Hi Bjorn, On 9/2/22 1:38 PM, Bjorn Helgaas wrote: > On Fri, Sep 02, 2022 at 10:24:05AM -0700, Sathyanarayanan Kuppuswamy wrote: >> On 9/2/22 7:58 AM, Bjorn Helgaas wrote: >>> From: Bjorn Helgaas <bhelgaas@xxxxxxxxxx> >>> >>> When disabling PTM, there's no need to clear the Root Select bit. We >>> disable PTM during suspend, and we want to re-enable it during resume. >>> Clearing Root Select here makes re-enabling more complicated. >> >> Currently, it looks like we disable PCI_PTM_CTRL_ROOT in pci_disable_ptm(), >> but not enable it in pci_enable_ptm(). Do you know this did not trigger an >> issue? > > For Root Ports and Switches, we enable PTM (and set Root Select when > appropriate) during enumeration in pci_ptm_init(). This is based on > the assumption that enabling PTM in Root Ports and Switches is a no-op > unless there's an Endpoint that generates PTM Requests. (It turns out > that's not quite true, because Kai-Heng's bug report [1] shows the > 08:00.0 Switch sending PTM Requests even though no Endpoint even has a > PTM Capability.) > > If we didn't enable PTM in Root Ports and Switches during enumeration, > we'd have to walk the whole path and enable them when enabling PTM for > an Endpoint. > > pci_enable_ptm() currently only works for Endpoints, which cannot be > PTM Roots, so it never has to set PCI_PTM_CTRL_ROOT. > > If we clear PCI_PTM_CTRL_ROOT in pci_disable_ptm(), it will never get > set again unless we re-enumerate the Root Port. Thanks for clarifying. > > Thanks for asking this, because it reminds me why I didn't add > pci_enable_ptm() calls in the resume paths! That would make them > parallel with the suspend paths, which would definitely be nice. But > we would have to rework pci_enable_ptm() to work for Root Ports and > Switch Ports as well. I think we *could* do that. What do you think? IMO, the code will look better if we keep the suspend and resume paths in sync. Since we are calling pci_disable_ptm() in suspend path, it makes sense to call pci_enable_ptm() in resume path. Making the pci_enable_ptm() handle root and upstream ports should not be very complicated, right? > > Regardless of that question, I think it's unnecessary to clear > PCI_PTM_CTRL_ROOT in pci_disable_ptm(), so we should leave it alone. I agree with you. We should not touch PCI_PTM_CTRL_ROOT in pci_disable_ptm(). > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=215453 > >> Also, you mentioned that it is complicated to enable it, can you add some >> details? >> >>> Per PCIe r6.0, sec 7.9.15.3, "When set, if the PTM Enable bit is also Set, >>> this Time Source is the PTM Root," so if PTM Enable is cleared, the value >>> of Root Select should be irrelevant. >>> >>> Preserve Root Select to simplify re-enabling PTM. >>> >>> Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx> >>> Cc: David E. Box <david.e.box@xxxxxxxxxxxxxxx> >>> --- >>> drivers/pci/pcie/ptm.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/pci/pcie/ptm.c b/drivers/pci/pcie/ptm.c >>> index 368a254e3124..b6a417247ce3 100644 >>> --- a/drivers/pci/pcie/ptm.c >>> +++ b/drivers/pci/pcie/ptm.c >>> @@ -42,7 +42,7 @@ void pci_disable_ptm(struct pci_dev *dev) >>> return; >>> >>> pci_read_config_word(dev, ptm + PCI_PTM_CTRL, &ctrl); >>> - ctrl &= ~(PCI_PTM_CTRL_ENABLE | PCI_PTM_CTRL_ROOT); >>> + ctrl &= ~PCI_PTM_CTRL_ENABLE; >>> pci_write_config_word(dev, ptm + PCI_PTM_CTRL, ctrl); >>> } >>> >> >> -- >> Sathyanarayanan Kuppuswamy >> Linux Kernel Developer -- Sathyanarayanan Kuppuswamy Linux Kernel Developer
