[Intel-wired-lan] [PATCH 5/5] PCI/PTM: Do not enable PTM automatically for Root and Switch Upstream Ports

Mika Westerberg
2026-02-24T11:11:00Z

Currently we enable PTM automatically for Root and Switch Upstream Ports
if the advertised capabilities support the relevant role. However, there
are few issues with this. First of all if there is no Endpoint that
actually needs the PTM functionality, this is just wasting link
bandwidth. There are just a couple of drivers calling pci_ptm_enable()
in the tree.

Secondly we do the enablement in pci_ptm_init() that is called pretty
early for the Switch Upstream Port before Downstream Ports are even
enumerated. Since the Upstream Port configuration affects the whole
Switch enabling it this early might cause the PTM requests to be sent
already. We actually do see effect of this:

pcieport 0000:00:07.1: pciehp: Slot(6-1): Card present
pcieport 0000:00:07.1: pciehp: Slot(6-1): Link Up
pci 0000:2c:00.0: [8086:5786] type 01 class 0x060400 PCIe Switch Upstream Port
pci 0000:2c:00.0: PCI bridge to [bus 00]
pci 0000:2c:00.0: bridge window [io 0x0000-0x0fff]
pci 0000:2c:00.0: bridge window [mem 0x00000000-0x000fffff]
pci 0000:2c:00.0: bridge window [mem 0x00000000-0x000fffff 64bit pref]
...
pci 0000:2c:00.0: PME# supported from D0 D1 D2 D3hot D3cold
pci 0000:2c:00.0: PTM enabled, 4ns granularity

At this point we have only enumerated the Switch Upstream Port and now
PTM got enabled which immediately triggers flood of these:

pcieport 0000:00:07.1: AER: Multiple Uncorrectable (Non-Fatal) error message received from 0000:00:07.1
pcieport 0000:00:07.1: PCIe Bus Error: severity=Uncorrectable (Non-Fatal), type=Transaction Layer, (Receiver ID)
pcieport 0000:00:07.1: device [8086:d44f] error status/mask=00200000/00000000
pcieport 0000:00:07.1: [21] ACSViol (First)
pcieport 0000:00:07.1: AER: TLP Header: 0x34000000 0x00000052 0x00000000 0x00000000
pcieport 0000:00:07.1: AER: device recovery successful
pcieport 0000:00:07.1: AER: Uncorrectable (Non-Fatal) error message received from 0000:00:07.1

In the above TLP Header the Requester ID is 0 which rightfully triggers
an error as we have ACS Source Validation enabled.

For this reason change the PTM enablement to happen at the time
pci_enable_ptm() is called. It will try to enable PTM first for upstream
devices before enabling for the Endpoint itself. For disable path we
need to keep count of how many times PTM has been enabled and disable
only on the last so change the dev->ptm_enabled to a counter (and rename
it to dev->ptm_enable_cnt analogous to dev->pci_enable_cnt).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
---
drivers/pci/pcie/ptm.c | 68 ++++++++++++++++++++++++------------------
include/linux/pci.h | 2 +-
2 files changed, 40 insertions(+), 30 deletions(-)

diff --git a/drivers/pci/pcie/ptm.c b/drivers/pci/pcie/ptm.c
index 2c848ae4f15f..a41ffd1914de 100644
--- a/drivers/pci/pcie/ptm.c
+++ b/drivers/pci/pcie/ptm.c
@@ -52,6 +52,7 @@ void pci_ptm_init(struct pci_dev *dev)
return;

dev->ptm_cap = ptm;
+ atomic_set(&dev->ptm_enable_cnt, 0);
pci_add_ext_cap_save_buffer(dev, PCI_EXT_CAP_ID_PTM, sizeof(u32));

pci_read_config_dword(dev, ptm + PCI_PTM_CAP, &cap);
@@ -85,10 +86,6 @@ void pci_ptm_init(struct pci_dev *dev)
dev->ptm_responder = 1;
if (cap & PCI_PTM_CAP_REQ)
dev->ptm_requester = 1;
-
- if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT ||
- pci_pcie_type(dev) == PCI_EXP_TYPE_UPSTREAM)
- pci_enable_ptm(dev);
}

void pci_save_ptm_state(struct pci_dev *dev)
@@ -129,26 +126,11 @@ void pci_restore_ptm_state(struct pci_dev *dev)
static int __pci_enable_ptm(struct pci_dev *dev)
{
u16 ptm = dev->ptm_cap;
- struct pci_dev *ups;
u32 ctrl;

if (!ptm)
return -EINVAL;

- /*
- * A device uses local PTM Messages to request time information
- * from a PTM Root that's farther upstream. Every device along the
- * path must support PTM and have it enabled so it can handle the
- * messages. Therefore, if this device is not a PTM Root, the
- * upstream link partner must have PTM enabled before we can enable
- * PTM.
- */
- if (!dev->ptm_root) {
- ups = pci_upstream_ptm(dev);
- if (!ups || !ups->ptm_enabled)
- return -EINVAL;
- }
-
switch (pci_pcie_type(dev)) {
case PCI_EXP_TYPE_ROOT_PORT:
if (!dev->ptm_root)
@@ -193,11 +175,35 @@ int pci_enable_ptm(struct pci_dev *dev)
int rc;
char clock_desc[8];

+ /*
+ * A device uses local PTM Messages to request time information
+ * from a PTM Root that's farther upstream. Every device along
+ * the path must support PTM and have it enabled so it can
+ * handle the messages. Therefore, if this device is not a PTM
+ * Root, the upstream link partner must have PTM enabled before
+ * we can enable PTM.
+ */
+ if (!dev->ptm_root) {
+ struct pci_dev *parent;
+
+ parent = pci_upstream_ptm(dev);
+ if (!parent)
+ return -EINVAL;
+ /* Enable PTM for the parent */
+ rc = pci_enable_ptm(parent);
+ if (rc)
+ return rc;
+ }
+
+ /* Already enabled? */
+ if (atomic_inc_return(&dev->ptm_enable_cnt) > 1)
+ return 0;
+
rc = __pci_enable_ptm(dev);
- if (rc)
+ if (rc) {
+ atomic_dec(&dev->ptm_enable_cnt);
return rc;
-
- dev->ptm_enabled = 1;
+ }

switch (dev->ptm_granularity) {
case 0:
@@ -239,27 +245,31 @@ static void __pci_disable_ptm(struct pci_dev *dev)
*/
void pci_disable_ptm(struct pci_dev *dev)
{
- if (dev->ptm_enabled) {
+ struct pci_dev *parent;
+
+ if (atomic_dec_and_test(&dev->ptm_enable_cnt))
__pci_disable_ptm(dev);
- dev->ptm_enabled = 0;
- }
+
+ parent = pci_upstream_ptm(dev);
+ if (parent)
+ pci_disable_ptm(parent);
}
EXPORT_SYMBOL(pci_disable_ptm);

/*
- * Disable PTM, but preserve dev->ptm_enabled so we silently re-enable it on
+ * Disable PTM, but preserve dev->ptm_enable_cnt so we silently re-enable it on
* resume if necessary.
*/
void pci_suspend_ptm(struct pci_dev *dev)
{
- if (dev->ptm_enabled)
+ if (atomic_read(&dev->ptm_enable_cnt))
__pci_disable_ptm(dev);
}

/* If PTM was enabled before suspend, re-enable it when resuming */
void pci_resume_ptm(struct pci_dev *dev)
{
- if (dev->ptm_enabled)
+ if (atomic_read(&dev->ptm_enable_cnt))
__pci_enable_ptm(dev);
}

@@ -268,7 +278,7 @@ bool pcie_ptm_enabled(struct pci_dev *dev)
if (!dev)
return false;

- return dev->ptm_enabled;
+ return atomic_read(&dev->ptm_enable_cnt);
}
EXPORT_SYMBOL(pcie_ptm_enabled);

diff --git a/include/linux/pci.h b/include/linux/pci.h
index 8aaa72dcb164..7e49d35d81a5 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -518,7 +518,7 @@ struct pci_dev {
unsigned int ptm_root:1;
unsigned int ptm_responder:1;
unsigned int ptm_requester:1;
- unsigned int ptm_enabled:1;
+ atomic_t ptm_enable_cnt; /* pci_enable_ptm() has been called */
u8 ptm_granularity;
#endif
#ifdef CONFIG_PCI_MSI
--
2.50.1