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

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Tue, Mar 16, 2021 at 04:51:40PM +0100, Rafael J. Wysocki 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>

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>