linux-kernel - Re: [PATCH v4 2/2] PCI/PM: Fix pci_pm_suspend

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Message-ID: <CAJZ5v0hs1HPaJNZticZfxz+CeAVuq-Rrw7J50-wv6-wWn5L_2Q@mail.gmail.com>
Date:   Mon, 25 Apr 2022 20:39:05 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     David Box <david.e.box@...ux.intel.com>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Jingar, Rajvi" <rajvi.jingar@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        "koba.ko@...onical.com" <koba.ko@...onical.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Russell Currey <ruscur@...sell.cc>,
        "Oliver O'Halloran" <oohall@...il.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v4 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM

On Mon, Apr 25, 2022 at 8:33 PM David E. Box
<david.e.box@...ux.intel.com> wrote:
>
> On Sat, 2022-04-23 at 10:01 -0500, Bjorn Helgaas wrote:
> > On Sat, Apr 23, 2022 at 12:43:14AM +0000, Jingar, Rajvi wrote:
> > > > -----Original Message-----
> > > > From: Bjorn Helgaas <helgaas@...nel.org>
> > > > On Thu, Apr 14, 2022 at 07:54:02PM +0200, Rafael J. Wysocki wrote:
> > > > > On 3/25/2022 8:50 PM, Rajvi Jingar wrote:
> > > > > > For the PCIe devices (like nvme) that do not go into D3 state still
> > > > > > need to
> > > > > > disable PTM on PCIe root ports to allow the port to enter a lower-
> > > > > > power PM
> > > > > > state and the SoC to reach a lower-power idle state as a whole. Move
> > > > > > the
> > > > > > pci_disable_ptm() out of pci_prepare_to_sleep() as this code path is
> > > > > > not
> > > > > > followed for devices that do not go into D3. This patch fixes the
> > > > > > issue
> > > > > > seen on Dell XPS 9300 with Ice Lake CPU and Dell Precision 5530 with
> > > > > > Coffee
> > > > > > Lake CPU platforms to get improved residency in low power idle states.
> > > > > >
> > > > > > Fixes: a697f072f5da ("PCI: Disable PTM during suspend to save power")
> > > > > > Signed-off-by: Rajvi Jingar <rajvi.jingar@...el.com>
> > > > > > Suggested-by: David E. Box <david.e.box@...ux.intel.com>
> > > > > > ---
> > > > > >   drivers/pci/pci-driver.c | 10 ++++++++++
> > > > > >   drivers/pci/pci.c        | 10 ----------
> > > > > >   2 files changed, 10 insertions(+), 10 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> > > > > > index 8b55a90126a2..ab733374a260 100644
> > > > > > --- a/drivers/pci/pci-driver.c
> > > > > > +++ b/drivers/pci/pci-driver.c
> > > > > > @@ -847,6 +847,16 @@ static int pci_pm_suspend_noirq(struct device
> > > > > > *dev)
> > > > > >       if (!pci_dev->state_saved) {
> > > > > >               pci_save_state(pci_dev);
> > > > > > +             /*
> > > > > > +              * There are systems (for example, Intel mobile chips
> > > > > > since
> > > > Coffee
> > > > > > +              * Lake) where the power drawn while suspended can be
> > > > significantly
> > > > > > +              * reduced by disabling PTM on PCIe root ports as this
> > > > > > allows the
> > > > > > +              * port to enter a lower-power PM state and the SoC to
> > > > > > reach a
> > > > > > +              * lower-power idle state as a whole.
> > > > > > +              */
> > > > > > +             if (pci_pcie_type(pci_dev) == PCI_EXP_TYPE_ROOT_PORT)
> > > > > > +                     pci_disable_ptm(pci_dev);
> > > >
> > > > Why is disabling PTM dependent on pci_dev->state_saved?  The point of
> > > > this is to change the behavior of the device, and it seems like we
> > > > want to do that regardless of whether the driver has used
> > > > pci_save_state().
> > >
> > > Because we use the saved state to restore PTM on the root port.
> > > And it's under this condition that the root port state gets saved.
> >
> > Yes, I understand that pci_restore_ptm_state() depends on a previous
> > call to pci_save_ptm_state().
> >
> > The point I'm trying to make is that pci_disable_ptm() changes the
> > state of the device, and that state change should not depend on
> > whether the driver has used pci_save_state().
>
> We do it here because D3 depends on whether the device state was saved by the
> driver.
>
>         if (!pci_dev->state_saved) {
>                 pci_save_state(pci_dev);
>
>                 /* disable PTM here */
>
>                 if (pci_power_manageable(pci_dev))
>                         pci_prepare_to_sleep(pci_dev);
>         }
>
>
> If we disable PTM before the check, we will have saved "PTM disabled" as the
> restore state. And we can't do it after the check as the device will be in D3.
>
> As to disabling PTM on all devices, I see no problem with this, but the
> reasoning is different. We disabled the root port PTM for power savings.

Right.  As per the comment explaining why it is disabled.

> >
> > When we're putting a device into a low-power state, I think we want to
> > disable PTM *always*, no matter what the driver did.  And I think we
> > want to do it for all devices, not just Root Ports.
> >
> > Bjorn
>