linux-kernel - Re: [PATCH 1/1] powerpc: Drop superfluous pci_dev_is

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Message-ID: <CAOSf1CEQB_Fz_yF0pgs6xqJJ2Say1a2XFjOedO2mE=Qn_BgbnQ@mail.gmail.com>
Date:   Tue, 14 Sep 2021 00:58:20 +1000
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 1/1] powerpc: Drop superfluous pci_dev_is_added() calls

On Sat, Sep 11, 2021 at 9:09 PM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Niklas Schnelle <schnelle@...ux.ibm.com> writes:
> > On powerpc, pci_dev_is_added() is called as part of SR-IOV fixups
> > that are done under pcibios_add_device() which in turn is only called in
> > pci_device_add() whih is called when a PCI device is scanned.
>
> Thanks for cleaning this up for us.
>
> > Now pci_dev_assign_added() is called in pci_bus_add_device() which is
> > only called after scanning the device. Thus pci_dev_is_added() is always
> > false and can be dropped.
>
> My only query is whether we can pin down when that changed.
>
> Oliver said:
>
>   The use of pci_dev_is_added() in arch/powerpc was because in the past
>   pci_bus_add_device() could be called before pci_device_add(). That was
>   fixed a while ago so It should be safe to remove those calls now.
>
> I trawled back through the history a bit but I can't remember/find which
> commit changed that, Oliver can you remember?

Yeah, on closer inspection that never happened. The re-ordering I was
thinking of was when the boot-time BAR assignments were moved in
3f068aae7a95 so they'd always occur before pci_bus_add_device() was
called. I think I got that change mixed up with commit 30d87ef8b38d
("powerpc/pci: Fix pcibios_setup_device() ordering") which moved some
of what what pcibios_device_add() did into pcibios_bus_add_device() to
harmonise the hot and coldplug paths.

As far as I can tell the pci_dev_is_added() check has been pointless
since the code was added in 6e628c7d33d9 ("powerpc/powernv: Reserve
additional space for IOV BAR according to the number of total_pe").
Even back then pci_device_add() was called first in both the normal
and OF based PCI probing paths so there's no circumstance where that
code would see the added flag set.

That patch was part of the PowerNV SRIOV support series which went
through quite a few iterations. My best guess is that check might have
been needed in an earlier version and was just carried forward until
it got merged. I didn't dig too deeply into the history though.

Reviewed-by: Oliver O'Halloran <oohall@...il.com>