Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

PCI Express on the Raspberry Pi 4

mloduchowski.com

Post reply on HN
Print view View original

91–100 of 166 posts

Re: PCI Express on the Raspberry Pi 4

#91

Post by benj111 »

monocasa wrote: post #85
segfaultbuserr wrote: post #5

It looks like an unreliable modification. Running a GHz-level interface with jumpers is almost impossible to control the impedance, it's a cool Proof-of-Concept though. But is it possible to bring the project to the next level? Is it possible to make a daughterboard with QFN connector? If so, one can make a pin-compatible daughterboard with an extension connector. To use it, just desolder the USB chip and solder a ne…

I've seen pcie literally run over a metal clothes hanger soldered to the board. It's extremely tolerant of terrible quality connections.

How did you reach the situation where the solution was soldering a clothes hanger to a board for pcie ???

Re: PCI Express on the Raspberry Pi 4

#92

Post by dgaudet »

strmpnk wrote: post #79
wang_li wrote: post #75

Aren't PCIe lanes shared? Why would I need to remove the USB 3.0 chip rather than just hooking right to the pins on the device where it's soldered in place? E: Apparently it's the PCI bus that is shared, not PCI Express lanes. Ty.

AFAICT lanes are not shared but there are chipsets which can break lanes out into other sets of lanes which are then routed back onto the original set of lanes. So if your CPU has 16 lanes you can hang a chip off of it which then provides more lanes which are then signaled back to the CPU over some subset of those lanes. It’s not clear if the lanes themselves can be multiplexed with packets from many devices but they…

motherboard features such as x16 or 2x8 are achieved with "pcie mux" chips. these are devices which select which of N pairs of differential wires is attached to the input/output differential pair. search for "pcie mux" will find many, such as [0]. if you look at the diagram you'll see that it connects wire pair A+/A- to either B+/B- or C+/C- based on the value of the SEL line.

these generally basic passive devices operating at analog signals level, no higher layer activity required. however some may exist which operate as "retimers", which do participate in the lowest layer of the PCIe electrical protocols (generally to extend reach). these are unlikely to be used for a typical x16 2x8 sort of motherboard feature though.

the example i picked here is 4 lanes, and you would need 4 such chips to do a x16 2x8. (spoiler: you mux lanes 8-15 from slot X to lanes 0-7 of slot Y, and there are both TX and RX pairs which need muxing.)

there do exist devices called "pcie switches" which operate at all layers of the pcie protocols, and allow for all sorts of sharing of the point-to-point links. examples at microsemi [1] ... for example a 48 lane switch could be used to connect two 16 lane GPUs to a 16 lane slot. this would allow either of the GPUs to burst to the full 16 lanes, or on average if both GPUs are communicating with the host then they would see 8 lanes of bandwidth. there's a picture of such a dual GPU card in this article [2], you can see the PCIe switch ASIC centered in between the two GPUs, above and to the right of the edge connector.

[0] http://www.ti.com/product/HD3SS3412

[1] https://www.microsemi.com/product-directory/ics/3724-pcie-sw...

[2] https://graphicscardhub.com/dual-gpu-graphics-cards/

Re: PCI Express on the Raspberry Pi 4

#93

Post by benj111 »

cesarb wrote: post #42
hvidgaard wrote: post #7

Earlier quoted context omitted.

Wouldn't the gigabit LAN be a better fit for this? If you want to make a cluster, you need to make some custom hardware to connect to, that can facilitate the communication. At this point you're likely spending more that if you just bought a real desktop for more performance. I can see the fun factor in hacking the system together, though.

> Wouldn't the gigabit LAN be a better fit for this? I might be showing my age but... Imagine a Beowulf cluster of these! (For those who don't know the reference, this used to be a common saying in slashdot back in the day.)

"I might be showing my age"

What, above 25?

Re: PCI Express on the Raspberry Pi 4

#95

Post by megous »

londons_explore wrote: post #78
megous wrote: post #76

Earlier quoted context omitted.

https://lb.raspberrypi.org/documentation/hardware/raspberryp... This says, there's OTG controller intended to be used in "peripheral device only" mode. I haven't found datasheet for BCM2711, so it's hard to tell.

> peripheral device only mode. I wonder why? The silicon required is far far simpler for host mode (since you only need a single memory buffer, and you fully dictate the timing, so can pass all the complex stuff up to software).

Maybe they don't have the VBUS circuitry setup for connecting self-powered devices in host mode.

Re: PCI Express on the Raspberry Pi 4

#98

Post by beatgammit »

buildbuildbuild wrote: post #72

Fun. If you want PCIe on a SBC without the soldering, I highly recommend perusing Hackerboards. I'm very happy with my RockPi 4 (4GB RAM, PCIe, USB 3, 6 cores), which I discovered through their excellent database. https://www.hackerboards.com/search_boarddb.php

I think you mean the RockPro64. The Rock64 only has 4 cores and no PCIe.

That being said, I missed the PCIe on the specs last time I was comparing SOCs and I had forgotten about hackerboards, thanks for the reminder!

Re: PCI Express on the Raspberry Pi 4

#99

Post by morphle »

yetihehe wrote: post #3

Hmm, how about using this for fast interconnect for making rpi clusters?

I suggest using the 16 x 12,5 Gbps serdes links of the MPF300 Polarfire FPGA ($159 if you desolder one from a development board) at 5 Gbps speeds to interconnect 16 RPi4. You have 64 ARM cores and 300 softcores on the FPGA with 264 Gbps theoretical bandwidth and 64, 256 to 512 GB of DDR3. Around $719 for the RPI+FPGA for 364 cores 64 GB, more for the extra DRAM. You can add GPU cards of course. If you make a 4-6 layer PCB, you could attach the 16 x 1G and HDMI to the FPGA as well for even more interconnect bandwitdh. Email me for the details or collaborating on building it.

Re: PCI Express on the Raspberry Pi 4

#100

Post by buildbuildbuild »

beatgammit wrote: post #98
buildbuildbuild wrote: post #72

Fun. If you want PCIe on a SBC without the soldering, I highly recommend perusing Hackerboards. I'm very happy with my RockPi 4 (4GB RAM, PCIe, USB 3, 6 cores), which I discovered through their excellent database. https://www.hackerboards.com/search_boarddb.php

I think you mean the RockPro64. The Rock64 only has 4 cores and no PCIe. That being said, I missed the PCIe on the specs last time I was comparing SOCs and I had forgotten about hackerboards, thanks for the reminder!

http://rockpi.org/ :)
Post reply on HN