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Date:   Tue, 29 Dec 2020 19:56:17 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Lukas Wunner <lukas@...ner.de>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Time to re-enable Runtime PM per default for PCI devcies?

On Sat, Dec 26, 2020 at 11:26 PM Heiner Kallweit <hkallweit1@...il.com> wrote:
>
> On 17.11.2020 17:57, Rafael J. Wysocki wrote:
> > On Tue, Nov 17, 2020 at 5:38 PM Bjorn Helgaas <helgaas@...nel.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.
> >
>
> To follow up on this discussion:
> We could call pm_runtime_forbid() conditionally, e.g. with the following
> condition. This would enable runtime pm per default for all non-ACPI
> systems, and it uses the BIOS date as an indicator for a hopefully
> not that broken ACPI implementation. However I could understand the
> argument that this looks a little hacky ..
>
> if (IS_ENABLED(CONFIG_ACPI) && dmi_get_bios_year() <= 2016)

dmi_get_bios_year() may not be a good indicator. Last time I used it
caused regression, because the value changed after BIOS update.
For example, my MacBook Pro is manufactured in 2011, but
dmi_get_bios_year() returns 2018 with latest BIOS.

Kai-Heng

>
>
>
> >>> 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.
> >
>

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