Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM

[Rafael J. Wysocki]

On Tue, May 17, 2022 at 4:48 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Mon, May 16, 2022 at 10:59:32PM +0200, Rafael J. Wysocki wrote:
> > On Mon, May 16, 2022 at 10:09 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > On Fri, May 13, 2022 at 10:00:48PM +0000, Jingar, Rajvi wrote:
> > > > > -----Original Message-----
> > > > > From: Bjorn Helgaas <helgaas@xxxxxxxxxx>
> > > > > Sent: Thursday, May 12, 2022 11:36 AM
> > > > > To: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> > > > > Cc: Jingar, Rajvi <rajvi.jingar@xxxxxxxxx>; Wysocki, Rafael J
> > > > > <rafael.j.wysocki@xxxxxxxxx>; Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; David Box
> > > > > <david.e.box@xxxxxxxxxxxxxxx>; Linux PCI <linux-pci@xxxxxxxxxxxxxxx>; Linux
> > > > > Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>; Linux PM <linux-
> > > > > pm@xxxxxxxxxxxxxxx>
> > > > > Subject: Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM
> > > > >
> > > > > On Thu, May 12, 2022 at 07:52:36PM +0200, Rafael J. Wysocki wrote:
> > > > > > On Thu, May 12, 2022 at 7:42 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > > > > > On Thu, May 12, 2022 at 03:49:18PM +0200, Rafael J. Wysocki wrote:
> > > > >
> > > > > > > > Something like this should suffice IMV:
> > > > > > > >
> > > > > > > > if (!dev_state_saved || pci_dev->current_state != PCI_D3cold)
> > > > > > > >
> > > > > > > >         pci_disable_ptm(pci_dev);
> > > > > > >
> > > > > > > It makes sense to me that we needn't disable PTM if the device is in
> > > > > > > D3cold.  But the "!dev_state_saved" condition depends on what the
> > > > > > > driver did.  Why is that important?  Why should we not do the
> > > > > > > following?
> > > > > > >
> > > > > > >   if (pci_dev->current_state != PCI_D3cold)
> > > > > > >     pci_disable_ptm(pci_dev);
> > > > > >
> > > > > > We can do this too.  I thought we could skip the power state
> > > > > > check if dev_state_saved was unset, because then we would know
> > > > > > that the power state was not D3cold.  It probably isn't worth
> > > > > > the hassle though.
> > > >
> > > > We see issue with certain platforms where only checking if device
> > > > power state in D3Cold is not enough and the !dev_state_saved check
> > > > is needed when disabling PTM. Device like nvme is relying on ASPM,
> > > > it stays in D0 but state is saved. Touching the config space wakes
> > > > up the device which prevents the system from entering into low power
> > > > state.
> > >
> > > Correct me if I'm wrong: for NVMe devices, nvme_suspend() has already
> > > saved state and put the device in some low-power state.  Disabling PTM
> > > here is functionally OK but prevents a system low power state, so you
> > > want to leave PTM enabled.
> > >
> > > But I must be missing something because pci_prepare_to_sleep()
> > > currently disables PTM for Root Ports.  If we leave PTM enabled on
> > > NVMe but disable it on the Root Port above it, any PTM Request from
> > > NVMe will cause an Unsupported Request error.
> > >
> > > Disabling PTM must be coordinated across PTM Requesters and PTM
> > > Responders.  That means the decision to disable cannot depend on
> > > driver-specific things like whether the driver has saved state.
> >
> > Setting state_saved generally informs pci_pm_suspend_noirq() that the
> > device has already been handled and it doesn't need to do anything to
> > it.
> >
> > But you are right that PTM should be disabled on downstream devices as
> > well as on the ports that those devices are connected to and it can be
> > done even if the given device has already been handled, so the
> > state_saved value is technically irrelevant.
> >
> > That's why I suggested to check if the power state is between D0 and
> > D3cold (exclusive) and only disable PTM if that is the case.  It is
> > pointless to disable PTM for devices in D3cold and it may be harmful
> > for devices that are left in D0.
>
> "... it may be harmful for devices that are left in D0" -- I want to
> understand this better.  It sounds like nvme_suspend() leaves the
> device in some device-specific low-power flavor of D0, and subsequent
> config accesses take it out of that low-power situation?

That's my understanding of it.

> If that's the case, it sounds a little brittle.  I don't think it's
> obvious that "pci_dev->state_saved was set by the driver" means "no
> config accesses allowed in pci_pm_suspend_noirq()."

Well, yes and no.  The device may be in D3cold then, so
pci_pm_suspend_noirq() should at least check that before accessing its
config space.

> And pci_pm_suspend_noirq() calls quirks via pci_fixup_device(), which are
> very likely to do config accesses.
>
> Maybe PTM needs to be disabled earlier, e.g., in pci_pm_suspend()?  I
> don't think PTM uses any interrupts, so there's probably no reason
> interrupts need to be disabled before disabling PTM.

That certainly is worth investigation.  For one, I don't see any
obvious downsides of doing so.