linux-kernel - Re: [PATCH -v2 1/3] PCI: introduce PCIe Device Serial Number Capability support

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Date:	Thu, 11 Jul 2013 14:18:05 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Yijing Wang <wangyijing@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Rafael <rjw@...k.pl>, Hanjun Guo <guohanjun@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>
Subject: Re: [PATCH -v2 1/3] PCI: introduce PCIe Device Serial Number Capability
 support

On 07/11/2013 04:09 PM, Bjorn Helgaas wrote:
> On Thu, Jul 11, 2013 at 3:51 AM, Don Dutile<ddutile@...hat.com>  wrote:
>> On 07/11/2013 05:43 AM, Yijing Wang wrote:
>>>
>>> Introduce PCIe Ext Capability Device Serial Number support,
>>> so we can use the unique device serial number to identify
>>> the physical device. During system suspend, if the PCIe
>>> device was removed and inserted a new same device, after
>>> system resume there is no good way to identify it, maybe
>>> Device Serial Number is a good choice if device support.
>>>
>>> Signed-off-by: Yijing Wang<wangyijing@...wei.com>
>>> ---
>>>    drivers/pci/pci.c   |   27 +++++++++++++++++++++++++++
>>>    drivers/pci/probe.c |    2 ++
>>>    include/linux/pci.h |    3 +++
>>>    3 files changed, 32 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>> index e37fea6..2e855b5 100644
>>> --- a/drivers/pci/pci.c
>>> +++ b/drivers/pci/pci.c
>>> @@ -2048,6 +2048,33 @@ void pci_free_cap_save_buffers(struct pci_dev *dev)
>>>    }
>>>
>>>    /**
>>> + * pci_device_serial_number - get device serial number
>>> + * @dev: the PCI device
>>> + *
>>> + * return the device serial number if device support,
>>> + * otherwise return 0.
>>> + */
>>> +u64 pci_device_serial_number(struct pci_dev *dev)
>>
>> See comment below:
>> void  pci_device_serial_number(struct pci_dev *dev)
>>
>>
>>> +{
>>> +       int pos;
>>> +       u64 sn;
>>> +       u32 lo, hi;
>>> +
>>> +       if (!pci_is_pcie(dev))
>>> +               return 0;
>>> +
>>
>> See comment below:
>> if (!pci_is_pcie(dev)) {
>>          dev->sn = 0;
>>          return;
>>
>> }
>>>
>>> +       pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DSN);
>>> +       if (!pos)
>>> +               return 0;
>>> +
>>> +       pci_read_config_dword(dev, pos + 4,&lo);
>>> +       pci_read_config_dword(dev, pos + 8,&hi);
>>> +       sn = ((u64)hi<<   32) | lo;
>>
>> See comment below:
>>          dev->sn = ((u64)hi<<   32) | lo;
>>          return;
>>
>>> +       return sn;
>>> +}
>>> +EXPORT_SYMBOL(pci_device_serial_number);
>>> +
>>> +/**
>>>     * pci_configure_ari - enable or disable ARI forwarding
>>>     * @dev: the PCI device
>>>     *
>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>> index 46ada5c..c4c1a2b 100644
>>> --- a/drivers/pci/probe.c
>>> +++ b/drivers/pci/probe.c
>>> @@ -1322,6 +1322,8 @@ static void pci_init_capabilities(struct pci_dev
>>> *dev)
>>>          /* Power Management */
>>>          pci_pm_init(dev);
>>>
>>> +       dev->sn = pci_device_serial_number(dev);
>>> +
>>
>> Finally, 'the comment below':
>> I know you were following Bjorn's suggestion, which I thought
>> was an improvement, but why not do above assignment in
>> pci_device_serial_number() ?
>> See above....
>
> pci_device_serial_number() would then have the side-effect of saving
> the result somewhere, and callers would have to know where to look.
Ah, like so many other features of a PCI device?  -- what are all those
flags/status bits in pdev for ??? ;-)

> Personally, I think it's simpler to return the serial number directly
> and avoid the side-effect, but maybe this is just bike-shedding.
>
What struck me about rtning a value is in pci_init_capabilities(),
it was the only function with a rtn value that had to be put into a pci_dev 
struct element,
while all the others stored their related capabilities in the pdev, or other struct
related to it.

So, maybe there should be a "pci_device_serial_number_init()" function
which does the storage in the pdev struct, and another (wrapper,
dare I shoot myself, inline)  that is "pci_device_serial_number()" that returns
the value of the pdev's sn element, if another module or other core component
want to get it. .... and do that in a future patch... :-/

> As long as we are bike-shedding, I'd just drop "sn" and do:
>
>      return ((u64)hi<<  32) | lo;
>
> And of course, you need spaces before "&hi" and "&lo" in the
> pci_read_config_dword() calls.

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