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Triple display with Raspberry Pi 4

Thu Jun 27, 2019 8:04 am

The Raspberry Pi 4 has three video/display interfaces with two micro HDMI ports, and one MIPI DSI connector.
I understand it's currently to drive two displays over HDMI with up to 1080p60 resolution each, and later maybe 4K30 will be supported. But it is also possible to connect an LCD display to the MIPI DSI interface to get three independent displays, or alternatively just one 4K display connected via HDMI plus the LCD display connected to the MIPI DSI connector. Thanks in advance.

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Re: Triple display with Raspberry Pi 4

Thu Jun 27, 2019 8:34 am

There is also DPI on GPIO pins so that makes it four now :-)
On older Pis I think all three (HDMI,DSI,DPI) worked at the same time in some way.

And there are also those cheap Fresco Logic (FL2000) USB3.0 video cards (popular for SDR) that may work now with Pi4 too.

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Re: Triple display with Raspberry Pi 4

Thu Jun 27, 2019 8:57 am

Does everybody forget good old composite video on the TRRS jack?

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Re: Triple display with Raspberry Pi 4

Thu Jun 27, 2019 10:11 am

Not currently, but the hardware potentially could feed 3 devices simultaneously.

There are 3 HVS (Hardware Video Scalar) channels, which each feed into one of the 5 pixel valves at a time, and from there on to output devices. There are a couple of restrictions (eg DSI0 and DPI share a pixel valve), but routing of HVS channel to PV is pretty flexible.

History has only ever required driving 2 displays simultaneously, and so the third HVS channel is assigned in the software to the transposer - it writes back to memory and is used should you wish to transpose (rotate) the display, and also currently for some of the composition with VLC.
If you don't need the transposer then HVS channel 2 would be available and could be used for a 3rd display.

HOWEVER, I see no need to do that in the firmware driver. A full DRM/KMS driver is in the pipeline for Pi4, and with the configuration of that through device tree I can see the possibility of exposing 3 displays.
Memory bandwidth and the V3D max composition size of 7680x7680 are going to limit what is practical, but it's far to early to say where those limits will sit.
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