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    SubjectRe: [PATCH RESEND v5 1/4] PCI: Clean up pci_scan_slot()
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    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@siemens.com>
    > > > Signed-off-by: Niklas Schnelle <schnelle@linux.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);
    }


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