Re: [PATCH] PCI: PM: Do not read power state in pci_enable_device_flags()

[Rafael J. Wysocki]

On Mon, Mar 22, 2021 at 3:32 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Tue, Mar 16, 2021 at 4:52 PM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > It should not be necessary to update the current_state field of
> > struct pci_dev in pci_enable_device_flags() before calling
> > do_pci_enable_device() for the device, because none of the
> > code between that point and the pci_set_power_state() call in
> > do_pci_enable_device() invoked later depends on it.
> >
> > Moreover, doing that is actively harmful in some cases.  For example,
> > if the given PCI device depends on an ACPI power resource whose _STA
> > method initially returns 0 ("off"), but the config space of the PCI
> > device is accessible and the power state retrieved from the
> > PCI_PM_CTRL register is D0, the current_state field in the struct
> > pci_dev representing that device will get out of sync with the
> > power.state of its ACPI companion object and that will lead to
> > power management issues going forward.
> >
> > To avoid such issues it is better to leave the current_state value
> > as is until it is changed to PCI_D0 by do_pci_enable_device() as
> > appropriate.  However, the power state of the device is not changed
> > to PCI_D0 if it is already enabled when pci_enable_device_flags()
> > gets called for it, so update its current_state in that case, but
> > use pci_update_current_state() covering platform PM too for that.
> >
> > Link: https://lore.kernel.org/lkml/20210314000439.3138941-1-luzmaximilian@xxxxxxxxx/
> > Reported-by: Maximilian Luz <luzmaximilian@xxxxxxxxx>
> > Tested-by: Maximilian Luz <luzmaximilian@xxxxxxxxx>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> Bjorn, can I take this, or do you want to take care of it yourself?

I'm taking the silence as consent, so the patch has been applied as
5.13 material with the R-by from Mika.

> > ---
> >
> > Max, I've added a T-by from you even though the patch is slightly different
> > from what you have tested, but the difference shouldn't matter for your case.
> >
> > ---
> >  drivers/pci/pci.c |   16 +++-------------
> >  1 file changed, 3 insertions(+), 13 deletions(-)
> >
> > Index: linux-pm/drivers/pci/pci.c
> > ===================================================================
> > --- linux-pm.orig/drivers/pci/pci.c
> > +++ linux-pm/drivers/pci/pci.c
> > @@ -1870,20 +1870,10 @@ static int pci_enable_device_flags(struc
> >         int err;
> >         int i, bars = 0;
> >
> > -       /*
> > -        * Power state could be unknown at this point, either due to a fresh
> > -        * boot or a device removal call.  So get the current power state
> > -        * so that things like MSI message writing will behave as expected
> > -        * (e.g. if the device really is in D0 at enable time).
> > -        */
> > -       if (dev->pm_cap) {
> > -               u16 pmcsr;
> > -               pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
> > -               dev->current_state = (pmcsr & PCI_PM_CTRL_STATE_MASK);
> > -       }
> > -
> > -       if (atomic_inc_return(&dev->enable_cnt) > 1)
> > +       if (atomic_inc_return(&dev->enable_cnt) > 1) {
> > +               pci_update_current_state(dev, dev->current_state);
> >                 return 0;               /* already enabled */
> > +       }
> >
> >         bridge = pci_upstream_bridge(dev);
> >         if (bridge)
> >
> >
> >