Limited Support For The AMD Pensando Elba SoC Might Finally Land Upstream In Linux 6.7

For a year and a half now Pensando has been working on enabling their Elba SoC support for the mainline Linux kernel - a process that coincidentally began just days after AMD announced it was acquiring Pensando. Over the past 18 months the AMD-Pensando Elba SoC enablement work has now been through 16 rounds of code review but still isn't over the finish line yet but some of the initial enablement code might finally land with Linux 6.7.

The AMD-Pensando Elba SoC is a data processing unit (DPU) intended for infrastructure offloading around storage and networking. The SoC consists of sixteen Arm Cortex-A72 cores, dual DDR4/DDR5 memory controllers, 32 lanes of PCIe Gen3 or Gen4 connectivity, up to dual 200 GbE or quad 100 GbE networking, storage and crypto offloading, and other features for DPU use-cases.

AMD-Pensando has continued iterating on the patches for mainline Linux kernel support for the Elba SoC but so far has been unsuccessful. Longtime Linux kernel developer and SoC subsystem maintainer Arnd Bergmann has summarized the current state of affairs for the Elba SoC patches following their 16th revision. Ultimately there is still no agreement on the SoC controller support for how it should be handled but is now suggesting the route of AMD upstreaming the basic SoC support -- as soon as Linux 6.7 -- while the SoC controller work will take longer. Arnd wrote:

"I'm sorry I've been out of the loop for so long, and I hope we can find a way to manage your SoC support soon. My impression is that the normal support patches (1, 3, 4, and 5) are largely uncontroversial, while the SoC controller support seems like we are still not converging onto something that is ready to merge, so I would suggest you split the two parts and send the basic support for inclusion in linux-6.7 while we continue to discuss the soc controller driver."

The Elba SoC controller is a SPI-connected device with various board control/status registers. Arnd went on to explain in his mailing list post:

"The main problem I still see is that this driver completely bypasses our normal kernel abstractions and instead creates a low-level passthrough interface for handling kernel functionality in userspace. This creates a liability both for the user ABI and the kernel implementation and prevents any

There is a chance that your design is in fact the best way to handle this particular hardware, but it is your job to write a convincing explanation of why this platform is different from all the others in the patch description. Your current one-paragraph text does not explain this at all.

I would suggest you prioritize getting the other patches included for the moment, but we can keep discussion the API design for this driver either in this thread or on the #armlinux IRC channel (irc.libera.chat) in parallel if you like. In order to help you here, I would need either the documentation of the SPI software interface, or the source code for the userspace tool."

So for Linux 6.7 this winter the AMD-Pensando Elba SoC support might see the basic support land without the SoC controller driver support. In any event at least the kernel code remains open and available for data centers already deploying the Pensando DPUs and this kernel upstreaming is unlikely to delay any of those deployments with most customers presumably just sticking to AMD's downstream kernel images / software builds.