Messages in this thread | | | Subject | How to expose various BMC chip controls ? | From | Benjamin Herrenschmidt <> | Date | Tue, 10 Apr 2018 19:17:30 +1000 |
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Hi Folks !
I would like to discuss something we need to solve for BMC chips before we start implementing a solution that you'll reject upstream :-)
So quick recap: the BMC chip is the management controller of your server, typically some kind of specialized ARM SoC which controls a variety of things ranging from power supply, fans, inventory, flash, to having even a built-in GPU connected to the main server processor via PCIe etc...
As part of the OpenBMC project, we have been contributing (along with others from Google etc...) a bunch of upstream drivers to support the BMC chips from Aspeed which are quite popular. There's work in progress to also upstream other vendor(s) chip support.
For most things, we can write appropriate drivers.
However, there's a range of things for which there is no good fit for dedicated per-function kernel drivers in the BMC:
The BMC chips have a variety of control bits and pieces (registers) that affect the host in all sorts of ways. A few semi-random examples:
- Scratch registers that the host BIOS can read via LPC that contain system specific details of interest to the BIOS itself. The content/format is specific to a given BIOS <-> BMC userspace pair.
- Similar ones related to the built-in VGA (the one the host sees on PCIe, it doesn't appear as a device on the BMC side). Used to pass some wiring information to the VGA BIOS and the Linux driver among others.
- Bits controlling which parts of the BMC the host can access remotely via either LPC or PCIe. (Visibility of the VGA to the host, of the BMC PCIe system-controller device, opening of various windows from the host into the BMC address space etc...)
- ... and a few more
Those things tend to be fairly system specific. Meaning that what's written in them (or read from them) and when it's written in these registers should be under userspace control on the BMC. There isn't necessarily many registers that need to be exposed that way, here too, which specific ones is rather system specific.
But /dev/mem isn't a solution :-)
So one simple idea I had, you tell me if it can fly, is to have some driver for the SoC as a whole (thinking of syscon here...) expose a selection of these things via sysfs.
The list being potentially system specific, it would be provided to the kernel via the device-tree (binding tbd) where we would effectively provide a list of names, register offset, start bit, bitfield size.
Any better idea ? It's a fairly simple problem, and the above solution would be very little code, but I just don't want us to go down a rabbit hole if some of you have religious objections to some of it :)
Cheers, Ben.
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