linux-kernel - Re: [PATCH RESEND v5 1/4] PCI: Clean up pci_scan

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Date:   Mon, 23 May 2022 10:40:53 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-s390@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH RESEND v5 1/4] PCI: Clean up pci_scan_slot()

On Fri, 2022-05-13 at 09:07 -0500, Bjorn Helgaas wrote:
> On Thu, May 12, 2022 at 04:56:42PM +0200, Niklas Schnelle wrote:
> > On Thu, 2022-05-05 at 10:38 +0200, 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>
> > > ---
> > 
> > Friendly ping :-)
> 
> Thanks and sorry for the delay.  I'm off today for my daughter's
> wedding reception but will get back to it next week.  Just to expose
> some of my thought process (and not to request more work from you!)
> I've been wondering whether b1bd58e448f2 ("PCI: Consolidate
> "next-function" functions") is really causing us more trouble than
> it's worth.  In some ways that makes the single next-function harder
> to read.  But I guess the hypervisor special case is not exactly a
> "next-function" thing -- it's a "keep scanning even if there's no fn
> 0" thing.
> 
> Bjorn

I've thought again about your comment. Personally what I like about
b1bd58e448f2 ("PCI: Consolidate "next-function" functions") is that it got rid of the next_fn function pointer complication. I agree though that on the other hand it removed a nice separation between the ARI and traditional cases. So I'm thinking maybe we should bring that part back. I think my patch as is makes it easier to see the equivalence to the existing code but then we could add a patch on top and turn it into the below, it's a bit more verbose but very easy to follow.

static int next_ari_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
{
…
}

static int next_trad_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
{
	if (fn >= 7)
		return -ENODEV;

	/* only multifunction devices may have more functions */
	if (dev && !dev->multifunction)
		return -ENODEV;

	return fn + 1;
}

static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
{
	if (pci_ari_enabled(bus)) {
		return next_ari_fn(bus, dev, fn);
	}
	return next_trad_fn(bus, dev, fn);
}