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

[Jarkko Nikula]

On 8/31/20 6:15 PM, Vaibhav Gupta wrote:
> 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.
This was fixed by c5eb1190074c ("PCI / PM: Allow runtime PM without 
callback functions"). So no need for empty runtime PM callbacks anymore.

--
Jarkko