Re: [PATCH 1/4] PCI/PTM: Preserve PTM Root Select

Sathyanarayanan Kuppuswamy
2022-09-02T21:11:18Z

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@google.com>
>>>
>>> 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@google.com>
>>> Cc: David E. Box <david.e.box@linux.intel.com>
>>> ---
>>> 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