Use another tablet as second display

I have an old ipad. I'm wondering if I can make it a secondary display for my Ubuntu laptop. Here records my efforts.

Basically, this involves:

• Step 1: Create a "headless" display - a display outputs to no where
  • The success of this step should result a display can be found and configured in "settings" as if a display is connected
  • How to:
    • Can be done by using the current GPU / driver with the hardware, then the solution depends on the hardware (I have success in this solution on Lenovo E480, see below)
    • Or can be done using "dummy driver"
• Step 2: Uses x11vnc to allow an VNC client to view and interact with "real" X display, the "headless" one

This involves:

• Use "vncserver" to start a VNC desktop (usually ":1") - separate from the default one (":0")
• Run VNC viewer on pad to view the desktop
• Use "x2x" to allow mouse traverse to the :1 desktop and control it.

Result:

• No window manager on :1 desktop (can try export DISPLAY=:1; gdm3)
• Not showing mouse pointer (However the touch screen of the pad can be used as mouse)
• Have to specially launch application in :1 display (DISPLAY=:1 google-chrome &)
• Cannot drag application across display

1. Find out display & GPU:

• `sudo lshw`, find out what hardware and driver is being used:
  • For my Lenovo E480: "product: Lexa PRO [Radeon 540/540X/550/550X / RX 540X/550/550X]... vendor: Advanced Micro Devices, Inc. [AMD/ATI].... driver=amdgpu"

2. Tools:

• xrandr : CLI interface for RandR extension https://www.x.org/releases/X11R7.7/doc/randrproto/randrproto.txt
  • `xrandr --verbose` list all information, can use to find an unused screen

3. Desired resolution (match the ipad): 1024X768

4. Make a headless display:

• (Tried, not work, read on)https://www.reddit.com/r/linuxquestions/comments/feqegp/headless_amdgpu/ suggests passing a kernel parameter - but it disables hardware outputs, so NO.
• (Tried, not work, read on) https://bbs.archlinux.org/viewtopic.php?id=191555 :
  • gtf - calculate modeline
  • xrandr --newmode - create new mode
  • xrandr --addmode (the output should be chosen from an unused, disconnected output, use 'xrandr --verbose` to find out)
  • xrandr --output (to enable output)
  • x11vnc - to export the output through VNC, I tried `x11vnc -nowf -nopw -noxdamage -clip 1024x768+0+0`
  • Result: worked, but heavily flawed, unusable
    • Display corrupt
    • Unable to drag window to the ipad screen
    • If I use x11vnc to expose a real display (laptop display), it works perfectly. So x11vnc works. Problem is with the display.
    • "This trick works on the more or less recent Intel drivers on an Optimus/Bumblebee system" - so maybe the problem with my laptop is I'm using "amdgpu" driver.
• (Tried, not work, read on) https://github.com/kbumsik/VirtScreen/issues/16 added this file:

# nvidia/nouveau/amdgpu device should be configured first before Intel GPU 

Section "Device" 

       Identifier "amdgpu0" 

       Driver     "amdgpu"  

EndSection 

# Then configure intel internal GPU 

Section "Device" 

       Identifier "intelgpu0" 

       Driver     "intel" 

       # You may put Option "VirtualHeads" "1" here but it seem you don't need to put this for Ubuntu 18.04.

It prevent user from login from graphic login interface. Failed.

• (Yes, this one works, read next section for details) https://github.com/kbumsik/VirtScreen/issues/16#issuecomment-649480599 This one seem to have very similar hardware so the solution looks promising... Yes, it works!
  • One issue - mouse pointer moving from laptop to VNC viewer does not show up. Solution: https://stackoverflow.com/questions/34282363/x11vnc-display-cursor-and-cursor-motion just add "-multiptr" option

• https://github.com/kbumsik/VirtScreen/issues/16#issuecomment-649480599
• https://stackoverflow.com/questions/34282363/x11vnc-display-cursor-and-cursor-motion

Only record the steps, more details see links above

1. Find PCI address (records down "02:00.0")

$ lspci | grep -E 'VGA|Display|3D' 

00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07) 

02:00.0 Display controller: Advanced Micro Devices, Inc. [AMD/ATI] Lexa PRO [Radeon 540/540X/550/550X / RX 540X/550/550X] (rev c0)

2. Create virtual display

$ cat /etc/modprobe.d/amdgpu.conf  

options amdgpu virtual_display=0000:02:00.0,1

3. Restart, set virtual display in settings

4. Check with command xrandr, find line similar to this:

Virtual-1-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm

5. Export display with x11vnc, '-multiptr' to make pointer visible (but it lags badly, so maybe just don't use)

x11vnc -clip xinerama0 -nopw -multiptr

6. I used RealVNC on an iPad2. It works ok.

• Address 'bing-e480.local:5900'