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

From: Boqun Feng <boqun.feng@gmail.com>
To: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>
Cc: "Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Wei Liu" <wei.liu@kernel.org>,
	"Dexuan Cui" <decui@microsoft.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
	linux-pci@vger.kernel.org,
	"Sunil Muthuswamy" <sunilmut@microsoft.com>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Arnd Bergmann" <arnd@arndb.de>, "Marc Zyngier" <maz@kernel.org>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>
Subject: Re: [PATCH v6 4/8] arm64: PCI: Support root bridge preparation for Hyper-V
Date: Fri, 20 Aug 2021 00:13:22 +0800	[thread overview]
Message-ID: <YR6DIkdkblL8NUP2@boqun-archlinux> (raw)
In-Reply-To: <20210726180657.142727-5-boqun.feng@gmail.com>

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
> 