Re: [RFC v2 6/7] PCI: arm64: Allow pci_config_window::parent to be NULL

From: Bjorn Helgaas <helgaas@kernel.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
	Arnd Bergmann <arnd@arndb.de>, Marc Zyngier <maz@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Andrew Lunn <andrew@lunn.ch>,
	Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>,
	Gregory Clement <gregory.clement@bootlin.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	Mike Rapoport <rppt@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jon Derrick <jonathan.derrick@intel.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-hyperv@vger.kernel.org
Subject: Re: [RFC v2 6/7] PCI: arm64: Allow pci_config_window::parent to be NULL
Date: Thu, 6 May 2021 17:25:30 -0500	[thread overview]
Message-ID: <20210506222530.GA1441653@bjorn-Precision-5520> (raw)
In-Reply-To: <20210503144635.2297386-7-boqun.feng@gmail.com>

Make your subject something like this so it matches previous practice:

  arm64: PCI: ...

The "::" notation probably comes from C++, but doesn't really apply in
C.  In C, we would say "cfg.parent" or "cfg->parent".

But pci_config_window and cfg->parent are probably too low-level for
the subject anyway.  Seems like it should mention Hyper-V, for
instance.

On Mon, May 03, 2021 at 10:46:34PM +0800, Boqun Feng wrote:
> This is purely a hack, for ARM64 Hyper-V guest, there is no
> corresponding ACPI device for the root bridge, so the best we can
> provide is an all-zeroed pci_config_window, and in this case make
> pcibios_root_bridge_prepare() act as the ACPI device is NULL.

Why is there no ACPI device?  Is this a needless arch dependency?  Or
is this related to using DT instead of ACPI?

The cover letter hints that this might be related to
PCI_DOMAINS_GENERIC=y, but that doesn't sound like a very convincing
reason (and the cover letter can provide an overview, but the commit
logs of individual patches shouldn't assume knowledge of the cover
letter).

> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  arch/arm64/kernel/pci.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
> index e9a6eeb6a694..f159df903ccb 100644
> --- a/arch/arm64/kernel/pci.c
> +++ b/arch/arm64/kernel/pci.c
> @@ -83,7 +83,7 @@ int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge)
>  {
>  	if (!acpi_disabled) {
>  		struct pci_config_window *cfg = bridge->bus->sysdata;
> -		struct acpi_device *adev = to_acpi_device(cfg->parent);
> +		struct acpi_device *adev = cfg->parent ? to_acpi_device(cfg->parent) : NULL;
>  		struct device *bus_dev = &bridge->bus->dev;
>  
>  		ACPI_COMPANION_SET(&bridge->dev, adev);
> -- 
> 2.30.2
> 

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