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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Tue, 17 Nov 2020 17:57:27 +0100
Message-ID: <CAJZ5v0ipMJ1gCB7okpROG_yAUi5Q8LknqeH+Jpdrjbb4D_vfuQ@mail.gmail.com>
Subject: Re: Time to re-enable Runtime PM per default for PCI devcies?
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Heiner Kallweit <hkallweit1@gmail.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Kai Heng Feng <kai.heng.feng@canonical.com>,
        Lukas Wunner <lukas@wunner.de>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Nov 17, 2020 at 5:38 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> [+to Rafael, author of the commit you mentioned,
> +cc Mika, Kai Heng, Lukas, linux-pm, linux-kernel]
>
> On Tue, Nov 17, 2020 at 04:56:09PM +0100, Heiner Kallweit wrote:
> > More than 10 yrs ago Runtime PM was disabled per default by bb910a7040
> > ("PCI/PM Runtime: Make runtime PM of PCI devices inactive by default").
> >
> > Reason given: "avoid breakage on systems where ACPI-based wake-up is
> > known to fail for some devices"
> > Unfortunately the commit message doesn't mention any affected  devices
> > or systems.

Even if it did that, it wouldn't have been a full list almost for sure.

We had received multiple problem reports related to that, most likely
because the ACPI PM in BIOSes at that time was tailored for
system-wide PM transitions only.

> > With Runtime PM disabled e.g. the PHY on network devices may remain
> > powered up even with no cable plugged in, affecting battery lifetime
> > on mobile devices. Currently we have to rely on the respective distro
> > or user to enable Runtime PM via sysfs (echo auto > power/control).
> > Some devices work around this restriction by calling pm_runtime_allow
> > in their probe routine, even though that's not recommended by
> > https://www.kernel.org/doc/Documentation/power/pci.txt
> >
> > Disabling Runtime PM per default seems to be a big hammer, a quirk
> > for affected devices / systems may had been better. And we still
> > have the option to disable Runtime PM for selected devices via sysfs.
> >
> > So, to cut a long story short: Wouldn't it be time to remove this
> > restriction?
>
> I don't know the history of this, but maybe Rafael or the others can
> shed some light on it.

The systems that had those problems 10 years ago would still have
them, but I expect there to be more systems where runtime PM can be
enabled by default for PCI devices without issues.