[RFC] PCI: Fix kernel panic of root-port-less PCIe enum due to ASPM

From: Serge Semin
Date: Thu Oct 06 2016 - 05:34:54 EST

Hello linux folks,

Sometime ago I discovered a kernel panic popping up when PCI subsystem was
trying to enumerate PCI express bus with ASPM service enabled. Here it is:

[ 5.089667] CPU 0 Unable to handle kernel paging request at virtual
address 00000060, epc == 80317004, ra == 80316ac8
[ 5.120952] Oops[#1]:
[ 5.528438] Call Trace:
[ 5.535640] [<80317004>] pcie_aspm_init_link_state+0x6c0/0x814
[ 5.552843] [<80300c44>] pci_scan_slot+0x140/0x148
[ 5.566957] [<80301dcc>] pci_scan_child_bus+0x50/0x1b0
[ 5.582096] [<80301944>] pci_scan_bridge+0x25c/0x694
[ 5.596724] [<80301e78>] pci_scan_child_bus+0xfc/0x1b0
[ 5.611862] [<80301944>] pci_scan_bridge+0x25c/0x694
[ 5.626488] [<80301e78>] pci_scan_child_bus+0xfc/0x1b0
[ 5.641628] [<8030215c>] pci_scan_root_bus+0x64/0x124
[ 5.656528] [<804ca298>] pcibios_scanbus+0xa8/0x188

I more than sure you are familiar with the issue, since I've found the
mailing discussion: "PCI: avoid NULL deref in alloc_pcie_link_state"

You closed the bugzilla ticket with the next statement:
"I'm closing this as invalid because the simulated machine where the problem
occurs has an invalid PCIe topology (an Upstream Port with no Downstream Port
or Root Port above it). As far as I know, there is no valid topology, e.g.,
a real hardware machine in the field, that would cause this failure."

I'm strongly disagree with it, since I've got at least two hardware with
PCIe-bus hierarchy as described in the mailing list. One of them is based on
Cavium Octeon III CN7020. Here is a ASCII-diagram of PCIe-bus:

| | \-00.1 Device [111d:808f]
| \-04.0-[06]----00.0 Device [126f:0750]

where 01:00.0 is an Upstream port of IDT PCIe-swtich.
/ # /usr/local/sbin/lspci -v -s 01:00.0
01:00.0 Class 0604: Device 111d:8061
Flags: bus master, fast devsel, latency 0
Memory at <unassigned> (32-bit, non-prefetchable) [size=2]
Memory at <unassigned> (32-bit, non-prefetchable) [size=2]
Bus: primary=01, secondary=02, subordinate=06, sec-latency=0
Memory behind bridge: 08000000-0dffffff
Expansion ROM at <unassigned> [disabled] [size=2]
Capabilities: [40] Express Upstream Port, MSI 00
Capabilities: [c0] Power Management version 3
Capabilities: [100] Advanced Error Reporting
Capabilities: [200] Virtual Channel
Kernel driver in use: pcieport

As you can see PCI-bus hierarchy doesn't have root port and the very first
upstream port is directly connected to Host-PCIe bridge of MCU, which of
course is not listed by the lspci utility.

Despite of Radim Kr?mÃ?, who suggested a fix, which would de-facto just
turned ASPM off, I found a quick solution, which disabled ASPM only in
the first link (Host-PCIe=>Upstream port) of PCIe-bus for such hierarchy.
ASPM for other PCIe-bus topologies shall work the way it was.

I hope the fix will be helpful.

Serge V. Semin
Leading Programmer
Embedded SW development group

Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>

drivers/pci/pcie/aspm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 0ec649d..a9295f29 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -522,7 +522,8 @@ static struct pcie_link_state *alloc_pcie_link_state(struct pci_dev *pdev)
link->pdev = pdev;
- if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ROOT_PORT) {
+ if ((pci_pcie_type(pdev) != PCI_EXP_TYPE_ROOT_PORT) &&
+ (!pci_is_root_bus(pdev->bus->parent))) {
struct pcie_link_state *parent;
parent = pdev->bus->parent->self->link_state;
if (!parent) {