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SubjectRe: [PATCH v6 4/8] arm64: PCI: Support root bridge preparation for Hyper-V
Catalin and Will,

Appreciate it that you can have a look at this one and patch #4, note
that there exists an alternative solution at[1].

The difference is the way used to pass the corresponding ACPI device
pointers for PCI host bridges: currently pci_config_window->parent is
used, and this patch and patch #4 allow the field to be NULL, because
Hyper-V's PCI host bridges don't have ACPI devices, while [1] changes to
use pci_host_bridge->private. And I'm OK with either way, I don't have a
strong opinion here ;-)

Looking forwards to your suggestion, Thanks!

Regards,
Boqun

[1]: https://lore.kernel.org/lkml/20210811153619.88922-1-boqun.feng@gmail.com/

On Tue, Jul 27, 2021 at 02:06:53AM +0800, Boqun Feng wrote:
> Currently at root bridge preparation, the corresponding ACPI device will
> be set as the companion, however for a Hyper-V virtual PCI root bridge,
> there is no corresponding ACPI device, because a Hyper-V virtual PCI
> root bridge is discovered via VMBus rather than ACPI table. In order to
> support this, we need to make pcibios_root_bridge_prepare() work with
> cfg->parent being NULL.
>
> Use a NULL pointer as the ACPI device if there is no corresponding ACPI
> device, and this is fine because: 1) ACPI_COMPANION_SET() can work with
> the second parameter being NULL, 2) semantically, if a NULL pointer is
> set via ACPI_COMPANION_SET(), ACPI_COMPANION() (the read API for this
> field) will return NULL, and since ACPI_COMPANION() may return NULL, so
> users must have handled the cases where it returns NULL, and 3) since
> there is no corresponding ACPI device, it would be wrong to use any
> other value here.
>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> arch/arm64/kernel/pci.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
> index 5148ae242780..2276689b5411 100644
> --- a/arch/arm64/kernel/pci.c
> +++ b/arch/arm64/kernel/pci.c
> @@ -90,7 +90,17 @@ int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge)
> return 0;
>
> cfg = bridge->bus->sysdata;
> - adev = to_acpi_device(cfg->parent);
> +
> + /*
> + * On Hyper-V there is no corresponding ACPI device for a root bridge,
> + * therefore ->parent is set as NULL by the driver. And set 'adev' as
> + * NULL in this case because there is no proper ACPI device.
> + */
> + if (!cfg->parent)
> + adev = NULL;
> + else
> + adev = to_acpi_device(cfg->parent);
> +
> bus_dev = &bridge->bus->dev;
>
> ACPI_COMPANION_SET(&bridge->dev, adev);
> --
> 2.32.0
>

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