Hi Bjorn, On Fri, Feb 28, 2025 at 11:45:09AM -0600, Bjorn Helgaas wrote: > On Tue, Nov 26, 2024 at 03:17:11PM -0800, Brian Norris wrote: > > From: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx> > > > > Unlike ACPI based platforms, there are no known issues with D3Hot for > > the PCI bridges in Device Tree based platforms. > > Can we elaborate on this a little bit? Referring to "known issues > with ACPI-based platforms" depends on a lot of domain-specific history > that most readers (including me) don't know. Well, to me, the background here is simply the surrounding code context, and the past discussions that I linked: https://lore.kernel.org/linux-pci/20240227225442.GA249898@bhelgaas/ The whole reason we need this patch is that: (a) there's some vaguely specified reason this function (which prevents standard-specified behavior) exists; and (b) that function includes a condition that allows all systems with a DMI/BIOS newer than year 2015 to use this feature. Digging a bit further, it seems like maybe the only reason this feature is prevented on DT systems is from commit ("9d26d3a8f1b0 PCI: Put PCIe ports into D3 during suspend"), where the subtext is that it was written by and for Intel in 2016, with an arbitrary time-based cutoff ("year this was being developed") that only works for DMI systems. DT systems do not tend to support DMI. If any of this is what you're looking for, I can try to copy/paste/summarize a few more of those bits, if it helps. > I don't think "ACPI-based" or "devicetree-based" are good > justifications for changing the behavior because they don't identify > any specific reasons. It's like saying "we can enable this feature > because the platform spec is written in French." AIUI, It's involved because of the general strategy of this function (per its comments, "recent enough PCIe ports"). So far, it sounds like that reason (presumably, old BIOS with poor power management code) doesn't really apply to a system based on device tree, where the power management code is mostly/entirely in the OS. But really, the original commit doesn't actually state reasons, so maybe the "known issues" phrasing could be weakened a bit, to avoid implying there were any stated reasons. > > Past discussions (Link [1]) determined the restrictions around D3 > > should be relaxed for all Device Tree systems. > > This is far too generic a statement for me to sign up to, especially > since "all Device Tree systems" doesn't say anything at all about how > any particular hardware works or what behavior we're relying on. > > We need to say something about what D3hot means (i.e., only message > and type 0 config requests accepted) and that we know anything below > the bridge is inaccessible in D3hot and why that's OK. E.g., maybe we > only care about wakeup requests and we know those still work with the > bridge in D3hot because XYZ. The context of this function is that it applies to situations where we are considering runtime all of the PCI hierarchy beneath the port. So yes, it applies in situations where we will wake device(s) by PMCSR, or they will request wakeup via PME, WAKE#, etc. Beyond that ... I don't really know what to say. This is how the spec says things should work, and AFAICT, the only reason we don't do that is because the feature writer was being conservative and happened to assume DMI is always present. Brian
