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

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

[Jean Delvare]

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 Delvare
SUSE L3 Support

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