CrazyErick
Posts: 15
Joined: Thu Feb 07, 2013 10:40 am

Pi4 dual display - controlling individually

Tue Mar 10, 2020 1:30 pm

Hi,

Most my seaching has yielded only HMDI not working or how to configure using both HDMI ports as one big display.

My question is - can one control each HDMI display seperately?

For example - could I run gui desktop on HDMI:0 and display seperate commands on HDMI:1?

So basically I want to control what goes to HDMI:0 and HDMI:1

Thanks,

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 30436
Joined: Sat Jul 30, 2011 7:41 pm

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 1:34 pm

At the moment the way to do this is to use the legacy graphics driver, but you do lose HW acceleration of things like the desktop.

Comment out the fkms line in config.txt, reboot and you should get two frame buffers, one with the desktop/console, the other to do with as you want.
Principal Software Engineer at Raspberry Pi Ltd.
Working in the Applications Team.

CrazyErick
Posts: 15
Joined: Thu Feb 07, 2013 10:40 am

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 2:54 pm

Thanks Jamesh,

But now I seem to have lost the second display

pi@raspberrypi:~ $ tvservice -s
state 0x6 [DVI DMT (87) RGB full 15:9], 800x480 @ 59.00Hz,

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 30436
Joined: Sat Jul 30, 2011 7:41 pm

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 3:43 pm

CrazyErick wrote:
Tue Mar 10, 2020 2:54 pm
Thanks Jamesh,

But now I seem to have lost the second display

pi@raspberrypi:~ $ tvservice -s
state 0x6 [DVI DMT (87) RGB full 15:9], 800x480 @ 59.00Hz,
Odd, is max_framebuffers set correctly in config.txt? Will need to be 2.
Principal Software Engineer at Raspberry Pi Ltd.
Working in the Applications Team.

CrazyErick
Posts: 15
Joined: Thu Feb 07, 2013 10:40 am

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 3:53 pm

My bad, its there

tvservice -s -list
state 0x6 [DVI DMT (87) RGB full 15:9], 800x480 @ 59.00Hz, progressive
2 attached device(s), display ID's are :
Display Number 2, type HDMI 0
Display Number 7, type HDMI 1


So how do I redirect to the second display then?
I assume I need to start a second X server?

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 30436
Joined: Sat Jul 30, 2011 7:41 pm

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 5:24 pm

I'm confused what you want? A separate desktop on each display? Not sure what purpose that would serve. What is your particular use case?
Principal Software Engineer at Raspberry Pi Ltd.
Working in the Applications Team.

CrazyErick
Posts: 15
Joined: Thu Feb 07, 2013 10:40 am

Re: Pi4 dual display - controlling individually

Tue Mar 10, 2020 5:41 pm

Thanks for responding jamesh.

So my plan is having some sort of interface on /dev/fb0 - Lets assume normal Xwindows desktop for now, or some GUI
On the second screen I want to display other graphics for example a browser window, or image, or some script or programatic output.

So I was hoping on

/dev/fb0 - normal x windows
/dev/fb1 - possibly some "xinit script managed output".

Does that make sense?

CrazyErick
Posts: 15
Joined: Thu Feb 07, 2013 10:40 am

Re: Pi4 dual display - controlling individually

Thu Mar 12, 2020 8:23 am

So update, I can output an image to /dev/fb1 but it breaks my x session on fb0.

My searching is not getting much luck.

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