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

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Date:   Mon, 11 Jul 2022 10:52:06 +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 16:50 +0200, Pierre Morel wrote:
> > > 

> 
> 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@...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.
> 
> 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.
> 

The fn value in this case iterates through the least significant 3 bits
of the geographical PCI address so yes this limits it to 7 functions.
My main qualm with the old code was that returning 0 for the end is
ambiguous because that is also a valid devfn.