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

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Message-ID: <YkwQAKcFU4CzYX5E@lahna>
Date:   Tue, 5 Apr 2022 12:46:40 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>
Subject: Re: [PATCH v1 0/3] ACPI: PCI: PM: Power up PCI devices with ACPI
 companions upfront

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@...ux.intel.com>