Re: [Qemu-devel] [PATCH 3/4] hw:acpi:pcihp: assume root PCI bus if bus has no ACPI_PCIHP_PROP_BSEL property

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 21, 2014 at 03:40:05PM +0100, Igor Mammedov wrote:
> when running with machine types older than 1.7 (i.e. without ACPI
> builtin tables), PCI bus won't have ACPI_PCIHP_PROP_BSEL property
> set.
> Taking in account that acpi hotplug handler in 1.7 and older
> machines is called only for root PCI bus, to make pcihp code
> compatible with legacy machine types assume that bus without
> ACPI_PCIHP_PROP_BSEL property has it equal to 0 and bail out
> if it's not root bus.
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>

I think that's not the best way to do this.
If bsel 0 *is* set on some bus, it should select it.
Fallback to bus 0 only if bsel value 0 is not set anywhere.
See e.g. how acpi_pcihp_find_hotplug_bus does it:

    if (!bsel && !find.bus) {
        find.bus = s->root;
    }

otherwise we introduce dependency on the logic that sets
bsel, this makes code fragile.

> ---
>  hw/acpi/pcihp.c |   10 +++-------
>  1 files changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
> index 6d34fe9..76dce8d 100644
> --- a/hw/acpi/pcihp.c
> +++ b/hw/acpi/pcihp.c
> @@ -63,14 +63,10 @@ static int acpi_pcihp_get_bsel(PCIBus *bus)
>  {
>      QObject *o = object_property_get_qobject(OBJECT(bus),
>                                               ACPI_PCIHP_PROP_BSEL, NULL);
> -    int64_t bsel = -1;
>      if (o) {
> -        bsel = qint_get_int(qobject_to_qint(o));
> +        return qint_get_int(qobject_to_qint(o));
>      }
> -    if (bsel < 0) {
> -        return -1;
> -    }
> -    return bsel;
> +    return 0;
>  }
>  
>  static void acpi_pcihp_test_hotplug_bus(PCIBus *bus, void *opaque)
> @@ -190,7 +186,7 @@ int acpi_pcihp_device_hotplug(AcpiPciHpState *s, PCIDevice *dev,
>  {
>      int slot = PCI_SLOT(dev->devfn);
>      int bsel = acpi_pcihp_get_bsel(dev->bus);
> -    if (bsel < 0) {
> +    if ((bsel == 0) && (dev->bus != s->root)) {
>          return -1;
>      }
>  
> -- 
> 1.7.1