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Subject: Re: Time to re-enable Runtime PM per default for PCI devcies?
To:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>
Cc:     "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>
References: <79940973-b631-90f9-dbc4-9579c6000816@gmail.com>
 <20201117163817.GA1397220@bjorn-Precision-5520>
 <CAJZ5v0ipMJ1gCB7okpROG_yAUi5Q8LknqeH+Jpdrjbb4D_vfuQ@mail.gmail.com>
From:   Heiner Kallweit <hkallweit1@gmail.com>
Message-ID: <b7bf02fd-c1aa-f430-524e-98922041ed81@gmail.com>
Date:   Sat, 26 Dec 2020 16:26:39 +0100
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On 17.11.2020 17:57, Rafael J. Wysocki wrote:
> 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.
> 

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)


>>> 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.
>