On Mon, Nov 16, 2020 at 9:58 PM David E. Box <david.e.box@xxxxxxxxxxxxxxx> wrote: > > On Mon, 2020-11-16 at 13:23 -0600, Bjorn Helgaas wrote: > > On Mon, Nov 16, 2020 at 06:53:09PM +0100, Rafael J. Wysocki wrote: > > > On Wed, Oct 7, 2020 at 7:10 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> > > > wrote: > > > > On Wed, Oct 07, 2020 at 06:53:16PM +0200, Rafael J. Wysocki > > > > wrote: > > > > > On Wed, Oct 7, 2020 at 6:49 PM David E. Box < > > > > > david.e.box@xxxxxxxxxxxxxxx> wrote: > > > > > > On Intel Platform Controller Hubs (PCH) since Cannon Lake, > > > > > > the Precision > > > > > > Time Measurement (PTM) capability can prevent PCIe root ports > > > > > > from power > > > > > > gating during suspend-to-idle, causing increased power > > > > > > consumption on > > > > > > systems that suspend using Low Power S0 Idle [1]. The issue > > > > > > is yet to be > > > > > > root caused but believed to be coming from a race condition > > > > > > in the suspend > > > > > > flow as the incidence rate varies for different platforms on > > > > > > Linux but the > > > > > > issue does not occur at all in other operating systems. For > > > > > > now, disable > > > > > > the feature on suspend on all Intel root ports and enable > > > > > > again on resume. > > > > > > > > > > IMV it should also be noted that there is no particular reason > > > > > why PTM > > > > > would need to be enabled while the whole system is > > > > > suspended. At > > > > > least it doesn't seem to be particularly useful in that state. > > > > > > > > Is this a hardware erratum? If not, and this is working as > > > > designed, > > > > it sounds like we'd need to apply this quirk to every device that > > > > supports PTM. That's not really practical. > > > > > > Why not? > > > > My objection was that the original patch is a quirk that applies only > > to Intel devices. > > > > If this is a generic thing that should be done for *all* devices that > > support PTM, that's fine, but it should not be a quirk, and it should > > not involve a list of Vendor or Device IDs. > > > > > It looks like the capability should be saved by pci_save_state() > > > (it > > > isn't ATM, which appears to be a mistake) and restored by > > > pci_restore_state(), so if that is implemented, the saving can be > > > combined with the disabling in principle. > > > > Yup, looks like a mistake. Maybe David can fix that at the same time > > (probably a separate patch, though). I don't have a way to test it, > > but he probably does. > > Yes, I can test save/restore of the PTM capability and submit a patch. > > > > > > > The bugzilla says "there is no erratum as this does not affect > > > > Windows," but that doesn't answer the question. What I want to > > > > know > > > > is whether this is a *hardware* defect and whether it will be > > > > fixed in > > > > future hardware. > > > > > > I cannot answer this question, sorry. > > > > > > ATM we only know that certain SoCs may not enter the deepest idle > > > state if PTM is enabled on some PCIe root ports during suspend. > > > > > > Disabling PTM on those ports while suspending helps and hence the > > > patch. > > > > > > It doesn't appear to qualify as a "hardware defect". > > > > > > > If it's a "wont-fix" hardware issue, we can just disable PTM > > > > completely on Intel hardware and we won't have to worry about it > > > > during suspend. > > > > > > I'm not following the logic here, sorry again. > > > > > > First of all, there are systems that never suspend, so why would > > > they > > > be affected by the remedy (whatever it is)? > > > > > > Second, it is not about the suspend failing entirely. It's about > > > being able to make the system draw less power while suspended. > > > > > > Generally, if someone said "I can make the system draw less power > > > while suspended if I disable PCIe feature X during suspend", would > > > you > > > disregard that? > > > > My questions were all prompted by the Intel-specific nature of the > > original patch, which suggests an ongoing maintenance burden. If it > > can be done generically, I have no problem with it. > > Okay. I'll add this to the save/restore patch then with the comment > that it saves power on some Intel platforms. I'd suggest doing two patches, then, one to save/restore the PTM capability and the other to add disabling it to the "save" path (with a comment as appropriate).
