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

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

[Bjorn Helgaas]

[+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.

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

[Bjorn Helgaas]

[+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.

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

[Vaibhav Gupta]

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.

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