Dual display support in legacy driver (HDMI + DSI on RPi3)

[aBUGSworstnightmare]

Legacy display driver in latest Raspian BUSTER release has support for dual display. If you want to use it you will need to make a few addtional steps though because it doesn't run 'out-of-the-box'.

Here's a small how-to for DSI + HDMI.
1) prepare a fresh uSD, boot and configure (initial setting for WiFi,Keyboard, etc) using DSI display only
2) add below to your 'config.txt'

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dtparam=audio=off
disable_overscan=1

enable_uart=off
start_x=1

max_framebuffers=2

#  desktop display will default to the LCD (fb0)
# This will force the specified display to be the first in the list, i.e. /dev/fb0
# Actually this is the full set:
# MAIN_LCD  0
# AUX_LCD   1
# HDMI0     2
# SDTV      3
# HDMI1 is only availabe on RPi4
# HDMI1     7
framebuffer_priority=0

# increase GPU memory
gpu_mem=112

3) replace the /usr/share/X11/xorg.conf.d/99-fbturbo.conf contents with the contents below

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sudo nano /usr/share/X11/xorg.conf.d/99-fbturbo.conf

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# This is a minimal sample config file, which can be copied to
# /etc/X11/xorg.conf in order to make the Xorg server pick up
# and load xf86-video-fbturbo driver installed in the system.
# When troubleshooting, check /var/log/Xorg.0.log for the debugging
# output and error messages.
#
# Run "man fbturbo" to get additional information about the extra
# configuration options for tuning the driver.

#Section "Device"
#        Identifier      "Allwinner A10/A13 FBDEV"
#        Driver          "fbturbo"
#        Option          "fbdev" "/dev/fb1"
#        Option          "SwapbuffersWait" "true"
#EndSection


Section "Device"
        Identifier      "fbturbo0"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb0"

        Option          "SwapbuffersWait" "true"
        Option          "debug" "true"
EndSection

Section "Device"
        Identifier      "fbturbo1"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb1"

        Option          "SwapbuffersWait" "true"
        Option          "debug" "true"
EndSection

Section "Device"
        Identifier      "fbdev0"
        Driver          "fbdev"
        Option          "fbdev" "/dev/fb0"
EndSection

Section "Device"
        Identifier      "fbdev1"
        Driver          "fbdev"
        Option          "fbdev" "/dev/fb1"
EndSection

Section "Monitor"
    Identifier "Monitor0"
    Option "Primary" "False"
EndSection

Section "Monitor"
    Identifier "Monitor1"
    Option "RightOf" "Monitor0"
    Option "Primary" "False"
EndSection

Section "Screen"
    Identifier "ScreenTurbo0"
    Monitor "Monitor0"
    Device "fbturbo0"
    Subsection "Display"
    EndSubSection
EndSection

Section "Screen"
    Identifier "ScreenTurbo1"
    Monitor "Monitor1"
    Device "fbturbo1"
    Subsection "Display"
    EndSubSection
EndSection

Section "Screen"
    Identifier "ScreenDev0"
    Monitor "Monitor0"
    Device "fbdev0"
    Subsection "Display"
    EndSubSection
EndSection

Section "Screen"
    Identifier "ScreenDev1"
    Monitor "Monitor1"
    Device "fbdev1"
    Subsection "Display"
    EndSubSection
EndSection

Section "ServerLayout"
    Identifier "Multihead"
    Screen  0 "ScreenTurbo0"
    Screen  1 "ScreenTurbo1" rightof "ScreenTurbo0"
    Option  "Xinerama" "true"
EndSection

Section "ServerLayout"
    Identifier "Singlehead0"
    Screen  0 "ScreenTurbo0"
EndSection

Section "ServerLayout"
    Identifier "Singlehead1"
    Screen  0 "ScreenTurbo1"
EndSection

Section "ServerFlags"
    Option "BlankTime"  "0"
    Option "StandbyTime"  "0"
    Option "SuspendTime"  "0"
    Option "OffTime"  "0"
    Option "DefaultServerLayout" "Singlehead0"
EndSection

4.) reboot
5.) check if two framebuffer devices were created

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ls /dev/fb*

this should give you the output '/dev/fb0 dev/fb1'

Checking if two frame buffer devices were created

fbtest.jpg (22.85 KiB) Viewed 17152 times

6) use raspi-config and select to boot to CLI and shutdown
8) connect your HDMI display and boot
9) at command line: start desktop with

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startx -- -layout Multihead

If everything is fine you should see your desktop spread across both displays

dual display with legacy driver on RPi3: HDMI + DSI

IMG_1026.jpg (74.46 KiB) Viewed 17152 times

[aBUGSworstnightmare]

Made further tests which include a CM3L setup as well.

Findings:
- Changing desktop background image
Changing the desktop background image on the 'fb1' framebuffer device is no longer possible (was possible during beta testing). Changing background image (right-click on desktop --> Desktop Preferences -> Picture only changes background on 'fb0' device (primary).

- Mouse pointer disappears
Mouse pointer sometimes disappears when mouse is moved to 'fb1' desktop. It was obserserved as well that mouse pointer is flickering sometime. If that happens mouse isn't displayed on 'fb1' desktop too. Quite frequent noted when connected from VNC viewer. Closing VNC viewer will not solve the problem; solvable only by reboot.

- mouse movement on 'fb1' device sometimes erratic
Moving the mouse between 'fb0' and 'fb1' results sometimes in erratic mouse movements on 'fb1' device (i.e. moving right to left all the time let's the mouse suddenly move up/down on 'fb1'

- 'hdmi_edid_file=1' command in config.txt fails
Extracted the EDID data of the monitor into a file. Added command above and rebooted the OS. Display on HDMI monitor ('fb'1 in this test) is incorrect (wrong resolution selected'). Legacy driver used standard (XGA) and not detailed timing from edit-file.

EDID data from 'edit.dat' file is parsed incorrectly

IMG_1027.jpg (98.25 KiB) Viewed 17121 times

detail view of the HDMI display; OS configured it to XGA resolution

IMG_1028.jpg (73.17 KiB) Viewed 17121 times

Note: EDID is parsed correctly when using this command in config.txt with new driver enabled (but HDMI as only display!).

Question: how to force a specfic timing to be used from EDID.dat file (as there can be several)?

[KaraokeAmerica]

Legacy display driver in latest Raspian BUSTER release has support for dual display. If you want to use it you will need to make a few addtional steps though because it doesn't run 'out-of-the-box'.
.
.
.
If everything is fine you should see your desktop spread across both displays

So........this worked for me on a Pi3B+! Awesome!

Since it doesn't specify what happens after the first boot, though, will this adjustment stick? Can I switch it back to booting to the GUI and will it simply "work" going forward if the second screen is plugged in?

[aBUGSworstnightmare]

Your primary screen should be DSI 7in display.
The driver will create frame buffer device for both of your screens, even when HDMI monitor is not connected.
If you always use the same HDMI screen you can add options for its resolution (hdmi_mode and hdmi_group) together with 'hdmi_force_hotplug=1'to your config.txt. This will allow you to connect the display at any time.

[bs_pi]

Hi,
This thread is HDMI + DSI for RPi3, but does it also work for RPi4? I was expecting that the RPi 4 would support HDMI + DSI out of the box but it does not. If I connect monitor to one of the HDMI ports and I have an DSI touchscreen, only the DSI shows on the Screen layout editor. Is this normal? Is the configuration changes suggested here the only way to support both outputs also on the RPi 4?
Thank you for your help!

[pi-tastic]

maybe they could add an option in the Raspberry Pi config application?

eg. DSI + Dual Screen ENABLE / DISABLE

why lines and lines of code?

maybe add swap file option as well.

eg. Swap File - 100m 1024m 2048m with radio buttons.

[6by9]

Hi,
This thread is HDMI + DSI for RPi3, but does it also work for RPi4? I was expecting that the RPi 4 would support HDMI + DSI out of the box but it does not. If I connect monitor to one of the HDMI ports and I have an DSI touchscreen, only the DSI shows on the Screen layout editor. Is this normal? Is the configuration changes suggested here the only way to support both outputs also on the RPi 4?
Thank you for your help!

The Pi4 does support HDMI0 + DSI out of the box, but not HDMI1 + DSI due to internal plumbing within the SoC. Please ensure you use the right HDMI connector.

[6by9]

maybe they could add an option in the Raspberry Pi config application?

eg. DSI + Dual Screen ENABLE / DISABLE

why lines and lines of code?

maybe add swap file option as well.

eg. Swap File - 100m 1024m 2048m with radio buttons.

The described way of doing this is only applicable to the legacy drivers, and is hideously inefficient in the way it splits the screen drawing between the two frame buffer devices.

The better way to achieve this is to use DRM/KMS, as is the default on Pi4. DRM/KMS renders a single framebuffer for the combined desktop, and then tells each display which bit to show. The one gotcha with this on Pi3 or below is that the 3D hardware is limited to a target frame buffer of a maximum 2048x2048. If your HDMI screen is 1920x1080, then you can't do side by side displays when extending your desktop as you run out of width One above the other will work.
(The Pi4 can render to up to 8192x8192, so dual 4k side by side still fits).

[aBUGSworstnightmare]

A FHD monitor (1920x1080pixels) in portrait mode with a DSi (SVGA. = 800x480pixels) in landscape should not brake the HW limit of a RPi3 3D framebuffer size (also valid for 2WXGA in portrait i.e.).

As the first post was resulting from tests wirh an earlier Buster release I still have to repeat it on latest.

So far I was unable to get dual display on RPI3 running FMKS driver (I think mainly caused by EDID issues).

[bs_pi]

Thank you @6by8, you are right, connecting the HDMI cable to the other port did the trick. However on the 'Screen Layout Editor' there is no option to have cloned screen, the only option is an extended desktop. Is there any way in which I could have both screens (HDMI and DSI) a cloned screen?
Thank you for your help.

[6by9]

Thank you @6by8, you are right, connecting the HDMI cable to the other port did the trick. However on the 'Screen Layout Editor' there is no option to have cloned screen, the only option is an extended desktop. Is there any way in which I could have both screens (HDMI and DSI) a cloned screen?
Thank you for your help.

That's mainly a limitation of the screen config tool. It's only a wrapper on top of xrandr.
The hardware can't send exactly the same composition to multiple destinations, but xrandr allows you to overlay the displays, and scale.

Dragging the two displays on top of each other which will clone them if they are the same resolution (this is generally the default).
If they are different resolutions then you need to resort to manual xrandr commands. There is the --scale XxY option, but xrandr doesn't allow the hardware to scale it, and uses GL to instead. This can lead to the system feeling a bit laggy depending on resolution. The HVS (composition hardware) would happily take the same source desktop framebuffer and render it fullscreen down each pipeline.

[pietievannieplaas]

Thank you for this article, works like a charm on my Pi3B

I would like to have the boot up go automatically to the GUI login display. After trying out quite a few suggestions and articles, I always end up with either only the touch screen working (using raspi-config), or I am logged in as root (adding startx command with the layout options to /etc/rc.local), with some PID error box.

It seems the only way to have dual screen setup working is with the -layout server option in startx, but I cannot find any help on where to specify the command line arguments for startx automatically. I ended up adding the line to a ~/.bash_login file, which does what I want.

Any better suggestions?

[radji]

Hi all

Ran all the attached code. My device list still only shows one framebuffer available.

Not sure where the error is. Any insight is appreciated. See attached log in zip file.

Xorg.0.zip

(4.36 KiB) Downloaded 162 times

[EasyBreathy]

Hey everyone,
I followed the instructions (on Rpi4) and it worked on the first try!
However, my touch input from the DSI mostly controls the cursor on the the HDMI screen.
How can I confine the touch input to the DSI? My end goal is for the HDMI screen to be output only.

[6by9]

Hey everyone,
I followed the instructions (on Rpi4) and it worked on the first try!
However, my touch input from the DSI mostly controls the cursor on the the HDMI screen.
How can I confine the touch input to the DSI? My end goal is for the HDMI screen to be output only.

Multiple screens are supported on Pi4 without doing anything. Following the instructions in the first post will reduce the performance of your Pi.

viewtopic.php?p=1724631#p1724631 for using xinput to map the touchscreen to one display.

[EasyBreathy]

Multiple screens are supported on Pi4 without doing anything. Following the instructions in the first post will reduce the performance of your Pi.

viewtopic.php?p=1724631#p1724631 for using xinput to map the touchscreen to one display.

Oh my, that's good to know, Thank you very much!
But in that case I have another question -
When I connected both the touchscreen and HDMI screen (to HDMI0) I wasn't getting any output to the HDMI.
How come it started working after going through the steps on this thread?

[6by9]

Multiple screens are supported on Pi4 without doing anything. Following the instructions in the first post will reduce the performance of your Pi.

viewtopic.php?p=1724631#p1724631 for using xinput to map the touchscreen to one display.

Oh my, that's good to know, Thank you very much!
But in that case I have another question -
When I connected both the touchscreen and HDMI screen (to HDMI0) I wasn't getting any output to the HDMI.
How come it started working after going through the steps on this thread?

I have made the assumption you're using an up to date Raspberry Pi OS (formerly known as Raspbian) image and not some other distribution.
In which case HDMI0 + DSI should work fine in initially putting up the rainbow screen on DSI and the same with a magenta top left corner on HDMI0. The Linux simple framebuffer then comes up on DSI, and once the FKMS drivers load you should then get the console on both displays.
If booting into X then it normally sets the 2 displays to both have the same origin at the top left, but they can be different sizes. Use the Screen Configuration app to move the displays around for an extended desktop.

[jkoz]

This post is still very much relevant today, as there are still some quirks to deal with when setting up a similar configuration. I created a new topic to continue this discussion, as my use case involved an SPI connected display on an RPi4. Here is the new post: /forums/viewtopic.php?f=63&t=306624