Re: [PATCH v6 1/5] PCI: Clean up pci_scan_slot()

[Pierre Morel <pmorel@linux.ibm.com>]

On 6/30/22 15:48, Niklas Schnelle wrote:
> On Thu, 2022-06-30 at 14:40 +0200, Pierre Morel wrote:
>>
>> On 6/28/22 16:30, Niklas Schnelle wrote:
>>> While determining the next PCI function is factored out of
>>> pci_scan_slot() into next_fn() the former still handles the first
>>> function as a special case. This duplicates the code from the scan loop.
>>>
>>> Furthermore the non ARI branch of next_fn() is generally hard to
>>> understand and especially the check for multifunction devices is hidden
>>> in the handling of NULL devices for non-contiguous multifunction. It
>>> also signals that no further functions need to be scanned by returning
>>> 0 via wraparound and this is a valid function number.
>>>
>>> Improve upon this by transforming the conditions in next_fn() to be
>>> easier to understand.
>>>
>>> By changing next_fn() to return -ENODEV instead of 0 when there is no
>>> next function we can then handle the initial function inside the loop
>>> and deduplicate the shared handling. This also makes it more explicit
>>> that only function 0 must exist.
>>>
>>> No functional change is intended.
>>>
>>> Cc: Jan Kiszka <jan.kiszka@siemens.com>
>>> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
>>> ---
>>>    drivers/pci/probe.c | 38 +++++++++++++++++++-------------------
>>>    1 file changed, 19 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>> index 17a969942d37..b05d0ed83a24 100644
>>> --- a/drivers/pci/probe.c
>>> +++ b/drivers/pci/probe.c
>>> @@ -2579,8 +2579,7 @@ struct pci_dev *pci_scan_single_device(struct pci_bus *bus, int devfn)
>>>    }
>>>    EXPORT_SYMBOL(pci_scan_single_device);
>>>    
>>> -static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
>>> -			    unsigned int fn)
>>> +static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
>>>    {
>>>    	int pos;
>>>    	u16 cap = 0;
>>> @@ -2588,24 +2587,26 @@ static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
>>>    
>>>    	if (pci_ari_enabled(bus)) {
>>>    		if (!dev)
>>> -			return 0;
>>> +			return -ENODEV;
>>>    		pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ARI);
>>>    		if (!pos)
>>> -			return 0;
>>> +			return -ENODEV;
>>>    
>>>    		pci_read_config_word(dev, pos + PCI_ARI_CAP, &cap);
>>>    		next_fn = PCI_ARI_CAP_NFN(cap);
>>>    		if (next_fn <= fn)
>>> -			return 0;	/* protect against malformed list */
>>> +			return -ENODEV;	/* protect against malformed list */
>>>    
>>>    		return next_fn;
>>>    	}
>>>    
>>> -	/* dev may be NULL for non-contiguous multifunction devices */
>>> -	if (!dev || dev->multifunction)
>>> -		return (fn + 1) % 8;
>>> +	if (fn >= 7)
>>> +		return -ENODEV;
>>> +	/* only multifunction devices may have more functions */
>>> +	if (dev && !dev->multifunction)
>>> +		return -ENODEV;
>>>    
>>> -	return 0;
>>> +	return fn + 1;
>>
>> No more % 8 ?
>> Even it disapear later shouldn't we keep it ?
> 
> The "% 8" became unnecessary due to the explicit "if (fn >= 7)"
> above.
> The original "% 8" did what I referred to in the commit message with
> "It [the function] also signals that no further functions need to be
> scanned by returning 0 via wraparound and this is a valid function
> number.". Instead we now explicitly return -ENODEV in this case.

Yes it goes with it.
With this code next_fn returns -ENODEV for fn = 8 instead of previously 
returning 1. (If I am right)

With the previous code, did we assume that next_fn is never called with 
fn > 7?
I guess yes as we test pci_ari_enabled first and without ARI we do not 
have more than 7 more functions. is it right?

For what I think this new code seems better as it does not make the 
assumption that it get called with fn < 8.

> 
>>
>>
>>
>>>    }
>>>    
>>>    static int only_one_child(struct pci_bus *bus)
>>> @@ -2643,26 +2644,25 @@ static int only_one_child(struct pci_bus *bus)
>>>     */
>>>    int pci_scan_slot(struct pci_bus *bus, int devfn)
>>>    {
>>> -	unsigned int fn, nr = 0;
>>>    	struct pci_dev *dev;
>>> +	int fn = 0, nr = 0;
>>>    
>>>    	if (only_one_child(bus) && (devfn > 0))
>>>    		return 0; /* Already scanned the entire slot */
>>>    
>>> -	dev = pci_scan_single_device(bus, devfn);
>>> -	if (!dev)
>>> -		return 0;
>>> -	if (!pci_dev_is_added(dev))
>>> -		nr++;
>>> -
>>> -	for (fn = next_fn(bus, dev, 0); fn > 0; fn = next_fn(bus, dev, fn)) {
>>> +	do {
>>>    		dev = pci_scan_single_device(bus, devfn + fn);
>>>    		if (dev) {
>>>    			if (!pci_dev_is_added(dev))
>>>    				nr++;
>>> -			dev->multifunction = 1;
>>> +			if (fn > 0)
>>> +				dev->multifunction = 1;
>>> +		} else if (fn == 0) {
>>> +			/* function 0 is required */
>>> +			break;
>>>    		}
>>> -	}
>>> +		fn = next_fn(bus, dev, fn);
>>> +	} while (fn >= 0);
>>>    
>>>    	/* Only one slot has PCIe device */
>>>    	if (bus->self && nr)
>>>
>>
>> Otherwise LGTM
>>
> 
> Thanks for taking a look!
> 

-- 
Pierre Morel
IBM Lab Boeblingen