On Wed, Aug 23, 2023 at 07:04:53AM +0200, Lukas Wunner wrote: > On Tue, Aug 22, 2023 at 07:02:43PM -0500, Bjorn Helgaas wrote: > > On Tue, Aug 22, 2023 at 12:11:10PM +0200, Rafael J. Wysocki wrote: > > > What we need to deal with here is basically non-compliant systems and > > > so we have to catch the various forms of non-compliance. > > > > Thanks for this, that helps. If pci_bridge_d3_possible() is a list of > > quirks for systems that are known to be broken (or at least not known > > to work correctly and avoiding D3 is acceptable), then we should > > document and use it that way. > > > > The current documentation ("checks if it is possible to move to D3") > > frames it as "does the bridge have the required features?" instead of > > "do we know about something broken in this bridge or this platform?" > > > > If something is broken, I would expect tests based on the device or > > DMI check. But several some are not obvious defects. E.g., > > "bridge->is_hotplug_bridge && !pciehp_is_native(bridge)" -- what > > defect are we finding there? What does the spec require that isn't > > happening? > > This particular check doesn't pertain to a defect, but indeed > follows from the spec: > > If hotplug control wasn't granted to the OS, the OS shall not put > the hotplug port in D3 behind firmware's back because the power state > affects accessibility of devices downstream of the hotplug port. > > Put another way, the firmware expects to have control of hotplug > and hotplug may break if the OS fiddles with the power state of the > hotplug port. > > Here's a bugzilla where this caused issues: > https://bugzilla.kernel.org/show_bug.cgi?id=53811 > > On the other hand Thunderbolt hotplug ports are required to runtime > suspend to D3 in order to save power. Sounds like there may be a requirement in a Thunderbolt spec about this, so maybe we could add that citation? I guess this goes with the "bridge->is_thunderbolt" check? > On Macs they're always handled > natively by the OS. Hence the code comment. And I guess this goes with the "System Management Mode" and "Thunderbolt on non-Macs" comments? A citation to the source behind "OS shall not put the hotplug port in D3 behind firmware's back" would be super helpful here. > A somewhat longer explanation I gave in 2016: > https://lore.kernel.org/all/20160617213209.GA1927@xxxxxxxxx/ > > Perhaps the code comment preceding that check can be rephrased to > convey its meaning more clearly... Thanks! I think it would be worth trying to separate out the "normal" things that correspond to the spec from the "quirk" things that work around defects. That's not material for *this* patch, though. It's also a little weird that pci_bridge_d3_possible() itself looks like it's invariant for the life of the system, but we call it several times (pci_pm_init(), pci_bridge_d3_update(), pcie_portdrv_probe(), etc). I guess this is because we save the result in dev->bridge_d3, but then pci_bridge_d3_update() updates dev->bridge_d3 based on other things, so the original value is lost. Maybe another bit or two could avoid those extra calls. Bjorn