linux-kernel - Re: [RFC v2 6/7] PCI: arm64: Allow pci_config

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

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@...il.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
>