[PATCH v2] PCI: Fix NULL pointer when find parent pcie_link_state

[Yijing Wang <wangyijing@huawei.com>]

https://bugzilla.kernel.org/show_bug.cgi?id=94361 reported
in ATCA platform, system had unusual pcie topology:

(root port)   (downstream port)   (upstream port)
+-1c.0-[02-0a]----00.0-[03-0a]--+-00.0-[04]--
|                               +-01.0-[05]-- (downstream port)
|                               +-02.0-[06]--
|                               +-03.0-[07]--
|                               +-08.0-[08]--
|                               +-09.0-[09]--
|                               \-0a.0-[0a]--
We assumed root port and downstream port always
have external link, and downstream port always has a
upstream port. So in this case, when we allocated
pcie_link_state for downstream port 02:00.0, it try
to get parent bus pcie_link_state,
parent = pdev->bus->parent->self->link_state;
because root bus self is NULL, system will crash here.

This patch fix this issue based on the following
assumption suggested by Bjorn.
1. Root port is always on the upstream end of a link.
2. The pcie hierarchy should alternate between links
and internal switch logic, there should be no adjacent
links or internal buses in pcie tree.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
---
 drivers/pci/pcie/aspm.c |   23 +++++++++++++++++++++--
 1 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 7d4fcdc..61b9d3f 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -546,6 +546,25 @@ static struct pcie_link_state *alloc_pcie_link_state(struct pci_dev *pdev)
 	return link;
 }
 
+/*
+ * We assume root port is always on the upstream end of
+ * a link, and the pcie hierarchy should alternate
+ * between links and internal switch logic.
+ */
+static bool pcie_has_external_link(struct pci_dev *pdev)
+{
+	if (!pci_is_pcie(pdev))
+		return false;
+
+	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_ROOT_PORT)
+		return true;
+
+	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_DOWNSTREAM)
+		if (!pdev->bus->self->link_state)
+			return true;
+
+	return false;
+}
 /*
  * pcie_aspm_init_link_state: Initiate PCI express link state.
  * It is called after the pcie and its children devices are scanned.
@@ -561,8 +580,8 @@ void pcie_aspm_init_link_state(struct pci_dev *pdev)
 
 	if (!pci_is_pcie(pdev) || pdev->link_state)
 		return;
-	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ROOT_PORT &&
-	    pci_pcie_type(pdev) != PCI_EXP_TYPE_DOWNSTREAM)
+
+	if (!pcie_has_external_link(pdev))
 		return;
 
 	/* VIA has a strange chipset, root port is under a bridge */
-- 
1.7.1