Re: A question about the patch: [PATCH] PCI/PM: Keep runtime PM enabled for unbound PCI devices

[Huang Ying <ying.huang@intel.com>]

On Thu, 2013-11-14 at 15:19 +0800, mike wrote:
> On 11/14/2013 01:59 PM, Huang Ying wrote:
> > On Thu, 2013-11-14 at 11:23 +0800, mike wrote:
> >> On 11/14/2013 03:20 AM, Alan Stern wrote:
> >>> On Wed, 13 Nov 2013, Bjorn Helgaas wrote:
> >>>
> >>>> [+cc Rafael, linux-pm]
> >>>>
> >>>> On Wed, Nov 13, 2013 at 6:09 AM, mike<qiudayu@linux.vnet.ibm.com>  wrote:
> >>>>> Hi Huang Ying,
> >>>>>
> >>>>> I see you are the author of this patch, commit id is:
> >>>>> 967577b062417b4e4b8e27b711220f4124f5153a
> >>>>>
> >>>>> I have a question while I try to understand this patch,
> >>>>> So I would very grateful if you or others can give me some reply.....
> >>>>>
> >>>>> ............
> >>>>> -       rc = ddi->drv->probe(ddi->dev, ddi->id);
> >>>>> +       pm_runtime_get_sync(dev);
> >>>>> +       pci_dev->driver = pci_drv;
> >>>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^
> >>>>> I see here you make the driver to initialize before probe,
> >>>>> But I have no idea of why you do this change.....
> >>>>>
> >>>>> and I look inside the code, it may be pm_runtime relate??
> >>> Yes, it is related to runtime PM.  In the PCI subsystem, runtime PM
> >>> doesn't do anything unless pci_dev->driver is set.  You can see this at
> >>> the start of pci_pm_runtime_suspend().
> >>>
> >>> Since we want the driver's probe routine to be able to carry out
> >>> runtime PM operations, we have to set pci_dev->driver before the probe
> >>> routine runs.
> >> Is there any situations , like in  probe state,  pci_dev->driver
> >> has been set. the  pci_pm_runtime_xxx() has passed
> >> pci_dev->driver NULL check, but at this point, probe fail
> >> occurs, and  pci_dev->driver to be set to NULL.
> >>
> >> What will happen ? Or this situation will never happen?
> >> I'm confuse about this.
> > I think that will never happen.  Before ->probe(), pm_runtime_get_sync()
> > is called, so pci_pm_runtime_xxx() will not be called until
> > pm_runtime_put_noidle() is called in ->probe().  And
> >   should be done as one of the latest actions in
> > ->probe(), after the normal probe actions succeeded.
> OK, just as your description, it seems OK.
> But this is really a issue as I explained in last email.
> 
> So I want to know if there are any side-effect of changing the code
> in pci_pm_runtime_xxx()
> 
>   if (!pci_dev->driver)
>          return 0;
>   to
> 
>   if (!dev->driver)
>          return 0;
> 

If you make this change, we can not put devices into low power state
(runtime suspend the device) in ->probe().  That is expected in some
circumstance.

Best Regards,
Huang Ying