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

[mike <qiudayu@linux.vnet.ibm.com>]

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 ?

Thanks
Mike
>
>> I know I always been a question guy, i apologize for spend a lot
>> time to reply this mail, but I really want to understand it,
> Never mind.
>
> Best Regards,
> Huang Ying
>
>