linux-kernel - Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend

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Date:   Mon, 16 May 2022 22:59:32 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Jingar, Rajvi" <rajvi.jingar@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        David Box <david.e.box@...ux.intel.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v5 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM

On Mon, May 16, 2022 at 10:09 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Fri, May 13, 2022 at 10:00:48PM +0000, Jingar, Rajvi wrote:
> >
> > > -----Original Message-----
> > > From: Bjorn Helgaas <helgaas@...nel.org>
> > > Sent: Thursday, May 12, 2022 11:36 AM
> > > To: Rafael J. Wysocki <rafael@...nel.org>
> > > Cc: Jingar, Rajvi <rajvi.jingar@...el.com>; Wysocki, Rafael J
> > > <rafael.j.wysocki@...el.com>; Bjorn Helgaas <bhelgaas@...gle.com>; David Box
> > > <david.e.box@...ux.intel.com>; Linux PCI <linux-pci@...r.kernel.org>; Linux
> > > Kernel Mailing List <linux-kernel@...r.kernel.org>; Linux PM <linux-
> > > pm@...r.kernel.org>
> > > 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@...nel.org> 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.