[1/5] PCI: Make sure all bridges reserve at least one bus number

When distributing extra buses between hotplug bridges we need to make
sure each bridge reserve at least one bus number, even if there is
currently nothing connected to it. For instance ACPI hotplug may bring
in additional devices to non-hotplug bridges later on. To make sure we
don't run out of bus numbers for non-hotplug bridges reserve one bus
number for them upfront before distributing buses for hotplug bridges.

Fixes: 1c02ea810065 ("PCI: Distribute available buses to hotplug-capable bridges")
Reported-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: stable@vger.kernel.org
---
 drivers/pci/probe.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

On Tue, Feb 13, 2018 at 5:31 PM, Mika Westerberg
<mika.westerberg@linux.intel.com> wrote:
> When distributing extra buses between hotplug bridges we need to make
> sure each bridge reserve at least one bus number, even if there is
> currently nothing connected to it. For instance ACPI hotplug may bring
> in additional devices to non-hotplug bridges later on. To make sure we
> don't run out of bus numbers for non-hotplug bridges reserve one bus
> number for them upfront before distributing buses for hotplug bridges.
>
> Fixes: 1c02ea810065 ("PCI: Distribute available buses to hotplug-capable bridges")
> Reported-by: Mario Limonciello <mario.limonciello@dell.com>
> Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> Cc: stable@vger.kernel.org

Just a few notation-related nits below.

> ---
>  drivers/pci/probe.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index ef5377438a1e..3e91d2191a7d 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2561,7 +2561,8 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>         for_each_pci_bridge(dev, bus) {
>                 cmax = max;
>                 max = pci_scan_bridge_extend(bus, dev, max, 0, 0);
> -               used_buses += cmax - max;
> +               /* Reserve one bus for each bridge */
> +               used_buses += cmax - max ?: 1;

I would write this as

used_buses++;
if (cmax - max > 1)
        used_buses += cmax - max - 1;

to make this and the new code below look analogous.

>         }
>
>         /* Scan bridges that need to be reconfigured */
> @@ -2584,12 +2585,14 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>                          * bridges if any.
>                          */
>                         buses = available_buses / hotplug_bridges;
> -                       buses = min(buses, available_buses - used_buses);
> +                       buses = min(buses, available_buses - (used_buses - 1));

You can write the inner expression as "available_buses - used_buses +
1" (without the extra parens).

>                 }
>
>                 cmax = max;
>                 max = pci_scan_bridge_extend(bus, dev, cmax, buses, 1);
> -               used_buses += max - cmax;
> +               /* One bus is already accounted so don't add it again */
> +               if (max - cmax > 1)
> +                       used_buses += (max - cmax) - 1;

The parens on the right-hand side are not necessary here AFAICS.

>         }
>
>         /*
> --

On Tue, Feb 13, 2018 at 06:43:00PM +0100, Rafael J. Wysocki wrote:
> On Tue, Feb 13, 2018 at 5:31 PM, Mika Westerberg
> <mika.westerberg@linux.intel.com> wrote:
> > When distributing extra buses between hotplug bridges we need to make
> > sure each bridge reserve at least one bus number, even if there is
> > currently nothing connected to it. For instance ACPI hotplug may bring
> > in additional devices to non-hotplug bridges later on. To make sure we
> > don't run out of bus numbers for non-hotplug bridges reserve one bus
> > number for them upfront before distributing buses for hotplug bridges.
> >
> > Fixes: 1c02ea810065 ("PCI: Distribute available buses to hotplug-capable bridges")
> > Reported-by: Mario Limonciello <mario.limonciello@dell.com>
> > Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > Cc: stable@vger.kernel.org
> 
> Just a few notation-related nits below.

Thanks for the comments! I'll update the patch accordingly.