PCI/PM: Avoid redundant delays on D3hot->D3cold

[ Upstream commit 4d98208 ]

When transitioning to D3cold, __pci_set_power_state() first transitions to
D3hot. If the device was already in D3hot, this adds 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 0aacdc9 ("PCI/PM: Clean up
pci_set_low_power_state()").

Fixes: 0aacdc9 ("PCI/PM: Clean up pci_set_low_power_state()")
Signed-off-by: Brian Norris <briannorris@google.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
[bhelgaas: reverse test to match other "dev->current_state == state" cases]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251003154008.1.I7a21c240b30062c66471329567a96dceb6274358@changeid
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: computersforpeace

drivers/pci/pci.c

@@ -1397,6 +1397,9 @@ 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 (dev->current_state == 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",