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

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Date:   Tue, 5 Apr 2022 14:07:03 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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

On Tue, Apr 5, 2022 at 1:45 PM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> 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.

Personally, I haven't seen any ACPI tables containing any _PRE yet.

> > 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>

Thanks!