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Subject[PATCH 4.18 084/197] PCI: Match Root Ports MPS to endpoints MPSS as necessary
4.18-stable review patch.  If anyone has any objections, please let me know.


From: Myron Stowe <>

[ Upstream commit 9f0e89359775ee21fe1ea732e34edb52aef5addf ]

In commit 27d868b5e6cf ("PCI: Set MPS to match upstream bridge"), we made
sure every device's MPS setting matches its upstream bridge, making it more
likely that a hot-added device will work in a system with an optimized MPS

Recently I've started encountering systems where the endpoint device's MPSS
capability is less than its Root Port's current MPS value, thus the
endpoint is not capable of matching its upstream bridge's MPS setting (see:
bugzilla via "Link:" below). This leaves the system vulnerable - the
upstream Root Port could respond with larger TLPs than the device can
handle, and the device will consider them to be 'Malformed'.

One could use the "pci=pcie_bus_safe" kernel parameter to work around the
issue, but that forces a user to supply a kernel parameter to get the
system to function reliably and may end up limiting MPS settings of other
unrelated, sub-topologies which could benefit from maintaining their larger

Augment Keith's approach to include tuning down a Root Port's MPS setting
when its hot-added endpoint device is not capable of matching it.

Signed-off-by: Myron Stowe <>
Signed-off-by: Bjorn Helgaas <>
Acked-by: Jon Mason <>
Cc: Keith Busch <>
Cc: Sinan Kaya <>
Cc: Dongdong Liu <>
Signed-off-by: Sasha Levin <>
Signed-off-by: Greg Kroah-Hartman <>
drivers/pci/probe.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1725,7 +1725,7 @@ int pci_setup_device(struct pci_dev *dev
static void pci_configure_mps(struct pci_dev *dev)
struct pci_dev *bridge = pci_upstream_bridge(dev);
- int mps, p_mps, rc;
+ int mps, mpss, p_mps, rc;

if (!pci_is_pcie(dev) || !bridge || !pci_is_pcie(bridge))
@@ -1753,6 +1753,14 @@ static void pci_configure_mps(struct pci
if (pcie_bus_config != PCIE_BUS_DEFAULT)

+ mpss = 128 << dev->pcie_mpss;
+ if (mpss < p_mps && pci_pcie_type(bridge) == PCI_EXP_TYPE_ROOT_PORT) {
+ pcie_set_mps(bridge, mpss);
+ pci_info(dev, "Upstream bridge's Max Payload Size set to %d (was %d, max %d)\n",
+ mpss, p_mps, 128 << bridge->pcie_mpss);
+ p_mps = pcie_get_mps(bridge);
+ }
rc = pcie_set_mps(dev, p_mps);
if (rc) {
pci_warn(dev, "can't set Max Payload Size to %d; if necessary, use \"pci=pcie_bus_safe\" and report a bug\n",
@@ -1761,7 +1769,7 @@ static void pci_configure_mps(struct pci

pci_info(dev, "Max Payload Size set to %d (was %d, max %d)\n",
- p_mps, mps, 128 << dev->pcie_mpss);
+ p_mps, mps, mpss);

static struct hpp_type0 pci_default_type0 = {

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