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

Pierre Morel
2022-06-30T14:46:01Z

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