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 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;

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,

Thanks
Mike


> circumstance.
>
> Best Regards,
> Huang Ying
>
>