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. > > 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. Bjorn
