[PATCH] SB600 for the Nemo board has non-zero devices on non-root bus

[Christian Zigotzky <chzigotzky@xenosoft.de>]

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On 30.11.2017 23:42, Bjorn Helgaas wrote:
 >
 > 00:11.0 claims to be a PCIe Root Port leading to [bus 05-06]. That
 > means there's a Link (presumably this A-Link II Express thing), and the
 > downstream end of the Link *should* be a PCIe Upstream Port on bus 05,
 > but no such device is visible.  I suppose the SB600 does implement
 > some sort of PCIe Port there, but keeps it invisible to software, and
 > at the same time, contains an invisible bridge that connects the Link
 > to all the conventional PCI devices on bus 05.
 >
 > When we scan bus 05, we do this:
 >
 >   pci_scan_child_bus_extend(bus=05)
 >     for (devfn = 0; devfn < 0x100; devfn += 8)
 >       pci_scan_slot(05, 00.0)
 >         pci_scan_single_device
 >           pci_scan_device(05, 00.0)           # fails; no 05:00.0
 >       pci_scan_slot(05, 01.0)
 >         only_one_child(bus=05)
 >           parent = 00:11.0
 >           pci_pcie_type(00:11.0) == ROOT_PORT # returns true
 >
 > Since only_one_child() sees that 00:11.0 is a Root Port, we give up
 > before we even get to the PCI_SCAN_ALL_PCIE_DEVS test.
 >
 > I *think* something like the patch below should make this work if you
 > use the "pci=pcie_scan_all" parameter.  We have some x86 DMI quirks
 > that set PCI_SCAN_ALL_PCIE_DEVS automatically.  I don't know how to do
 > something similar on powerpc, but maybe you do?
 >

Hi Bjorn,

I tested your new patch today. It boots with the boot argument 
"pci=pcie_scan_all". Well done! :-)

It doesn't boot without the boot argument "pci=pcie_scan_all".

Many thanks for your help.

Cheers,
Christian


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commit 75eaf674066590e79b3e03d32488871fc881ab40
Author: Bjorn Helgaas <bhelgaas@google.com>
Date:   Thu Nov 30 15:22:39 2017 -0600

    PCI: Make PCI_SCAN_ALL_PCIE_DEVS work for Root Ports as well as Downstream
    
    Previously PCI_SCAN_ALL_PCIE_DEVS (set by quirks or the "pci=pcie_scan_all"
    kernel parameter) only affected Switch Downstream Ports, not Root Ports.
    
    Simplify and restructure only_one_child() so PCI_SCAN_ALL_PCIE_DEVS means
    we scan for all possible devices below Root Ports as well as Switch
    Downstream Ports.
    
    Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 14e0ea1ff38b..9e57d4ef0c1f 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2217,20 +2217,28 @@ static int only_one_child(struct pci_bus *bus)
 {
 	struct pci_dev *parent = bus->self;
 
-	if (!parent || !pci_is_pcie(parent))
+	if (!parent)
+		return 0;
+
+	/*
+	 * 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 (pci_is_pcie(parent) && parent->has_secondary_link)
 		return 1;
+
 	return 0;
 }