AMD SB-RMI Driver Coming For Linux 5.15

AMD continues pushing new code out for Linux in better exposing their platform's capabilities in the open-source world. The latest AMD driver work now queued via "-next" branches for introduction this autumn in Linux 5.15 is SB-RMI sensor support.

AMD SB-RMI is the Side-Band Remote Management Interface for out-of-band communication between the AMD SoC/CPU and the baseboard management controller (BMC) via the Advanced Platform Management Link (APML / SBI). Queued via the hwmon-next Git branch is an initial AMD SB-RMI sensor driver for Linux.

This SB-RMI driver (simply named SBRMI) allows for reading the CPU socket power consumption, power limit, and maximum power limit. Additionally, the driver allows setting/limiting the maximum power limit of a CPU socket. Though given SB-RMI, it's important to reiterate this is just for server platforms with a supported BMC configuration and just not any AMD platform / desktop. Capable dual socket servers are supported by the driver.

The SB-RMI driver exposes the current socket power, the read/write current socket cap, and maximum cap via the hwmon sysfs interface. The code is ready to go once the Linux 5.15 merge window opens up in the coming weeks.

It's nice seeing progress being made by AMD engineers on exposing these additional capabilities under Linux. For EPYC processors there is currently RAPL support while Zen voltage/current support was previously dropped from Linux with the mainline k10temp driver over lack of public AMD documentation. There was also the AMD energy monitoring driver that was removed from the Linux kernel earlier this year over a disagreement on handling of the CPU metrics being restricted to root-only for security purposes. Hopefully there is progress made on those fronts particularly for desktop/mobile users moving forward.

This SB-RMI work does appear to be motivated at least in part over AMD's growing OpenBMC support just like the SB-TSI sensor support for Linux that landed a few kernels ago.