It probably goes without saying that hardware hackers were excited when the Raspberry Pi 4 was announced, but it wasn’t just because there was a new entry into everyone’s favorite line of Linux SBCs. The new Pi offered a number of compelling hardware upgrades, including an onboard PCI-Express interface. The only problem was that the PCIe interface was dedicated to the USB 3.0 controller; but that’s nothing a hot-air rework station couldn’t fix.
We’ve previously seen steady-handed hackers remove the USB 3.0 controller on the Pi 4 to connect various PCIe devices with somewhat mixed results, but [Colin Riley] has raised the bar by successfully getting a PCIe multiplier board working with the diminutive Linux computer. While there are still some software kinks to work out, the results are very promising and he already has a few devices working.
Getting that first PCIe port added to the Pi 4 is already fairly well understood, so [Colin] just had to follow the example set by hackers such as [Tomasz Mloduchowski]. Sure enough, when he plugged the port multiplier board in (after a bit of what he refers to as “professional wiggling”), the appropriate entry showed up in
But there was a problem. While the port multiplier board was recognized by the kernel, nothing he plugged into it showed up. Checking the kernel logs, he found messages relating to bus conflicts, and one that seemed especially important: “devices behind bridge are unusable because [bus 02] cannot be assigned for them“. To make a long story short, it turns out that the Raspbian kernel is specifically configured to only allow a single PCI bus.
Fortunately, it’s an easy fix once you know what the problem is. Using the “Device Tree Compiler” tool, [Colin] was able to edit the Raspbian Device Tree file and change the PCI “bus-range” variable from
<0x0 0x1> to
<0x0 0xff>. From there, it was just a matter of plugging in different devices and seeing what works. Simple things such as USB controllers were no problem, but getting ARM Linux support for the NVIDIA GTX 1060 he tried will have to be a topic for another day.