Re: [PATCH v4 2/2] PCI/PM: Fix pci_pm_suspend_noirq() to disable PTM

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc other folks interested in PTM from https://lore.kernel.org/r/20220408153159.106741-1-kai.heng.feng@canonical.com]

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@intel.com>
> > Suggested-by: David E. Box <david.e.box@linux.intel.com>

> > ---
> >   v1 -> v2: add Fixes tag in commit message
> >   v2 -> v3: move changelog after "---" marker
> >   v3 -> v4: add "---" marker after changelog
> > ---
> >   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().

Bjorn