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TheMindVirus
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Raspberry Pi GPU Concept (" Model D ")

Wed Feb 26, 2020 5:31 pm

I have a few ideas that I'd like to share with the community, in the form of a product specification/guide.
The structure of the information on this page is subject to change as it is still a work in progress.
I hope you can understand this is too much for me to take on individually, so this is a project for the community.

The ultimate goal is to form a fully modular and recyclable computing ecosystem, right down to the components that make the hardware and software we use. This is to minimise the amount of components that have to be scrapped because they become incompatible.
FPGA's are examples of chips which are reconfigurable and can be used to adapt to changes in hardware and software.

The goal for this topic is to address the issues of supply and demand of the Pi0 and provide an alternative for people who want to build a Cluster of Raspberry Pi's. This is perhaps because people are interested to learn about datacenters, or want faster 3D graphics so that they have access to the virtual world. With this interest often comes the expectation of hardware to support real-time big-data streaming, VR experiences and the Virtual Internet (Cyberspace).

What I am proposing is a series of Compute Cards, each is a Grid Node offering a different piece of retro/future computing to the system.

The baseline is a Pi ARM system for control, FPGA for routing and custom logic, then an option of something like a Motorola 68k, RISC-V chip or IBM PowerPC (etc. a long list but focus on one at a time per card). It showcases each system for what it is, why it was created, how it works and what if it had software using principles from today. Either the physical chip or an FPGA hardware emulation could be implemented. An example of future hardware would involve emulating a device which can perform matrix/multidimensional instructions like graphics cards can.

Here are a few product prototypes (your ideas for the design may vary from this one):
PiD Prototypes.jpg
PiD Prototypes.jpg (127.93 KiB) Viewed 4762 times
PiD Board & Schematic.jpg
PiD Board & Schematic.jpg (95.41 KiB) Viewed 4762 times
The product idea is intended as an educational tool for people to learn how a Graphical Processing Unit (GPU) works inside a computer.
It would see users developing the tool to support new applications and enhance the graphics capability of the Pi as an external GPU.
It could see use as a cheap, low-power, more versatile option for performing intelligence tasks in Datacenters and Compute Clusters.
It is also gives a purpose for the Compute Module 1 & 3, besides being an industrial approach to the Raspberry Pi.

Compute Cards are devices which provide new processing architectures and instructions to the existing computing experience.
For example, a Pi Compute Card could provide all the resources of an ARM System-on-Chip (SoC) to an existing desktop (with only x86, x64).
A Coprocessor is a device which works alongside a main processor, to achieve more work in the same timeframe (in parallel).
A GPU is a type of Processor dedicated to solving graphics tasks. It is sometimes found as part of a SoC and as a separate Coprocessor Card.

One potential name I thought about is "Model D".
I chose this name for the following reasons:
  • PiD is the circumference of a circle
  • D is for Datacenter/Desktop
  • D is also for Delta (the others are Model Alpha, Model Beta and Model 0)
  • Pi3D/Pi4D emphasise the dimensions of space and time that GPU’s draw on the screen
However, it might be already trademarked by Behringer, who make the Model D synthesizer.
It's informally known as the "Rube Goldberg Contraption", as first mentioned by pagenotfound.
It is a nickname given to a machine which is perhaps more complex than it needs to be to perform its simple task.


Here is an example route-to-market for the product:
PYNQ-Z1.jpg
PYNQ-Z1.jpg (16.98 KiB) Viewed 4047 times
The goal is to make FPGA computing simpler and more accessible for beginners and enthusiasts.
It revolves around creating a Raspberry Pi out of the existing PYNQ-Z1 dev board. The processor is custom configured inside a Xilinx Zynq FPGA.
PYNQ is a platform using Python to make coding on FPGA's simpler. The Z1 is a collaboration between Xilinx and Digilent for manufacturing.
The board can already run Linux and has a lot of the hardware pre-designed. The Zynq Ultrascale should also have the PCIe Endpoint Block.

Resources
What is PYNQ?: http://www.pynq.io/home.html
Xilinx Firmware: https://github.com/Xilinx/PYNQ/tree/mas ... ds/Pynq-Z1
PYNQ-Z1 Reference Manual: https://reference.digilentinc.com/_medi ... ynq-rm.pdf
Digilent Manufacturer Specification: https://reference.digilentinc.com/refer ... q-z1/start

Tasks
  • Visualise a block diagram of a Raspberry Pi and map it onto the hardware on a PYNQ-Z1 (or similar board)
  • Design a custom board in PCB design software, using the PYNQ-Z1 as a template (schematics provided in the links above)
  • Print the custom board (or buy a dev board), get the hardware operational and see if it can run elements of Raspbian (e.g. Pixel Desktop)
  • See what else the Zynq FPGA can do - the hardware is configurable in Verilog/VHDL and has a HDMI Input port (and PCIe Endpoint Block)
Additional Specifications
  • Make the board smaller than the PYNQ, either Pi Model B size or as a PC Card (same shape defined by PCIe/PCI/ISA specifications)
  • Use the audio circuitry and other hardware design from the PYNQ-Z1 Reference Manual in the links above (or use your own if it works)
  • Use minimal components to bring the price down as close to $35 as possible (without too much sacrifice)
  • Remove unnecessary headers (e.g. Arduino Headers which can work across USB instead)
Other Examples
ZynqBerry PYNQ-Z2 IceZero
Last edited by TheMindVirus on Thu Apr 30, 2020 12:54 am, edited 42 times in total.
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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 6:44 pm

Can you please

1. clearly state what the ultimate purpose of your Rube Goldberg contraption is

2. provide a labeled drawing

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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 7:30 pm

It seems that you are effectively turning a Pi Zero into a DisplayLink device, and making it compatible with DisplayLink 2.0 would actually be something useful to have. That way all the drivers and gubbins on the host side sending the image is already include in Linux which would save a lot of work, and it would also be usable with Windows and other PC's.

The main problem with that plan is that DisplayLink support on a Pi seems to be badly broken at present.

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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 7:47 pm

Raspberry Pi GPU.png
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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 8:19 pm

It would probably help if there was a Raspberry Pi Compute Module with USB3.0. But there isn't (yet). Also you won't get gigabit ethernet working on one. The best you'll get is about 350Mb/s via the single USB2.0 channel.

Things may change if a CM4 comes out.
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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 10:03 pm

OK, thar's better. I mean, your first post started like this
...Pi0 being powered by a Pi Keyboard Hub. The Keyboard is powered by a USB KVM switch which is powered by a Microsoft Surface Charger. The Surface is connected to the KVM switch as well as the Pi0 itself using a USB OTG adapter. A USB WiFi adapter is also plugged into the Pi Keyboard..
and proceeded to mention a few more components, HW and SW...

Now that it's reduced to the essential parts, I think I get it. While I realize this is probably meant for fun and learning, you are aware that the built in graphics hardware of a modern PC motherboard may beat the Pi's GPU, especially considering the overhead of getting your shaders to the point where they can be executed?

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Re: Raspberry Pi GPU Concept

Wed Feb 26, 2020 10:17 pm

Of course at this stage a lot of host machine hardware and existing GPU's would outperform this card. However, this card is built for learning how it works and educating people on graphics hardware. It may even provide more opportunities for expansion than existing GPU's because it is a full-blown Linux Coprocessor with a custom OS running on it. The PCIe structure keeps it installed in place and prevents movement so that it can potentially reach its full bandwidth capability. Initially it just uses USB hub chips because they are easier to get hold of.

In reply to previous comments:

A Raspberry Pi Compute Module 4 would be amazing. The CM in the picture is just a placeholder, but the new Raspberry Pi 4 could be a working alternative for prototyping. It has both power and ethernet running on one cable. It's a simpler version of the above the design (USB eGPU!). I would hope that one day the USB hub eventually gets replaced by a System Management Controller (SMC) using the full PCIe-x4/x16 bandwidth.

One step beyond: Add an FPGA, educate people on VHDL by making custom matrix operations, make it so easy to use that even a child could start making custom shaders using it.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 1:14 am

Just saw the post about DisplayLink 2.0 and realised it's not the same as DisplayPort...
There was once a thread about it: viewtopic.php?t=202518
Really nice products though, this GPU could use it as a fallback like Microsoft Basic Display Adapter if there's no signal from the Pi.

New shields could fit into other PCI card slots but they attach directly to the Pi GPU (instead of the host machine) on that flying ribbon cable ^^^.
If you want the rear USB ports to be accessible on the back of the PC, a USB extension PCI bracket is available to buy from most online electronics stores.

The Pi doesn't need the extra +12V molex power connectors like traditional GPU's do, unless it was to be used on a motor driver card.
The on-board powered USB hub may however need extra power to drive peripherals (if it cannot be drawn from the PCIe slot itself).

Instead of removing the SD card to change software, it would be ideal if instead the SD card was available from the Host Machine while the Pi is sleeping. It would need a power signal (On/Reset button!) to wake it up again and boot the new software. The Intel Xeon Phi used to work like this but the design was overcomplicated. It could be as straight-forward to use as a Pi. (avoided the obvious Xeon Pi pun)

Raspberry Pi Slice anyone?
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 6:38 am

This is a bit of a rough render, but you get the idea. The Pi Slice GPU.
RPiSliceGPU (2).png
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 12:30 pm

There's a major problem that no-one so far has mentioned: there is only one USB port on a zero/CM.

The moment you put it into gadget mode you lose all host functionality so no acess to your Gigabit ethernet and your "USB Boot & media".

I also have doubts about your plan to connect everything via a "PCIex1 USB hub" as that'll lead to your devices being connected to the host PC not the zero/CM which will have to go through the host to access them so no USB boot etc.

Interesting idea (and one I considered briefly when the CM launched) but not as straight forward as you imagine.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 2:21 pm

TheMindVirus wrote:
Wed Feb 26, 2020 10:17 pm
A Raspberry Pi Compute Module 4 would be amazing. The CM in the picture is just a placeholder, but the new Raspberry Pi 4 could be a working alternative for prototyping.
Your ultimate plans seem a little ambitious to me, which will probably have some people thinking you're in the realms of fantasy there and it's just not credible, but a Pi 4B would seem a good starting point.

The Pi 4B has two HDMI outputs, 4K support, USB 3.0, and can be used in gadget mode via USB-C and, with some pretty major tinkering, can provide PCIe. Plus there's all the rest a 4B has.

Maybe start with something more achievable and build up to what you'd really like after than.

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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 3:03 pm

The PCIe-x1 USB Hub is also a placeholder for an interface chip with the Host PC. It is there to provide power and Ethernet. PCIe is too high bandwidth for the Pi so we use a bus that is within its capability instead.

The rest of the USB ports come from the Pi. If there is only one available, it can be expanded by an additional USB hub. No need to over complicate it with additional chips, but the Gigabit Ethernet could either come from a Pi4-style chipset or an onboard 2-port switch.

The HDMI and Audio are placeholders for either DisplayLink or DisplayPort Hubs (for more screens at once from one port) but since the Pi4 already has a HDMI and Audio port, it makes sense to include these in the current design.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 3:14 pm

TheMindVirus wrote:
Thu Feb 27, 2020 3:03 pm
The PCIe-x1 USB Hub is also a placeholder for an interface chip with the Host PC. It is there to provide power and Ethernet. PCIe is too high bandwidth for the Pi so we use a bus that is within its capability instead.

The rest of the USB ports come from the Pi. If there is only one available, it can be expanded by an additional USB hub. No need to over complicate it with additional chips, but the Gigabit Ethernet could either come from a Pi4-style chipset or an onboard 2-port switch.

The HDMI and Audio are placeholders for either DisplayLink or DisplayPort Hubs (for more screens at once from one port) but since the Pi4 already has a HDMI and Audio port, it makes sense to include these in the current design.

As already mentioned if you configure the RPi Zeros USB Port to "Gadget Mode" it is a Slave, therefore a USB Hub is also a Slave, USB needs a Master / Slave scenario.

The 4B is better because the Power Port can be configured to "Gadget Mode", which means powering via the Header Pins, but it will give you 4 USB Ports etcetera.


Still feel that when you put it all together it will not be a capable GPU as you envisage, but at least you will learn a lot !!
The information is out there....you just have to let it in.

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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 3:38 pm

I hope it can be an educational tool for STEM students to learn how their GPU works.
Good to see people are trying it already, this is an attempt at attaching a Pi to a custom PCI Bracket:
https://www.thingiverse.com/thing:2720009

The prototype is just a Pi4, a USB Hub PCIe Card, a HDMI Extension Bracket and a few cables. The next generation Pi’s could be more and more capable of this task.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 4:27 pm

TheMindVirus wrote:
Thu Feb 27, 2020 3:38 pm
I hope it can be an educational tool for STEM students to learn how their GPU works.
Good to see people are trying it already, this is an attempt at attaching a Pi to a custom PCI Bracket:
https://www.thingiverse.com/thing:2720009

The prototype is just a Pi4, a USB Hub PCIe Card, a HDMI Extension Bracket and a few cables. The next generation Pi’s could be more and more capable of this task.
I think a PCIe card that holds a compute module is a nice idea. At present, it might be more reasonable to create a similar carrier for a Pi 4B. Maybe it would be possible to mechanically position the 4B so the Ethernet and USB ports are available at the back of the PC and then route the two micro-HDMI connections via adapters as two full-sized HDMI ports on a bracket. The carrier itself could be a USB3 adapter and communication with the PC could still be done through gadget mode using the USB-C port on the 4B.

It would be nice if the card included the ability to reset the PC and boot the PC from an image on the Pi.

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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 5:05 pm

ejolson wrote:
Thu Feb 27, 2020 4:27 pm
TheMindVirus wrote:
Thu Feb 27, 2020 3:38 pm
I hope it can be an educational tool for STEM students to learn how their GPU works.
Good to see people are trying it already, this is an attempt at attaching a Pi to a custom PCI Bracket:
https://www.thingiverse.com/thing:2720009

The prototype is just a Pi4, a USB Hub PCIe Card, a HDMI Extension Bracket and a few cables. The next generation Pi’s could be more and more capable of this task.
I think a PCIe card that holds a compute module is a nice idea. At present, it might be more reasonable to create a similar carrier for a Pi 4B. Maybe it would be possible to mechanically position the 4B so the Ethernet and USB ports are available at the back of the PC and then route the two micro-HDMI connections via adapters as two full-sized HDMI ports on a bracket. The carrier itself could be a USB3 adapter and communication with the PC could still be done through gadget mode using the USB-C port on the 4B.

It would be nice if the card included the ability to reset the PC and boot the PC from an image on the Pi.
I'm not sure organisations would take it on if it were able to boot different software on the host machine, but nice idea. It would be possible if the Pi4 with USB-C was adapted to be a USB Mass Storage Device as well as USB Ethernet Adapter. The Beaglebone used to do both at the same time, the drivers were even included on the Mass Storage. The card is therefore intended for developers and educators, it should ideally not be used for production unless it is made secure (similar to the actual Pi).

I was talking to a friend of mine about the audio jack, it seems the second ring of Tip-Ring-Ring-Sleeve is used for composite video. Is anyone exploring the possibility of this becoming a microphone input so that 3.5mm headsets could start working? The workaround right now is to use an audio hat, but needs special adapters.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 5:23 pm

TheMindVirus wrote:
Thu Feb 27, 2020 3:38 pm
I hope it can be an educational tool for STEM students to learn how their GPU works.
I'm not sure how many STEM students would be interested in what you are proposing, or would need all the advanced gubbins which will come with it, PCIe connectivity and the like.

I can see the merit in having a means to easily specify shaders and the like so people can see the effect they have, learn how to do it etc. But it seems to me that could perhaps be better done using just a Pi 4B, using one display as the desktop, the other to see the results of what they are doing.

I don't really see the point of connecting it all to a PC's PCIe bus. If one wanted to control it from a PC one could use standard networking and a browser.

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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 5:35 pm

hippy wrote:
Thu Feb 27, 2020 5:23 pm
TheMindVirus wrote:
Thu Feb 27, 2020 3:38 pm
I hope it can be an educational tool for STEM students to learn how their GPU works.
I'm not sure how many STEM students would be interested in what you are proposing, or would need all the advanced gubbins which will come with it, PCIe connectivity and the like.

I can see the merit in having a means to easily specify shaders and the like so people can see the effect they have, learn how to do it etc. But it seems to me that could perhaps be better done using just a Pi 4B, using one display as the desktop, the other to see the results of what they are doing.

I don't really see the point of connecting it all to a PC's PCIe bus. If one wanted to control it from a PC one could use standard networking and a browser.
True, you can just use a Pi4 on a USB port for this. Maybe this is the way that GPU's will be heading with some encouragement. The point is to replicate the current format of a GPU and then develop it towards something which can be used externally without hardware faults occurring.
The other reason that it's PCIe is because it is locked in place, you can fairly safely assume that it won't be removed while the system is running.

Yes, I am aware that this exists: https://pcsupport.lenovo.com/gb/en/solutions/acc500012
It could do so much more if it was a Pi.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 6:41 pm

hippy wrote:
Thu Feb 27, 2020 5:23 pm
I don't really see the point of connecting it all to a PC's PCIe bus. If one wanted to control it from a PC one could use standard networking and a browser.
That's largely why I never went further than a brief though expermient on this back when the CM launched. Then the zero came along and invalidated that idea. Why bother with a custom PCIe card when you can just hang a zero off a USB port?

Having a bunch of CM or Zeros on a PCIe card doesn't seem to add much unless all you want is the compute power with no access to the outside world except via the host PC.

The limiting factor is the single USB port on the Pi's SoC. Yes it can do both host and device modes, yes it can swap between them. What it can't do is both at the same time and swapping between modes will disconnect stuff that requires the other one.

Yes, it's possible to add USB host and ethernet via SPI but that'll be slower still.

To those suggesting a 4B, yes it can do device and host at the same time but the "as device" connection will be restricted to USB 2 speeds at best. Plus both the power required and heat generated are higher.

Moving on to the OP's idea to use the zero/CM/4B as a GPU for the host PC, the interesting/challenging work there is going to be in writing the drivers. You'll need to present to the host PC as a GPU but route the comms between PC and Pi over ethernet (Yes ethernet, you're emulating that over USB) at which point you might as well skip the additional expense and complexity of a custom PCIe card and just connect over a real ethernet connection. Or, as has been suggested, emulate the displaylink protocol/hardware over USB.

Plus on the Pi end. you'll need to to detect and handle power off, sleep, hybernate, etc of the host PC or you'l end up with corrupt storage. You might be able to run a small number of Zeros off the PC's 5v standby line but there won't be enough current there to run even a single 4B with peripherals (likely a maximum of 2.5A less whatever the motherboard is using).

Sorry to rain on everybody's parade.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 6:48 pm

TheMindVirus wrote:
Thu Feb 27, 2020 5:35 pm
True, you can just use a Pi4 on a USB port for this. Maybe this is the way that GPU's will be heading with some encouragement. The point is to replicate the current format of a GPU and then develop it towards something which can be used externally without hardware faults occurring.
The other reason that it's PCIe is because it is locked in place, you can fairly safely assume that it won't be removed while the system is running.
You don't need PCIe for that. Just mount the chosen Pi model inside the PC's case and connect it via a spare front panel USB header on the motherboard.
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Re: Raspberry Pi GPU Concept

Thu Feb 27, 2020 8:15 pm

TheMindVirus wrote:
Thu Feb 27, 2020 5:05 pm
I was talking to a friend of mine about the audio jack, it seems the second ring of Tip-Ring-Ring-Sleeve is used for composite video. Is anyone exploring the possibility of this becoming a microphone input so that 3.5mm headsets could start working? The workaround right now is to use an audio hat, but needs special adapters.
No RPi model to date has had an analogue input of any variety. Until that changes, any "exploring" of repurposing a contact on the 3.5mm jack is pointless.
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Re: Raspberry Pi GPU Concept

Fri Feb 28, 2020 1:00 am

To those that say exploring these new avenues in hardware design is pointless, I say that this is precisely how new discoveries are made. There is a lot we can learn from experiments like this (especially in this case about graphics and how to process memory more efficiently in 2D, 3D, ND instead of just linearly).

I personally would like to see this product alongside the Compute Kit as a solution for Compute Modules, potentially convincing me to buy a module. It may not be the fastest card or the most practical solution, but people still buy the form factor because of its appearance and functional value.
It also provides more real estate for tech companies to put their chips on the board, especially in the field of High Performance Computing (HPC).
I just want one because I'm a fan of Rube Goldberg machines, probably why I have a load of VST's that do the same thing with unique character.
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Re: Raspberry Pi GPU Concept

Fri Feb 28, 2020 12:17 pm

TheMindVirus wrote:
Fri Feb 28, 2020 1:00 am
To those that say exploring these new avenues in hardware design is pointless, I say that this is precisely how new discoveries are made.
I'm not saying it's pointless just that it's more complex and therefore more expensive than other options. And that there are limitations in the current hardware that make some aspects of your proposal impossible to impliment.
There is a lot we can learn from experiments like this (especially in this case about graphics and how to process memory more efficiently in 2D, 3D, ND instead of just linearly).
Sure. But you can learn all that in other ways too. On the Pi or by using a $20 graphics card in your PC.
I personally would like to see this product alongside the Compute Kit as a solution for Compute Modules, potentially convincing me to buy a module. It may not be the fastest card or the most practical solution, but people still buy the form factor because of its appearance and functional value.
That's a pretty niche market.
It also provides more real estate for tech companies to put their chips on the board, especially in the field of High Performance Computing (HPC).
Pi as an HPC? That keeps coming up. Doesn't matter how many you cluster together, it'll never fall into that category. It does work as a learning tool though.
I just want one because I'm a fan of Rube Goldberg machines, probably why I have a load of VST's that do the same thing with unique character.
That's as good a reason as any.

You need to do more research on what the current Pi hardware can and cannot do. And on what drivers would be required and how to produce them. I suspect it'll be much more work than you're expecting.

At the software level what you're basicaly proposing is a GPU connected to the host PC via ethernet. I'm not sure that any of the mainstream GPU companies makes such a thing.
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Re: Raspberry Pi GPU Concept

Fri Feb 28, 2020 1:13 pm

TheMindVirus wrote:
Fri Feb 28, 2020 1:00 am
To those that say exploring these new avenues in hardware design is pointless, I say that this is precisely how new discoveries are made.
I don't think anyone is saying it's pointless. Rather that it would seem to appeal to a limited market, that there may be easier ways to do it, or ways which may give it an appeal to a larger audience.

My main point was that any specialised gizmo will mostly appeal to those who already want that and can afford it. Something which is purely Pi 4B based will invite in a larger audience who would not invest in something more specialised, may attract and lead people into something they never thought they had any interest in.
TheMindVirus wrote:
Fri Feb 28, 2020 1:00 am
I personally would like to see this product alongside the Compute Kit as a solution for Compute Modules, potentially convincing me to buy a module.
It seems unlikely to me that the Foundation would provide such a product themselves but I doubt they would stand in the way of anyone doing it themselves and would offer every encouragement in doing that and best wishes for such a venture.

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TheMindVirus
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Re: Raspberry Pi GPU Concept

Sat Feb 29, 2020 5:34 pm

Thanks everyone for the kind words of support for the venture.
I may try making my first prototype PCI PCB as a custom compute module board as small as I can make it. Different components will become available during that time.

Meanwhile AMD have just announced this:
https://www.anandtech.com/show/15554/a ... ting-at-6w
It’s a 6W version of the Ryzen APU.
It would make a perfect Pi Pro imo.
Alastair Cota
Embedded Firmware Developer
[ERR]: Sense of Humour Failure

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