dgaff
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Multi-Seat use of RPi 4

Tue Jun 25, 2019 2:53 pm

Hey all,

Like literally everyone else, I'm mega pumped for the new version! Already ordered a few. I have a question about a project I'm going to try to pull off before the end of the year and would love some advice / parts lists from those with ideas:

I want a huge wall of displays (ideally 7-10 inch screens) connected to an array of Pi's doing a bunch of stuff. I imagine coordinating that stuff via something like a redis queue the workers listen to. To save money, I'd love to reduce parts as much as possible - if, for example, I can have a single Pi running multiple monitors running independent work, that would be ideal. Ideally, I'd write up the software such that, as a display is detected on the device, it's registered in redis, and a new X session is pinned to the screen automatically, at which point the work starts firing off (i.e., I'd love to do up-front work on the software end to make configuring things as close to plug and play as possible later on). So, a few questions for the community:

1. How many monitors do you think you could squeeze onto the RPi 4, both in terms of physical connections as well as computational overhead? What sort of hardware setup would that number of monitors require? The answer almost all the time to similar questions for the RPi 3 was "1 monitor 1 Pi" but maybe that's changed with the new ports we have? Is there technically a better mini computer to run this off for more monitors per device that I should check out?
2. How would this work on the software end? From some initial looking, it would seem that http://multi-seat.com/raspbian-dual-monitors/ and https://wiki.archlinux.org/index.php/Xorg_multiseat seem to be the natural place to start the adventure - is there something easier to work with / better in some way that should also make the list?

Thanks in advance!

hippy
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Re: Multi-Seat use of RPi 4

Tue Jun 25, 2019 7:28 pm

dgaff wrote:
Tue Jun 25, 2019 2:53 pm
From some initial looking, it would seem that http://multi-seat.com/raspbian-dual-monitors/ ... seem to be the natural place to start the adventure
That guide worked well for setting up dual monitors on my Pi 3B under Stretch, using HDMI plus DisplayLink, but I don't know how it fares with the new frame buffer handling. It also prevented hardware acceleration from working which may be an issue.

It might be a case of try it and see.

incognitum
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 2:06 am

Multi-seat works in principle, with the exception of audio.
Currently that is only supported on one of the micro HDMI ports, not on both.

bunklung
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 11:18 am

incognitum wrote:
Fri Jun 28, 2019 2:06 am
Multi-seat works in principle, with the exception of audio.
Currently that is only supported on one of the micro HDMI ports, not on both.
Seems like a nothing-burger.

I don't think HDMI ARC works anyhow, so you are limited to one microphone input. This means the second user would already need something extra.

Enter the cheap USB audio dongle. Now, if you want 5.1 audio and HD audio pass through you might as well buy another RPi 4B at that point if you want a high end audio for the second user.

bunklung
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 12:33 pm

hippy wrote:
Tue Jun 25, 2019 7:28 pm
dgaff wrote:
Tue Jun 25, 2019 2:53 pm
From some initial looking, it would seem that http://multi-seat.com/raspbian-dual-monitors/ ... seem to be the natural place to start the adventure
That guide worked well for setting up dual monitors on my Pi 3B under Stretch, using HDMI plus DisplayLink, but I don't know how it fares with the new frame buffer handling. It also prevented hardware acceleration from working which may be an issue.

It might be a case of try it and see.
I don't know how related 3D acceleration vs hardware video encoding/decoding in regards to two displays, but one of the engineers made a statement how only one decode would work, but two should work provided they make changes to the frame buffer. Sorry, I might be botching that response since I am going from memory. It was in the general thread I think.

Having only one screen be accelerated would be a bummer for only one user, let alone two.

*Edit* Found it in a different thread, could be limited to just HEVC:

https://www.raspberrypi.org/forums/view ... 9#p1485769

incognitum
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 2:06 pm

bunklung wrote:
Fri Jun 28, 2019 11:18 am
incognitum wrote:
Fri Jun 28, 2019 2:06 am
Multi-seat works in principle, with the exception of audio.
Currently that is only supported on one of the micro HDMI ports, not on both.
Seems like a nothing-burger.

I don't think HDMI ARC works anyhow, so you are limited to one microphone input. This means the second user would already need something extra.
Not everybody needs a microphone.
But software like VLC complained (loudly, spamming messages to screen) that it could not find a playback device during my testing.
And the average user does expect that to work..
Enter the cheap USB audio dongle.
Well, with two USB port assigned to seat 1 for keyboard and mouse, and the other two assigned to seat 2 for keyboard and mouse, there isn't any to plug that dongle in.
Unless you also add a USB hub, or mess with wireless input devices, which I would rather avoid.

bunklung
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 2:48 pm

incognitum wrote:
Fri Jun 28, 2019 2:06 pm
bunklung wrote:
Fri Jun 28, 2019 11:18 am
incognitum wrote:
Fri Jun 28, 2019 2:06 am
Multi-seat works in principle, with the exception of audio.
Currently that is only supported on one of the micro HDMI ports, not on both.
Seems like a nothing-burger.

I don't think HDMI ARC works anyhow, so you are limited to one microphone input. This means the second user would already need something extra.
Not everybody needs a microphone.
But software like VLC complained (loudly, spamming messages to screen) that it could not find a playback device during my testing.
And the average user does expect that to work..
Enter the cheap USB audio dongle.
Well, with two USB port assigned to seat 1 for keyboard and mouse, and the other two assigned to seat 2 for keyboard and mouse, there isn't any to plug that dongle in.
Unless you also add a USB hub, or mess with wireless input devices, which I would rather avoid.
The average user won't be the person setting up dual seat.

Some keyboards have hubs (like the official Pi keyboard) and if you already need to purchase a USB dongle for sound, what's another cheap hub in the shopping cart?

incognitum
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Re: Multi-Seat use of RPi 4

Fri Jun 28, 2019 3:01 pm

bunklung wrote:
Fri Jun 28, 2019 2:48 pm
The average user won't be the person setting up dual seat.
But is that because it is currently unnecessarily complicated to setup, or is there really no demand for it?
I do think multiseat setups offer obvious cost saving advantages for anyone in need for more than one workstation, e.g. schools.

bunklung
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Re: Multi-Seat use of RPi 4

Sat Jun 29, 2019 11:09 pm

incognitum wrote:
Fri Jun 28, 2019 3:01 pm
bunklung wrote:
Fri Jun 28, 2019 2:48 pm
The average user won't be the person setting up dual seat.
But is that because it is currently unnecessarily complicated to setup, or is there really no demand for it?
I do think multiseat setups offer obvious cost saving advantages for anyone in need for more than one workstation, e.g. schools.
I agree 100%. It would be great if dual seat was a simple check box. This would be great for schools/governments on a budget.

The hardware limitations would still be an issue for some, 3D acceleration, sound, # of USB ports etc.

incognitum
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Re: Multi-Seat use of RPi 4

Sat Jun 29, 2019 11:28 pm

bunklung wrote:
Sat Jun 29, 2019 11:09 pm
The hardware limitations would still be an issue for some, 3D acceleration, sound, # of USB ports etc.
Haven't tested any demanding 3D accelerated stuff like games, but graphics wise it is good enough for normal desktop use.
Playing video simultaneously on both seats using normal desktop applications like Chromium and VLC worked ok, apart from VLC printing "I miss an audio device" errors on top.
multiseat-configuration.jpg
multiseat-configuration.jpg (94.25 KiB) Viewed 2035 times
Something that will probably be fixable in software.

bunklung
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Re: Multi-Seat use of RPi 4

Sat Jun 29, 2019 11:42 pm

incognitum wrote:
Sat Jun 29, 2019 11:28 pm
bunklung wrote:
Sat Jun 29, 2019 11:09 pm
The hardware limitations would still be an issue for some, 3D acceleration, sound, # of USB ports etc.
Haven't tested any demanding 3D accelerated stuff like games, but graphics wise it is good enough for normal desktop use.
Playing video simultaneously on both seats using normal desktop applications like Chromium and VLC worked ok, apart from VLC printing "I miss an audio device" errors on top.

multiseat-configuration.jpg

Something that will probably be fixable in software.
Very nice! Perhaps you could make a turn key script for everyone or do a nice write up as it relates to the RPi 4? :)

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jbeale
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Re: Multi-Seat use of RPi 4

Sun Jun 30, 2019 12:13 am

It's impressive that a single Pi4 can work reasonably enough for two simultaneous users each using their own display.
As far as classroom use goes, I somehow thought that a normal desktop monitor would be at least 2x or 3x as expensive as a Pi, meaning for a new install you don't achieve anything like 2 seats for the price of 1 simply by reducing the number of Pi boards needed. But maybe you do if you already have the monitors, keyboards, mice, and cables.

...and anyway I guess it is a bit faster than the first timesharing system I used... 0.64 MIPS total across maybe 20 users, on an IBM System/370 :-)

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Gavinmc42
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Re: Multi-Seat use of RPi 4

Sun Jun 30, 2019 1:24 am

...and anyway I guess it is a bit faster than the first timesharing system I used... 0.64 MIPS total across maybe 20 users, on an IBM System/370
Well with all those uarts we could connect a 1/2 dozen Zeros and have a bunch of serial terminals.
So one Pi4 and some Zero's could do 8 seats?
That just has to lower the cost of seats.

All we need now is some $10 screens?
But wait I get them at secondhand shops :D
The HDMI to DVI cable/adaptor usually costs more :lol:
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incognitum
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Re: Multi-Seat use of RPi 4

Sun Jun 30, 2019 1:37 am

jbeale wrote:
Sun Jun 30, 2019 12:13 am
As far as classroom use goes, I somehow thought that a normal desktop monitor would be at least 2x or 3x as expensive as a Pi, meaning for a new install you don't achieve anything like 2 seats for the price of 1 simply by reducing the number of Pi boards needed. But maybe you do if you already have the monitors, keyboards, mice, and cables.
Yes, many already have older monitors laying around.
May only need a HDMI<->DVI adapter to put them to good use.
Last edited by incognitum on Sun Jun 30, 2019 1:49 am, edited 1 time in total.

incognitum
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Re: Multi-Seat use of RPi 4

Sun Jun 30, 2019 1:48 am

bunklung wrote:
Sat Jun 29, 2019 11:42 pm
Very nice! Perhaps you could make a turn key script for everyone or do a nice write up as it relates to the RPi 4? :)
If you don't need 3D acceleration (which is currently a bit problematic as the fkms driver only exposes one dri device), the following is easiest:

- start a terminal

Code: Select all

sudo leafpad /boot/config.txt
Comment out the fkms line (change "dtoverlay=vc4-fkms-v3d" to "#dtoverlay=vc4-fkms-v3d")
Save the file, close leafpad AND REBOOT.


- create a file /etc/udev/rules.d/99-seat.rules to assign the keyboard and mouse connected to the two USB ports on the right to seat1 and the others will default to seat0

Code: Select all

sudo leafpad /etc/udev/rules.d/99-seat.rules

Code: Select all

# Assign the blue USB 3 ports to the second seat

TAG=="seat", ATTR{phys}=="usb-0000:01:00.0-1.1*", ENV{ID_SEAT}="seat1"
TAG=="seat", ATTR{phys}=="usb-0000:01:00.0-1.2*", ENV{ID_SEAT}="seat1"
SUBSYSTEM=="graphics", KERNEL=="fb[0-9]", TAG+="master-of-seat"
SUBSYSTEM=="graphics", KERNEL=="fb1", ENV{ID_SEAT}="seat1"
- Modify /usr/share/X11/xorg.conf.d/99-fbturbo.conf

Code: Select all

sudo leafpad /usr/share/X11/xorg.conf.d/99-fbturbo.conf

Code: Select all

Section "Device"
        Identifier      "fb0"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb0"
	MatchSeat       "seat0"
        Option          "SwapbuffersWait" "true"
EndSection

Section "Device"
        Identifier      "fb1"
        Driver          "fbturbo"
        Option          "fbdev" "/dev/fb1"
        MatchSeat       "seat1"
        Option          "SwapbuffersWait" "true"
EndSection
- Uncheck "auto-login as pi user" in the Raspberry Pi configuration utility.
- Add multiple users to the systems and add them to the gpio and video groups like:

Code: Select all

sudo adduser user1
*choose password and press enter a couple times*
sudo adduser user1 gpio
sudo adduser user1 video
When you reboot again now, you will be prompted with a login screen on each screen.
Make sure you login under a different username on each screen.

ejolson
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Re: Multi-Seat use of RPi 4

Thu Jul 04, 2019 11:42 am

jbeale wrote:
Sun Jun 30, 2019 12:13 am
But maybe you do if you already have the monitors, keyboards, mice, and cables.
In many school contexts this is a very reasonable assumption. Surplus monitors and keyboards seem to accumulate without bound. So do the old PCs, but in an effort to decrease effort and increase cost, IT support refuses to support PCs more than about five years old. There used to be just enough money left over to buy chalk, but now we're stuck with whiteboard markers and whiteboards.

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Gavinmc42
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 8:18 am

The Pi4 audio is not using the PWM now?
So a simple PWM to audio hat could be made for a second audio output?

Hey is the audio on both HDMI channels?
Plugged into TV's there would be separate audio?
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incognitum
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 8:50 am

Gavinmc42 wrote:
Fri Jul 05, 2019 8:18 am
Hey is the audio on both HDMI channels?
Currently only on one at a time.

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Gavinmc42
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 10:08 am

Currently only on one at a time.
Hardware or software limitation?
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Raspberries are not Apples or Oranges

incognitum
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 10:11 am

Gavinmc42 wrote:
Fri Jul 05, 2019 10:08 am
Hardware or software limitation?
No idea.
Audio is currently handled by the firmware, closed source and such. So as outsider I do not know.

Perhaps things will be better when the switch to normal full kms is made, instead of firmware kms.

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Gavinmc42
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 10:38 am

Wonder if SonicsPi outputs to HDMI?
I mostly use old DVI monitors and have only played a little with SonicPi.
Plugging Zero's into those uarts, how many SonicPi can be got running at once?
Full orchestra, done with a few kids?
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jamesh
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 1:43 pm

Gavinmc42 wrote:
Fri Jul 05, 2019 10:08 am
Currently only on one at a time.
Hardware or software limitation?
Software. I'm looking at this at the moment.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
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bunklung
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 2:41 pm

jamesh wrote:
Fri Jul 05, 2019 1:43 pm
Gavinmc42 wrote:
Fri Jul 05, 2019 10:08 am
Currently only on one at a time.
Hardware or software limitation?
Software. I'm looking at this at the moment.
I am very impressed of how interactive and open you all are to feedback and questions.

Most kinds of business platforms are never this interactive and straight up with communications. I think this is one of the reasons why the RPi is so successful.

jamesh
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 3:21 pm

bunklung wrote:
Fri Jul 05, 2019 2:41 pm
jamesh wrote:
Fri Jul 05, 2019 1:43 pm
Gavinmc42 wrote:
Fri Jul 05, 2019 10:08 am

Hardware or software limitation?
Software. I'm looking at this at the moment.
I am very impressed of how interactive and open you all are to feedback and questions.

Most kinds of business platforms are never this interactive and straight up with communications. I think this is one of the reasons why the RPi is so successful.
We are a very small company (RPF(Trading)). We had an all hands on Wednesday. We all fitted in to one room, albeit boardroom sized. This means communications work quite well, no layers of management to fight through etc. We try to be as open as possible, but obviously there is loads of fun stuff we cannot talk about!
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rpdom
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Re: Multi-Seat use of RPi 4

Fri Jul 05, 2019 3:47 pm

jamesh wrote:
Fri Jul 05, 2019 3:21 pm
We are a very small company (RPF(Trading)). We had an all hands on Wednesday. We all fitted in to one room, albeit boardroom sized. This means communications work quite well, no layers of management to fight through etc. We try to be as open as possible, but obviously there is loads of fun stuff we cannot talk about!
Sounds like an ideal working environment. I would love to work somewhere like that, even in a minor role. Not all small companies are so communicative and open though.

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