2024-09-10
Ways ATX power supply control could work on server motherboards
Yesterday I talked about how ATX power supply control seems to work on desktop motherboards, which is relatively straightforward; as far as I can tell from various sources, it's handled in the chipset (on modern Intel chipsets, in the PCH), which is powered from standby power by the ATX power supply. How things work on servers is less clear. Here when I say 'server' I mean something with a BMC (Baseboard management controller), because allowing you to control the server's power supply is one of the purposes of a BMC, which means the BMC has to hook into this power management picture.
There appear to be a number of ways that the power control and management could or may be done and the BMC connected to it. People on the Fediverse replying to my initial question gave me a number of possible answers:
- The power management still happens in the chipset and the
BMC has a PCIe lane to the chipset
that it uses to have the chipset do power things.
- The power management happens in the chipset but the physical
power button is wired to the BMC and the BMC controls the 'power
button wire' that goes to the chipset.
To power on or off the server the BMC basically pretends there's
a person doing the appropriate things with the power button,
and then lets the chipset handle it.
(The BMC has various ways to tell what the current state of the power supply is.)
- The BMC is where parts of the chipset functionality are implemented, including the power button parts.
I found documentation for some of Intel's older Xeon server chipsets (with provisions for BMCs) and as of that generation, power management was still handled in the PCH and described in basically the same language as for desktops. I couldn't spot a mention of special PCH access for the BMC, so BMC control over server power might have been implemented with the 'BMC controls the power button wire' approach.
I can also imagine hybrid approaches. For example, you could in theory give the BMC control over the 'turn power on' wire to the power supplies, and route the chipset's version of that line to the BMC, in addition to routing the power button wire to the BMC. Then the BMC would be in a position to force a hard power off even if something went wrong in the chipset (or a hard power on, although if the chipset refuses to trigger a power on there might be a good reason for that).
(Server power supplies aren't necessarily 'ATX' power supplies as such, but I suspect that they all have similar standby power, 'turn power on', and 'is the PSU power stable' features as ATX PSUs do. Server PSUs often clearly aren't plain ATX units because they allow the BMC to obtain additional information on things like the PSU's state, temperature, current power draw, and so on.)
Our recent experience with BMCs that wouldn't let their servers power on when they should have suggests that on these servers (both Dell R340s), the BMC has some sort of master control or veto power over the normal 'return to last state' settings in the BIOS. At the same time, the 'what to do after AC power returns' setting is in the BIOS, not in the BMC, so it seems that the BMC is not the sole thing controlling power.
(I tried to take a look at how this was done in OpenBMC, but rapidly got lost in a twisty maze of things. I think at least some of the OpenBMC supported hardware does this through I2C commands, although what I2C device it's talking to is a good question. Some of the other hardware appears to have GPIO signal definitions for power related stuff, including power button definitions.)