linux-kernel - Re: [RFC v4 3/7] arm64: PCI: Support root bridge preparation for Hyper-V PCI

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Date:   Wed, 14 Jul 2021 14:46:45 -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>,
        Catalin Marinas <catalin.marinas@....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>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....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: [RFC v4 3/7] arm64: PCI: Support root bridge preparation for
 Hyper-V PCI

On Wed, Jul 14, 2021 at 06:27:33PM +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.

It would be nice to have a hint about why we don't actually need the
ACPI companion device in this case.

> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  arch/arm64/kernel/pci.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
> index 1006ed2d7c60..3b81ac42bc1f 100644
> --- a/arch/arm64/kernel/pci.c
> +++ b/arch/arm64/kernel/pci.c
> @@ -84,7 +84,13 @@ 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);
> +		/*
> +		 * On Hyper-V there is no corresponding APCI 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.
> +		 */
> +		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);

s/APCI/ACPI/ above.

I think this would be more readable like this:

  struct pci_config_window *cfg = bridge->bus->sysdata;
  ...

  if (acpi_disabled)
    return 0;

  /*
   * On Hyper-V there is no corresponding ACPI device for a root
   * ...
   */
  cfg = bridge->bus->sysdata;
  if (!cfg->parent)
    return 0;

  adev = to_acpi_device(cfg->parent);
  bus_dev = &bridge->bus->dev;
  ACPI_COMPANION_SET(&bridge->dev, adev);
  ...

This could be done in two steps: the first to restructure the code
without making any functional change, and a second to return when
there's no cfg->parent.  If you do it in one step, the patch will be
much harder to read.

Bjorn