How to run HDMI and Official Raspberry Pi Display (DSI) simultaneously?

Question

Hallo to all RPi lovers,

I've been trying for a while to clone the video from Official Raspberry Pi Display (connects through DSI) to the HDMI display. Both work fine when alone but not together.

I am pretty much sure, that it is possible from hardware side, as the omxplayer can play video, on HDMI display, while DSI Display is on and functioning.

Looking deeper in the problem I am able to state following:

1. Raspberry Pi doesn't create a framebuffer for second device, there is only /dev/fb0, no matter how many monitors are plugged

2. As the consequence of first, fbcp and other tools, that can play with framebuffer around, won't work.

3. As far as I understood the module bcm2708_fb is in charge of HDMI framebuffer however here is the lsmod:

Module             Size Used by
cfg80211         419759 0
rfkill            16659 2 cfg80211
bcm2835_gpiomem    3023 0
evdev             10226 6
joydev             9047 0
snd_bcm2835       19739 3
snd_pcm           74833 1 snd_bcm2835
r8712u           167991 0
snd_timer         18164 1 snd_pcm
snd               52116 9 snd_bcm2835,snd_timer,snd_pcm
uio_pdrv_genirq    2966 0
rpi_ft5406         2826 0
uio                8228 1 uio_pdrv_genirq
rpi_backlight      1988 0
i2c_dev            6040 0
fuse              81721 3
ipv6             341380 30

and modprobe bcm2708_fb gives in dmsg following error:

[ 3597.206111] bcm2708_fb soc:fb: Unknown ioctl 0x40187a22
[ 3721.217242] bcm2708_fb soc:fb: Unknown ioctl 0x40187a22
[ 3757.719256] bcm2708_fb soc:fb: Unknown ioctl 0x40187a22
[ 3881.152850] bcm2708_fb soc:fb: Unknown ioctl 0x40187a22

Just to imagine here is a part of config that defines the device:

Section "Device"
        Identifier      "Allwinner A10/A13 FBDEV"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb0"

        Option          "SwapbuffersWait" "true"
EndSection

But Xorg configuration is the least of the Problem.

Would be happy to get any advices or questions and thanking for them in advance.

PS. To use built-in RCA, GPIO-VGA adapter, USB external graphic cards is not an option. Aim is stable and clear - to clone HDMI and DSI

The Device is Raspberry Pi 2, (Jessy and Wheezy)

PPS. Is there is the way to input code here as code with no highlighting?

shabaz · Answer

I'm pretty sure that would require GPU firmware effort, see here for some discussion not entirely unrelated: https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=120541

The discussion there is concerned about having two framebuffers and for the GPU to read them both, but I can imagine for your scenario the firmware would also need changes to read the same framebuffer and direct to DSI (after any resolution scaling) and to HDMI too. Or for you to replicate the content into two framebuffers appropriately, but it still needs the GPU change mentioned at that link.

GPU firmware changes ideally require official RPF engineer interaction as far as I understand, I could be wrong (very likely)..

From what I've read, not many people are experts in writing code for the GPU.

balearicdynamics · Answer

Shabaz are you sure that it is not possible changing the X11 Linux settings? I think - not yet tried - that the HDMI screen can be saw as a second display...

If possible it will be great, don't you think it its the worth to investigate in this direciton ?

balearicdynamics · Answer

(sorry for this fragmented post...) The reason of my suspects just originate from this Wikipedia article: https://en.wikipedia.org/wiki/Display_Serial_Interface So, as the LCD is as a matter of fact a serial display interface, it should not impact with the HDMI output if there are not some specific hardware impeachment.

Enrico

shabaz · Answer

It could be possible to use the same framebuffer (nothing is impossible with software : ) but would require the GPU firmware to know to translate that to both displays, hence GPU firmware mods would be needed.

According to this page https://www.raspberrypi.org/blog/the-eagerly-awaited-raspberry-pi-display/ omxplayer had specific mods to do this without dumping to a framebuffer but they don't state what the mods were. I've never used omxplayer, but if it is open source then that may provide some clues.

Robert Peter Oakes · Answer

It has been confirmed in many forums that you can use both displays simultainiously but not as a mirror or secondary console (YET). you can have one as a console and access the other as shabaz has indicated by using the frame buffer. Raspberry PI org has recognized this feature requirement and is indicating they are working on it for a later release of OS.

shabaz · Answer

To complicate matters further, there is the text here: https://www.adafruit.com/products/2453 According to that, Adafruit's DPI display doesn't simultaneously work with the HDMI output. So we also can't assume that if the DPI based VGA functions with HDMI, that other DPI displays will work with HDMI : ( Looks like the code implementation for enabling DPI based VGA might have been very specific or implemented a framebuffer that Adafruit for some reason decided to not use? I have no idea to be honest. So many combinations : (

How to run HDMI and Official Raspberry Pi Display (DSI) simultaneously?

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