[PATCH 2/2] PCI: Disable Precision Time Measurement during suspend

[David E. Box]

On Intel client platforms that support suspend-to-idle, like Ice Lake,
root ports that have Precision Time Management (PTM) enabled can prevent
the port from being fully power gated, causing higher power consumption
while suspended.  To prevent this, after saving the PTM control register,
disable the feature.  The feature will be returned to its previous state
during restore.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=209361
Reported-by: Len Brown <len.brown@xxxxxxxxx>
Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Signed-off-by: David E. Box <david.e.box@xxxxxxxxxxxxxxx>
---
 drivers/pci/pci.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 6fd4ae910a88..a2b40497d443 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -21,6 +21,7 @@
 #include <linux/module.h>
 #include <linux/spinlock.h>
 #include <linux/string.h>
+#include <linux/suspend.h>
 #include <linux/log2.h>
 #include <linux/logic_pio.h>
 #include <linux/pm_wakeup.h>
@@ -1543,7 +1544,7 @@ static void pci_save_ptm_state(struct pci_dev *dev)
 {
 	int ptm;
 	struct pci_cap_saved_state *save_state;
-	u16 *cap;
+	u16 *cap, ctrl;
 
 	if (!pci_is_pcie(dev))
 		return;
@@ -1560,6 +1561,17 @@ static void pci_save_ptm_state(struct pci_dev *dev)
 
 	cap = (u16 *)&save_state->cap.data[0];
 	pci_read_config_word(dev, ptm + PCI_PTM_CTRL, cap);
+
+	/*
+	 * On Intel systems that support suspend-to-idle, additional
+	 * power savings can be gained by disabling PTM on root ports,
+	 * as this allows the port to enter a deeper pm state.
+	 */
+	if (pm_suspend_target_state == PM_SUSPEND_TO_IDLE &&
+	    pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
+		ctrl = *cap & ~(PCI_PTM_CTRL_ENABLE | PCI_PTM_CTRL_ROOT);
+		pci_write_config_word(dev, ptm + PCI_PTM_CTRL, ctrl);
+	}
 }
 
 static void pci_restore_ptm_state(struct pci_dev *dev)
-- 
2.20.1