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hardware/platforms/Meraki/MR24/README.md

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MR24 PCIe speculation
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MR24

Working on MR24 (3x3 bgn + 3x3 abgn, APM82181)

PCIe speculation

On 30 Aug 2018, 91.13.120.222 added a comment to the WikiDevi article for the MR24. They noted that the mPCIe slots on the MR24 were non-standard, saying simply:

The two minipcie ports are non-standard as only the supplied Wi-Fi cards will work in them.

With only superficial knowledge of how device initialization works in OpenWrt, it may appear this is correct. For example, see this post entitlted PCI-E External Storage methods?. This person wanted to add storage to their router, but unfortunately no PCI IDs even came up that correspond to a USB 3.0 or SATA controller via PCIe ( [168c:0030] is one of the Atheros WLAN cards, and [111d:8039] is the IDT PCIe switch 1. The solver of that thread goes on to say that it is likely not initializing because:

The PCIe bus on some Atheros chips is not a full implementation. It only has the functionality required to support wifi cards.

... but as this board contains no Atheros-based PCI hosts, this is impossible! Indeed, other boards like the Netgear WNDR4700 2 have other PCI devices that they can use despite having nearly identical hardware.

With some knowledge it's clear that in order for the devices to show up, the device tree has to be populated in order to bring things up correctly after the kernel loads.

U-boot, which this router runs on, has a few ways of demonstrating that it knows about and is working on devices:

  • It allows you to manipulate them before boot using some u-boot commands (see e.g. this mailing list entry)

  • It actually lists out PCI device IDs (as vendor:product pairs) at boot-time -- see how it shows up in the MR24 u-boot log and in the WNDR4700 u-boot log. (The WNDR4700 is mentioned as it has a very similar processor, same architecture, a very similar PCI switch AND processor, but additionally has the ubiquitous Renasas PCI to USB 3.0 chip 2.)

Footnotes

  1. Search through /usr/share/misc/pci.ids (most Linux machines have this) to identify devices. ↩

  2. http://en.techinfodepot.shoutwiki.com/wiki/Netgear_WNDR4700 ↩ ↩2