DSI and Hyperpixel4 Pi3B+

[Azkali]

DSI and Hyperpixel4

I have a Hyperpixel 4" Touchscreen running fine on a Pi3B+. I'm trying to setup a dual monitor with a 5" DSI touchscreen (from DFRobot)

But I can have either one or the other at the same time ( when enabling/disabling enable_dpi_lcd in /boot/config.txt ).

Is it even possible to achieve this configuration ? The finality is to use this config on a pi4 ( I haven't received mine yet so I'm testing it on a Pi3B+ for now )

Here is my /boot/config.txt

pi@raspberrypi ~ % cat /boot/config.txt
[all]
# dtoverlay=vc4-fkms-v3d ## Can print to DSI screen wih fakeKMS
# dtoverlay=vc4-kms-v3d ## Unable to print to both screen with fullKMS

dtoverlay=hyperpixel4

dtparam=touchscreen-swapped-x-y
dtparam=touchscreen-inverted-x

overscan_left=0
overscan_right=0
overscan_top=0
overscan_bottom=0

framebuffer_width=800
framebuffer_height=480

# enable_dpi_lcd=1
# display_default_lcd=1

dpi_group=2
dpi_mode=87
dpi_output_format=0x7f216

display_rotate=3
dpi_timings=480 0 10 16 59 800 0 15 113 15 0 0 0 60 0 32000000 6
hdmi_timings=480 0 10 16 59 800 0 15 113 15 0 0 0 60 0 32000000 6

dtparam=audio=off
disable_overscan=1

enable_uart=off
start_x=1

max_framebuffers=2
framebuffer_priority=1

gpu_mem=112

Output of tvservice :

pi@raspberrypi ~ % tvservice -l                                       
2 attached device(s), display ID's are : 
Display Number 0, type Main LCD
Display Number 3, type Composite

[Gadgetoid]

I believe DPI and DSI are mutually exclusive on all models of Pi. This certainly wont work on anything pre Pi4 and information on the Pi4 is still pretty scant- the closest I get is that, from a hardware perspective, the DSI0 and DPI connections "share a pixel valve"- source: https://www.raspberrypi.org/forums/viewtopic.php?t=243812 and that of the three "pixel valves" one is assigned to HDMI, one to DSI0/DPI and one as a writeback to memory to perform transpose/rotate operations.

On Pi 4 your best bet is to use an HDMI display alongside with DPI or DSI since these should work together.

[Azkali]

Mmhh just checked out there is an open issue on the RPI : https://github.com/raspberrypi/firmware/issues/1094 so technically it should be possible