Re: [PATCH v1 0/3] ACPI: PCI: PM: Power up PCI devices with ACPI companions upfront

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

Hi Rafael,

On Mon, Apr 04, 2022 at 05:20:30PM +0200, Rafael J. Wysocki wrote:
> Hi All,
> 
> There are cases in which the power state of a PCI device depends on an ACPI
> power resource (or more of them) in such a way that when the given power
> resource is in the "off" state, the PCI device depending on it is in D3cold.
> 
> On some systems, the initial state of these power resources is "off", so the
> kernel should not access the config space of PCI devices depending on them,
> until the power resources in question are turned "on", but currently that is
> not respected during PCI device enumeration.  Namely, the PCI device
> enumeration code walks the entire bus and enumerates all of the devices it
> can find, including the ones whose initial power state in principle depends on
> the ACPI power resources in the "off" state.

I guess these devices do not have _PRE() method either.

> Apparently, most of the time, the config space of such devices is accessible
> regardless of the state of the ACPI power resource associated with the PCI
> device, so the device enumeration is successful, but there are two potential
> issues related to this behavior.  First off, even if the given PCI device
> is accessible when the ACPI power resource depended on by it is "off",
> changing its configuration may confuse the platform firmware and lead to
> problems when the ACPI power resource in question is turned "on".  Second,
> the PCI device may not be actually accessible at all when the ACPI power
> resource depended on by it is "off", in which case it won't be found during
> the PCI enumeration of devices.
> 
> This patch series addresses that problem by turning "on" all ACPI power
> resources depended on by PCI devices before attempting to access the config
> space of those devices for the first time.

Makes sense.

For the series,

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