linux-kernel - Re: [PATCH 1/2] i2c: i801: Fix runtime PM

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Date:   Mon, 31 Aug 2020 20:45:30 +0530
From:   Vaibhav Gupta <vaibhavgupta40@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Jean Delvare <jdelvare@...e.de>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        linux-i2c@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        stable@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci@...r.kernel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c: i801: Fix runtime PM

On Fri, Aug 28, 2020 at 11:26:40AM -0500, Bjorn Helgaas wrote:
> [+cc Vaibhav]
> 
> On Wed, Jun 27, 2018 at 04:23:40PM -0500, Bjorn Helgaas wrote:
> > [+cc Rafael, linux-pm, linux-kernel]
> > 
> > On Wed, Jun 27, 2018 at 10:15:50PM +0200, Jean Delvare wrote:
> > > Hi Jarkko,
> > > 
> > > On Tue, 26 Jun 2018 17:39:12 +0300, Jarkko Nikula wrote:
> > > > Commit 9c8088c7988 ("i2c: i801: Don't restore config registers on
> > > > runtime PM") nullified the runtime PM suspend/resume callback pointers
> > > > while keeping the runtime PM enabled. This causes that device stays in
> > > > D0 power state and sysfs /sys/bus/pci/devices/.../power/runtime_status
> > > > shows "error" when runtime PM framework attempts to autosuspend the
> > > > device.
> > > > 
> > > > This is due PCI bus runtime PM which checks for driver runtime PM
> > > > callbacks and returns with -ENOSYS if they are not set. Fix this by
> > > > having a shared dummy runtime PM callback that returns with success.
> > > > 
> > > > Fixes: a9c8088c7988 ("i2c: i801: Don't restore config registers on runtime PM")
> > > 
> > > I don't want to sound like I'm trying to decline all responsibility for
> > > a regression I caused, but frankly, if just using SIMPLE_DEV_PM_OPS()
> > > breaks runtime PM, then it's the PM model which is broken, not the
> > > i2c-i801 driver.
> > > 
> > > I will boldly claim that the PCI bus runtime code is simply wrong in
> > > returning -ENOSYS in the absence of runtime PM callbacks, and it should
> > > be changed to return 0 instead. Or whoever receives that -ENOSYS should
> > > not treat it as an error - whatever makes more sense.
> > > 
> > > Having to add dummy functions in every PCI driver that doesn't need to
> > > do anything special for runtime PM sounds plain stupid. It should be
> > > pretty obvious that a whole lot of drivers are going to use
> > > SIMPLE_DEV_PM_OPS() because it exists and seems to do what they want,
> > > and all of them will be bugged because the PCI core is doing something
> > > silly and unexpected.
> > > 
> > > So please let's fix it at the PCI subsystem core level. Adding Bjorn
> > > and the linux-pci list to Cc.
> > 
> > Thanks Jean.  What you describe does sound broken.  I think the PM
> > guys (cc'd) will have a better idea of how to deal with this.
> 
> Did we ever get anywhere with this?  It seems like the thread petered
> out.
This does seems worrying. I remember, few days earlier you pointed out a driver
i2c-nvidia-gpuc.c. In the code, gpu_i2c_suspend() is an empty-body function. And
comment mentioned that empty stub is necessary for runtime_pm to work.

And this driver also uses UNIVERSAL_DEV_PM_OPS.