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 16:37 +0800, mike wrote:
> On 11/14/2013 04:25 PM, Huang Ying wrote:
> > On Thu, 2013-11-14 at 16:12 +0800, mike wrote:
> >> On 11/14/2013 03:53 PM, Huang Ying wrote:
> >>> 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
> >> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >> This means dev->driver is NULL ?? but pci_dev->driver is set???
> >>
> >> Because if use pci_dev->driver can put into low power state, means
> >>
> >> pci_dev->driver is set, but in the situation, use dev->driver will can't,
> >>
> >> means dev->driver = null, but I have not find any case that
> >>
> >> dev->driver = null, but pci_dev->driver != null;
> > Sorry I make a mistake here.  The dev->driver != null in
> > local_pci_probe().  We use pci_dev->driver instead of dev->driver in
> > pci_pm_runtime_xxx() because we want device to be kept in normal power
> > state (D0) and SUSPENDED state when unbound.The
> > pm_runtime_put/get_sync in pci_device_remove/local_pci_probe will not
> > change the power state of the device because of the check in
> > pci_pm_runtime_xxx().
> Yes, you are right, but what am I confuse is that, why check dev->driver
> in pci_pm_runtime_xxx() can't keep the device in normal power
> state (D0) and SUSPENDED state when unbound.
> 
> May be logic issue ?

Because dev->driver is set before local_pci_probe() and cleared after
pci_device_remove().  But we need a flag to be changed in
local_pci_probe() and pci_device_remove().

Best Regards,
Huang Ying