Debian: Hardware Limitations for Third Monitor

Question

I have one laptop with both HDMI and VGA ports. I connected two external monitors. However, I can activate any combination of two monitors simultaneously, yet never all three together: in the settings it says "Changes cannot be applied", "this could be due to hardware limitations". Output from xrandr:

Screen 0: minimum XXX, current XXX, maximum XXX

LVDS-1 connected (normal left inverted right x axis y axis)

a bounch of lines with all resolutions

VGA-1 connected primary XXX left (normal left inverted right x axis y axis) XXXmm x XXXmm

  a bounch of lines with all resolutions

HDMI-1 connected XXX left (normal left inverted right x axis y axis) XXXmm x XXXmm

  a bounch of lines with all resolutions

a bounch of lines with a number of other "ports-XXX disconnected"

Exact Model: Dell Latitude E5520. Graphic Card: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09).

Output of xrandr --listproviders:

Providers: number : 1 Provider 0: id: 0x49 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 8 associated providers: 0 name:modesetting

output of xrandr:

 Screen 0: minimum 320 x 200, current 2160 x 1920, maximum 8192 x 8192 LVDS-1 connected (normal left inverted right x axis y axis) 1366x768 60.05 + 40.01
1360x768 59.80 59.96
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 56.25
840x525 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 59.98
800x450 59.95 59.82
640x512 60.02
720x450 59.89
700x450 59.96 59.88
640x480 60.00 59.94
720x405 59.51 58.99
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98
576x432 60.06
640x360 59.86 59.83 59.84 59.32
512x384 60.00
512x288 60.00 59.92
480x270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320x180 59.84 59.32
VGA-1 connected primary 1080x1920+1080+0 left (normal left inverted right x axis y axis) 531mm x 299mm 1920x1080 60.00*+ 1600x1200 60.00
1680x1050 59.95
1280x1024 75.02 60.02
1440x900 59.89
1280x960 60.00
1152x864 75.00
1024x768 75.03 60.00
832x624 74.55
800x600 75.00 60.32
640x480 75.00 59.94
720x400 70.08
HDMI-1 connected 1080x1920+0+0 left (normal left inverted right x axis y axis) 531mm x 299mm 1920x1080 60.00*+ 50.00 59.94
1920x1080i 60.00 50.00 59.94
1600x1200 60.00
1680x1050 59.88
1280x1024 60.02
1440x900 59.90
1280x960 60.00
1280x720 60.00 50.00 59.94
1024x768 60.00
800x600 60.32 56.25
720x576 50.00
720x576i 50.00
720x480 60.00 59.94
720x480i 60.00 59.94
640x480 66.67 60.00 59.94
DP-1 disconnected (normal left inverted right x axis y axis) HDMI-2 disconnected (normal left inverted right x axis y axis) HDMI-3 disconnected (normal left inverted right x axis y axis) DP-2 disconnected (normal left inverted right x axis y axis) DP-3 disconnected (normal left inverted right x axis y axis)

Any help would be greatly appreciated.

Answer 1

All the Intel 2nd generation iGPUs seem to have only 2 pixel pipelines, which means they can only produce content for two separate displays at the same time, if I've understood correctly.

It might be possible to use a single pipeline to provide the same display content to two displays (i.e. clone mode) if both displays can accept the same resolution and refresh rate.

Since VGA connector uses analog signals, it also needs a RAMDAC. Some GPUs might have e.g. four pixel pipelines but only two RAMDACs, limiting you to a maximum of only two displays with analog connectors (VGA or DVI-A); the rest would need to be connected using some form of digital signaling (DVI-D, HDMI or DisplayPort, or internal LVDS on laptops).