linux-kernel - Re: [PATCH v6 4/8] arm64: PCI: Support root bridge preparation for Hyper-V

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Date:   Mon, 23 Aug 2021 10:43:44 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, Arnd Bergmann <arnd@...db.de>,
        Marc Zyngier <maz@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Will Deacon <will@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH v6 4/8] arm64: PCI: Support root bridge preparation for
 Hyper-V

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@...il.com>

Acked-by: Catalin Marinas <catalin.marinas@....com>