linux-kernel - [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold

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Message-ID: <20251003154008.1.I7a21c240b30062c66471329567a96dceb6274358@changeid>
Date: Fri,  3 Oct 2025 15:40:09 -0700
From: Brian Norris <briannorris@...omium.org>
To: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Brian Norris <briannorris@...gle.com>,
	stable@...r.kernel.org,
	Brian Norris <briannorris@...omium.org>
Subject: [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold

From: Brian Norris <briannorris@...gle.com>

When transitioning to D3cold, __pci_set_power_state() will first
transition a device to D3hot. If the device was already in D3hot, this
will add excess work:
(a) read/modify/write PMCSR; and
(b) excess delay (pci_dev_d3_sleep()).

For (b), we already performed the necessary delay on the previous D3hot
entry; this was extra noticeable when evaluating runtime PM transition
latency.

Check whether we're already in the target state before continuing.

Note that __pci_set_power_state() already does this same check for other
state transitions, but D3cold is special because __pci_set_power_state()
converts it to D3hot for the purposes of PMCSR.

This seems to be an oversight in commit 0aacdc957401 ("PCI/PM: Clean up
pci_set_low_power_state()").

Fixes: 0aacdc957401 ("PCI/PM: Clean up pci_set_low_power_state()")
Cc: <stable@...r.kernel.org>
Signed-off-by: Brian Norris <briannorris@...gle.com>
Signed-off-by: Brian Norris <briannorris@...omium.org>
---

 drivers/pci/pci.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b0f4d98036cd..7517f1380201 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1539,6 +1539,9 @@ static int pci_set_low_power_state(struct pci_dev *dev, pci_power_t state, bool
 	   || (state == PCI_D2 && !dev->d2_support))
 		return -EIO;

+	if (state == dev->current_state)
+		return 0;
+
 	pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
 	if (PCI_POSSIBLE_ERROR(pmcsr)) {
 		pci_err(dev, "Unable to change power state from %s to %s, device inaccessible\n",
-- 
2.51.0.618.g983fd99d29-goog