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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Linux ACPI <linux-acpi@vger.kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux PCI <linux-pci@vger.kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>
Subject: [PATCH v1 0/3] ACPI: PCI: PM: Power up PCI devices with ACPI companions upfront
Date:   Mon, 04 Apr 2022 17:20:30 +0200
Message-ID: <21439956.EfDdHjke4D@kreacher>
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Precedence: bulk

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.

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.

The first two patches introduce the requisite machinery and the actual change
of behavior is done in the last patch.

Thanks!