote output, allowing the primary display to extend onto the remote
> outputs."

Having never seen this before myself, I also did some Googling and came
up with this:

https://unix.stackexchange.com/questions/209947/xrandr-where-does-virtual1-output-come-from

https://bbs.archlinux.org/viewtopic.php?id=180904

> My questions are:
> 1. Why or how can I have more CRTCs than officially supported outputs for
> the Intel UHD 620?

It seems that when you add a VIRTUALn output, a virtual CRTC is also
created but this doesn't physically exist on your card.  It's a
software CRTC for a monitor that also only exists in software.

> 2. What does the intel-virtual-output utility do for me, a hybrid Intel/AMD
> user?

Can't answer this as I am not sure about the utility.  The links above
say the VIRTUALn outputs are a feature of Xorg and not specific to any
driver (although it does require driver support to actually work).

> 3. Why do I need VIRTUAL1?

The links above say that if a monitor is disconnected while in use,
Xorg replaces it with a VIRTUALn device as a placeholder.  If you
disable the monitors with xrandr before disconnecting them, then I
imagine you won't get a VIRTUALn device.

Another use appears to be for remote access.  You can set up a virtual
display and then use remote-desktop software to make that virtual
monitor viewable on a different computer.  This could be another a way
of having more than four monitors on a machine with Intel graphics,
except that you need another machine to display the fourth and
subsequent monitors, and it's over the network so will be slow.

> 4. What would happen if I removed the intel-virtual-output utility?

I suspect nothing, if this is all part of Xorg.  It looks like you can
just disable the VIRTUAL1 output with xrandr though if you don't need
it.

Cheers,
Adam.