linux-kernel - Re: [PATCH v6 1/5] PCI: Clean up pci_scan

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Message-ID: <7741c617519b786750fbe04029fbb6295c8d6c85.camel@linux.ibm.com>
Date:   Thu, 30 Jun 2022 15:48:02 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Jan Kiszka <jan.kiszka@...mens.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/5] PCI: Clean up pci_scan_slot()

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@...mens.com>
> > Signed-off-by: Niklas Schnelle <schnelle@...ux.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.

> 
> 
> 
> >   }
> >   
> >   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!