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

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Date:   Fri, 13 May 2022 09:07:23 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
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 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