Dual Monitor Support

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On some hardware, it is possible to have 2 monitors display separate images.

Contents

Limitations

There are a few limitations to the possible combinations.

LCDCs

Most SoCs have 2 LCDCs (LCD Controllers). These engines read out from the Display Backend (a sort of framebuffer compositor), and serializes the pixels, according to very specific timing (display mode timing). In order to get dual display support, you need to first have 2 CRTCs and the ability to get that serialized digital signal onto a monitor.

Connectors/Encoders

Not everything can be switched from one LCDC to another, so they usually end up being tied to the first LCDC, limiting the possible combinations.

LCD

A directly connected panel is always tied directly to an LCDC. It can therefor not be switched between LCDCs.

HDMI

If your SoC has a HDMI encoder, then HDMI can be freely switched between LCDCs.

VGA

VGA is a combination of 3 DACs (or TVECs - TV Encoders), for Red Green and Blue, and a horizontal and vertical synchronization pulse. While the DACs are freely switchable, the sync signals are wired directly to an LCDC. It is therefor not possible to switch a VGA connection between LCDCs.

TV

An analog TV signal is either CVBS (composite - 1 DAC), S-Video (2 DACs), RGB (3 DACs) or YCbCr (3 DACs). Timing is embedded into the DACs analog signal and analog TV connections can therefor be switched freely between LCDCs.

FEX changes

Currently, you will need appropriate changes to the FEX file to be able to set up dual monitor support. Our `disp_init` section of the Fex Guide shows the possible values.

The following example sets a dual monitor mode with the same 1280x720 resolutions to both VGA (/dev/fb0) and HDMI (/dev/fb1).

[disp_init]
disp_mode = 2
screen0_output_type = 4
screen1_output_type = 3
screen0_output_mode = 11
screen1_output_mode = 5

This sets the dual monitor mode and the same 1280x720 screen resolution for both VGA (/dev/fb0) and HDMI (/dev/fb1) monitors.

Xorg Configuration

Single XServer

You can use something like the following /etc/X11/xorg.conf file ("fbturbo" can be also replaced with "fbdev" there):

Section "Device"
        Identifier      "FBDEV 0"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb0"
EndSection

Section "Device"
        Identifier      "FBDEV 1"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb1"
EndSection

Section "Screen"
        Identifier      "VGA"
        Device          "FBDEV 0"
        Monitor         "Monitor name 0"
EndSection

Section "Screen"
        Identifier      "HDMI"
        Device          "FBDEV 1"
        Monitor         "Monitor name 1"
EndSection

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen          0 "VGA"
        Screen          1 "HDMI" RightOf "VGA"
EndSection

Separate XServers

Stick the following into /etc/X11/xorg.conf

Section "Device"
    Identifier "Framebuffer Graphics Driver"
    Driver "fbturbo"
    Option "fbdev" "/dev/fb0"
EndSection

And the following into /etc/X11/xorg2.conf

Section "Device"
    Identifier "Framebuffer Graphics Driver"
    Driver "fbturbo"
    Option "fbdev" "/dev/fb1"
EndSection

You then need to start X with the -config argument pointing to the right config file.

For example, for XFCE:

(startxfce4 -- :0 -config /etc/X11/xorg.conf -novtswitch &>/dev/null) &

And

(startxfce4 -- :1 -config /etc/X11/xorg2.conf -sharevts -novtswitch &>/dev/null) &
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