Subject: Re: [PATCH v2 3/6] powerpc/pci: use pci_is_pcie() to simplify code

From: Gavin Shan <[email-masked]>
Date: Fri, 11 Oct 2013 15:56:42 +0800

On Fri, Oct 11, 2013 at 03:28:02PM +0800, Yijing Wang wrote:
>On 2013/10/11 14:53, Gavin Shan wrote:
>> On Fri, Oct 11, 2013 at 02:33:58PM +0800, Yijing Wang wrote:
>>> On 2013/10/11 14:16, Gavin Shan wrote:
>>>> On Fri, Oct 11, 2013 at 04:49:56PM +1100, Benjamin Herrenschmidt wrote:
>>>>> On Fri, 2013-09-06 at 14:30 -0600, Bjorn Helgaas wrote:
>>>>>> On Thu, Sep 05, 2013 at 03:55:27PM +0800, Yijing Wang wrote:
>> 
>> .../...
>> 
>>>>>>> Use pci_is_pcie() to simplify code.
>>>>>>> diff --git a/arch/powerpc/kernel/eeh.c b/arch/powerpc/kernel/eeh.c
>>>>>>> index 55593ee..6ebbe54 100644
>>>>>>> --- a/arch/powerpc/kernel/eeh.c
>>>>>>> +++ b/arch/powerpc/kernel/eeh.c
>>>>>>> @@ -189,8 +189,7 @@ static size_t eeh_gather_pci_data(struct eeh_dev *edev, char * buf, size_t len)
>>>>>>>  	}
>>>>>>>  
>>>>>>>  	/* If PCI-E capable, dump PCI-E cap 10, and the AER */
>>>>>>> -	cap = pci_find_capability(dev, PCI_CAP_ID_EXP);
>>>>>>> -	if (cap) {
>>>>>>> +	if (pci_is_pcie(dev)) {
>>>>>>>  		n += scnprintf(buf+n, len-n, "pci-e cap10:\n");
>>>>>>>  		printk(KERN_WARNING
>>>>>>>  		       "EEH: PCI-E capabilities and status follow:\n");
>>>>>
>>>>> So we remove reading of "cap", but slightly further down the code does:
>>>>>
>>>>> 		for (i=0; i<=8; i++) {
>>>>> 			eeh_ops->read_config(dn, cap+4*i, 4, &cfg);
>>>>> 			n += scnprintf(buf+n, len-n, "%02x:%x\n", 4*i, cfg);
>>>>> 			printk(KERN_WARNING "EEH: PCI-E %02x: %08x\n", i, cfg);
>>>>> 		}
>>>>>
>>>>> Which actually *uses* the value of "cap" ... oops :-)
>>>>>
>>>>
>>>> It's my fault and I should have looked into the changes more closely.
>>>> How about changing it like this:
>>>>
>>>> 	cap = pci_is_pcie(dev) ? pci_pcie_cap(dev) :
>>>> 	      pci_find_capability(dev, PCI_CAP_ID_EXP);
>>>> 	if (cap) {
>>>> 		...
>>>> 	}
>>>>
>>>> It would save some PCI-CFG access cycles for most cases :-)
>>>
>>> Hi Gavin,  it's not your fault, it's my fault. :)
>>>
>>> Because pci_pcie_cap(dev) == dev->pcie_cap == pci_find_capability(dev, PCI_CAP_ID_EXP);
>>>
>>> so I think it's ok to use dev->pcie_cap instead of stale "cap".
>>>
>> 
>> Yijing, There has one exception: dev->pcie_cap isn't updated yet.
>
>In my idea, dev->pcie_cap(here is pci_dev->pcie_cap) will update in set_pcie_port_type() function,
>and this function always be called after allocate pci device. We get pci_dev by eeh_dev_to_pci_dev(),
>I think pci_dev has been initialized completely.
>
>> This function has possibility to be invoked before that. However,
>> we don't have the binding (eeh device <-> PCI device) for the case.
>> So the piece of code shouldn't be running
>
>In PCI core, I knew
>
>pci_scan_device()
>   pci_setup_device()
>       set_pcie_port_type()
>            pci_dev->pcie_cap = pci_find_capability(pdev, PCI_CAP_ID_EXP);
>
>In powerpc, I also found
>
>of_scan_pci_dev()
>   of_create_pci_dev()
>       set_pcie_port_type()
>	    pci_dev->pcie_cap = pci_find_capability(pdev, PCI_CAP_ID_EXP);
>> 
>> However, it's a bit safer to have pci_find_capability(dev, PCI_CAP_ID_EXP)
>> as well even though we needn't it for 99.9% cases if you agree :-)
>
>I agree, this function is not the performance bottleneck,
>safety is more important. :)
>So if Bjorn and Benjamin think it's not safe, it's ok to drop it. :)
>

No, it's not what I mean. Anyway, "v3" looks good to me.
At least, it can save PCI-CFG access cycles find locate
the PCIe capability position :-)

Thanks,
Gavin