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Date:   Mon, 4 Jan 2021 18:39:44 +0100
From:   Lukas Wunner <lukas@wunner.de>
To:     Heiner Kallweit <hkallweit1@gmail.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        "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>,
        "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>
Subject: Re: Time to re-enable Runtime PM per default for PCI devcies?
Message-ID: <20210104173944.GA31187@wunner.de>
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On Thu, Dec 31, 2020 at 10:38:12AM +0100, Heiner Kallweit wrote:
> On 31.12.2020 05:07, Lukas Wunner wrote:
> > FWIW, if platform_pci_power_manageable() returns true, it can probably
> > be assumed that allowing runtime PM by default is okay.  So as a first
> > step, you may want to call that instead of adding a new callback.
> 
> I don't think that's sufficient. Most likely all the broken old systems
> return true for platform_pci_power_manageable().

platform_pci_power_manageable() is not a global flag, but rather
a per-device flag whether the platform is capable of power-managing
that device.  E.g. for the ACPI platform, it indicates that objects
such as _PS0 or _PS3 are present in the device's namespace.

My point is that if the platform can power-manage a device,
then it ought to be safe to enable runtime PM by default for it.

If you insist on a "big hammer" approach by turning on runtime PM
by default for everything, you risk regressions.  You can avoid
that by going for a smart approach which enables runtime PM in
cases when it's safe.

Thanks,

Lukas