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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Mar 16, 2021 at 4:52 PM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> 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@gmail.com/
> Reported-by: Maximilian Luz <luzmaximilian@gmail.com>
> Tested-by: Maximilian Luz <luzmaximilian@gmail.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Bjorn, can I take this, or do you want to take care of it yourself?

> ---
>
> 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)
>
>
>