From: Boqun Feng <boqun.feng@gmail.com>
Date: Tue, 27 Jul 2021 02:06:53 +0800
Patch-mainline: v5.15-rc1
Subject: arm64: PCI: Support root bridge preparation for Hyper-V
Git-commit: 7d40c0f70d92291605c4498b8ee4b3a3c3ba07b1
References: bsc#1197291

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.

Link: https://lore.kernel.org/r/20210726180657.142727-5-boqun.feng@gmail.com
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Olaf Hering <ohering@suse.de>
---
 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
--- 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);