So OpenBMC is fine (happy for them!), but having open firmware is much deeper and broader than that: yes, it's the service processor (in contrast to the BMC which is a closed part on Dell machines) -- but it's also the root-of-trust and (especially) the host CPU itself. We at Oxide have open source software from first instruction out of the AMD PSP; I elaborated more on our approach in my OSFC 2022 talk.[0]

[0] https://www.osfc.io/2022/talks/i-have-come-to-bury-the-bios-...

Dell now ships with OpenBMC iDRACs and such. How does what you mention differ from the RoT in Dells?

https://www.dell.com/en-us/blog/hardware-root-trust/

Dell uses trusted platform modules (TPM). It's a separate chipset than the BMC chipset.

For a mostly open source solution, not only would you need open source BMC firmware, you must have an open source UEFI/BIOS/boot firmware like CoreBoot, LinuxBoot, Oreboot, Uboot, etc.