SnowsongWolf
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Joined: Sun Sep 15, 2019 2:52 am

Dual HDMI Output on RPi 4

Sun Sep 15, 2019 3:13 am

Long story short, I have an RPi 4 with 2 HDMI displays. Both displays are working, however I have the same output on both. Which is to say there is only fb0 showing in /dev. How do I get individual framebuffers for each monitor? I'm running Raspbian buster lite (no desktop). My Google-fu is usually strong but I've been searching for a few hours with no luck.

SnowsongWolf
Posts: 4
Joined: Sun Sep 15, 2019 2:52 am

Re: Dual HDMI Output on RPi 4

Sun Sep 15, 2019 4:52 am

Nevermind, finally figured it out. I had to disable the optional driver loaded in the [rpi4] section of /boot/config.txt

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PeterO
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Re: Dual HDMI Output on RPi 4

Sun Sep 15, 2019 6:20 am

That doesn't sound right. It should work twin headed "out of the box".

What exactly did you change in config.txt ?

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jamesh
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Re: Dual HDMI Output on RPi 4

Sun Sep 15, 2019 8:23 pm

With the FKMS driver re-enabled (with it off you lose 3d acceleration), you get one frame buffer, but you can make it incorporate the two displays. Go to the screen configuration utility, and you will see just one display, but that is in fact one over the other. CLick and drag the display around and you will see its actually two displays, and you can then set them up as you want, including different orientations, gaps between etc.
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DirkS
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Re: Dual HDMI Output on RPi 4

Sun Sep 15, 2019 8:28 pm

jamesh wrote:
Sun Sep 15, 2019 8:23 pm
Go to the screen configuration utility
OP is running Lite (no desktop)...

KenT2
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Re: Dual HDMI Output on RPi 4

Mon Sep 16, 2019 6:52 am

DirkS wrote:
Sun Sep 15, 2019 8:28 pm
jamesh wrote:
Sun Sep 15, 2019 8:23 pm
Go to the screen configuration utility
OP is running Lite (no desktop)...
use the xrandr command

jamesh
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Re: Dual HDMI Output on RPi 4

Mon Sep 16, 2019 6:26 pm

KenT2 wrote:
Mon Sep 16, 2019 6:52 am
DirkS wrote:
Sun Sep 15, 2019 8:28 pm
jamesh wrote:
Sun Sep 15, 2019 8:23 pm
Go to the screen configuration utility
OP is running Lite (no desktop)...
use the xrandr command
Which is all the Screen Saver configuration tool does under the skin,
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DirkS
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Re: Dual HDMI Output on RPi 4

Mon Sep 16, 2019 8:31 pm

KenT2 wrote:
Mon Sep 16, 2019 6:52 am
DirkS wrote:
Sun Sep 15, 2019 8:28 pm
jamesh wrote:
Sun Sep 15, 2019 8:23 pm
Go to the screen configuration utility
OP is running Lite (no desktop)...
use the xrandr command
Not installed on Lite...
It's part of the package x11-xserver-utils

jamesh
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Re: Dual HDMI Output on RPi 4

Mon Sep 16, 2019 8:34 pm

TBH, if you are not using the desktop, legacy graphics mode will probably be fine, and gives you two framebuffers rather than one over two displays.
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