linux-kernel - Re: Time to re-enable Runtime PM per default for PCI devcies?

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Date:   Thu, 31 Dec 2020 10:38:12 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
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>,
        Kai Heng Feng <kai.heng.feng@...onical.com>,
        "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 31.12.2020 05:07, Lukas Wunner wrote:
> On Wed, Dec 30, 2020 at 11:56:04PM +0100, Heiner Kallweit wrote:
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -3024,7 +3024,9 @@ void pci_pm_init(struct pci_dev *dev)
>>  	u16 status;
>>  	u16 pmc;
>>  
>> -	pm_runtime_forbid(&dev->dev);
>> +	if (pci_acpi_forbid_runtime_pm())
>> +		pm_runtime_forbid(&dev->dev);
>> +
> 
> Generic PCI code usually does not call ACPI-specific functions directly,
> but rather through a pci_platform_pm_ops callback.
> 
> 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(). So yes, most likely
we'd need a new callback if we want to have the platform ops abstraction.
But it could be an optional callback, something like: forbid_runtime_pm
The question is just: is it worth it?

By the way: pci_set_platform_pm() returns an error if a callback isn't
set, but no existing caller bothers to check the return code.

> Thanks,
> 
> Lukas
> 
Heiner