linux-kernel - Re: [PATCH v1] PCI: Make PCI_SCAN_ALL_PCIE_DEVS work for Root as well as Downstream Ports

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Message-ID: <20171202191827.GB18780@bhelgaas-glaptop.roam.corp.google.com>
Date:   Sat, 2 Dec 2017 13:18:27 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     linux-pci@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Christian Zigotzky <chzigotzky@...osoft.de>
Subject: Re: [PATCH v1] PCI: Make PCI_SCAN_ALL_PCIE_DEVS work for Root as
 well as Downstream Ports

On Fri, Dec 01, 2017 at 06:27:10PM -0600, Bjorn Helgaas wrote:
> From: Bjorn Helgaas <bhelgaas@...gle.com>
> 
> PCIe Downstream Ports normally have only a Device 0 below them.  To
> optimize enumeration, we don't scan for other devices *unless* the
> PCI_SCAN_ALL_PCIE_DEVS flag is set by set by quirks or the
> "pci=pcie_scan_all" kernel parameter.
> 
> Previously PCI_SCAN_ALL_PCIE_DEVS only affected scanning below Switch
> Downstream Ports, not Root Ports.
> 
> But the "Nemo" system, also known as the AmigaOne X1000, has a PA Semi Root
> Port whose link leads to an AMD/ATI SB600 South Bridge.  The Root Port is a
> PCIe device, of course, but the SB600 contains only conventional PCI
> devices with no visible PCIe port.
> 
> Simplify and restructure only_one_child() so that we scan for all possible
> devices below Root Ports as well as Switch Downstream Ports when
> PCI_SCAN_ALL_PCIE_DEVS is set.
> 
> This is enough to make Nemo work with "pci=pcie_scan_all".  We would also
> like to add a quirk to set PCI_SCAN_ALL_PCIE_DEVS automatically on Nemo so
> users wouldn't have to use the "pci=pcie_scan_all" parameter, but we don't
> have that yet.
> 
> Link: https://lkml.kernel.org/r/CAErSpo55Q8Q=5p6_+uu7ahnw+53ibVDNRXxrzRV9QnUr_9EUfw@mail.gmail.com
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=198057
> Reported-and-Tested-by: Christian Zigotzky <chzigotzky@...osoft.de>
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>

Applied to pci/enumeration for v4.16.

> ---
>  drivers/pci/probe.c |   25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 14e0ea1ff38b..303c0cb0550c 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2215,22 +2215,27 @@ static unsigned next_fn(struct pci_bus *bus, struct pci_dev *dev, unsigned fn)
>  
>  static int only_one_child(struct pci_bus *bus)
>  {
> -	struct pci_dev *parent = bus->self;
> +	struct pci_dev *bridge = bus->self;
>  
> -	if (!parent || !pci_is_pcie(parent))
> +	/*
> +	 * Systems with unusual topologies set PCI_SCAN_ALL_PCIE_DEVS so
> +	 * we scan for all possible devices, not just Device 0.
> +	 */
> +	if (pci_has_flag(PCI_SCAN_ALL_PCIE_DEVS))
>  		return 0;
> -	if (pci_pcie_type(parent) == PCI_EXP_TYPE_ROOT_PORT)
> -		return 1;
>  
>  	/*
> -	 * PCIe downstream ports are bridges that normally lead to only a
> -	 * device 0, but if PCI_SCAN_ALL_PCIE_DEVS is set, scan all
> -	 * possible devices, not just device 0.  See PCIe spec r3.0,
> -	 * sec 7.3.1.
> +	 * A PCIe Downstream Port normally leads to a Link with only Device
> +	 * 0 on it (PCIe spec r3.1, sec 7.3.1).  As an optimization, scan
> +	 * only for Device 0 in that situation.
> +	 *
> +	 * Checking has_secondary_link is a hack to identify Downstream
> +	 * Ports because sometimes Switches are configured such that the
> +	 * PCIe Port Type labels are backwards.
>  	 */
> -	if (parent->has_secondary_link &&
> -	    !pci_has_flag(PCI_SCAN_ALL_PCIE_DEVS))
> +	if (bridge && pci_is_pcie(bridge) && bridge->has_secondary_link)
>  		return 1;
> +
>  	return 0;
>  }
>  
>