linux-kernel - Re: [PATCH 1/2] ACPI / hotplug / PCI: Avoid doing too much for spurious notifies

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Date:	Sat, 07 Sep 2013 01:45:31 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	Jiang Liu <liuj97@...il.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/2] ACPI / hotplug / PCI: Avoid doing too much for spurious notifies

On Friday, September 06, 2013 08:46:28 AM Yinghai Lu wrote:
> On Fri, Sep 6, 2013 at 6:43 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Sometimes we may get a spurious device check or bus check notify for
> > a hotplug device and in those cases we should avoid doing all of the
> > configuration work needed when something actually changes.  To that
> > end, check the return value of pci_scan_slot() in enable_slot() and
> > bail out early if it is 0.
> >
> > This turns out to help reduce the amount of diagnostic output from
> > the ACPIPHP subsystem and speed up boot on at least one system that
> > generates multiple device check notifies for PCIe ports during
> > boot.
> >
> > Reported-by: Alex Williamson <alex.williamson@...hat.com>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/pci/hotplug/acpiphp_glue.c |   35 +++++++++++++++++++++++++++--------
> >  1 file changed, 27 insertions(+), 8 deletions(-)
> >
> > Index: linux-pm/drivers/pci/hotplug/acpiphp_glue.c
> > ===================================================================
> > --- linux-pm.orig/drivers/pci/hotplug/acpiphp_glue.c
> > +++ linux-pm/drivers/pci/hotplug/acpiphp_glue.c
> > @@ -542,12 +542,12 @@ static void __ref enable_slot(struct acp
> >         struct acpiphp_func *func;
> >         int max, pass;
> >         LIST_HEAD(add_list);
> > +       int nr_found;
> >
> >         list_for_each_entry(func, &slot->funcs, sibling)
> >                 acpiphp_bus_add(func_to_handle(func));
> >
> > -       pci_scan_slot(bus, PCI_DEVFN(slot->device, 0));
> > -
> > +       nr_found = pci_scan_slot(bus, PCI_DEVFN(slot->device, 0));
> >         max = acpiphp_max_busnr(bus);
> >         for (pass = 0; pass < 2; pass++) {
> >                 list_for_each_entry(dev, &bus->devices, bus_list) {
> > @@ -566,8 +566,11 @@ static void __ref enable_slot(struct acp
> >                         }
> >                 }
> >         }
> > -
> >         __pci_bus_assign_resources(bus, &add_list, NULL);
> > +       /* Nothing more to do here if there are no new devices on this bus. */
> > +       if (!nr_found && (slot->flags & SLOT_ENABLED))
> > +               return;
> > +
> >         acpiphp_sanitize_bus(bus);
> >         acpiphp_set_hpp_values(bus);
> >         acpiphp_set_acpi_region(slot);
> >
> 
> why not just returning early before size bridges and assign unassign resources?

Because we still need to do that even if pci_scan_slot() returns 0.

There may be new devices down in the hierarchy and we need to look for them.

Thanks,
Rafael

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