PCI PM: Make pci_set_power_state() handle devices with no PM support

There is a problem with PCI devices without any PM support (either
native or through the platform) that pci_set_power_state() always
returns error code for them, even if they are being put into D0.
However, such devices are always in D0, so pci_set_power_state()
should return success when attempting to put such a device into D0.
It also should update the current_state field for these devices as
appropriate. This modification is necessary so that the standard
configuration registers of these devices are successfully restored by
pci_restore_standard_config() during the "early" phase of resume.

In addition, pci_set_power_state() should check the value of
current_state before calling the platform to change the power state
of the device to avoid doing that unnecessarily.

Signed-off-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Ingo Molnar <[e-mail]>
Acked-by: Jesse Barnes <[e-mail]>

drivers/pci/pci.c

···
 	u16 pmcsr;
 	bool need_restore = false;
 
+	/* Check if we're already there */
+	if (dev->current_state == state)
+		return 0;
+
 	if (!dev->pm_cap)
 		return -EIO;
 
···
 	 * Can enter D0 from any state, but if we can only go deeper 
 	 * to sleep if we're already in a low power state
 	 */
-	if (dev->current_state == state) {
-		/* we're already there */
-		return 0;
-	} else if (state != PCI_D0 && dev->current_state <= PCI_D3cold
+	if (state != PCI_D0 && dev->current_state <= PCI_D3cold
 	    && dev->current_state > state) {
 		dev_err(&dev->dev, "invalid power transition "
 			"(from state %d to %d)\n", dev->current_state, state);
···
 		 */
 		return 0;
 
-	if (state == PCI_D0 && platform_pci_power_manageable(dev)) {
+	/* Check if we're already there */
+	if (dev->current_state == state)
+		return 0;
+
+	if (state == PCI_D0) {
 		/*
 		 * Allow the platform to change the state, for example via ACPI
 		 * _PR0, _PS0 and some such, but do not trust it.
 		 */
-		int ret = platform_pci_set_power_state(dev, PCI_D0);
+		int ret = platform_pci_power_manageable(dev) ?
+			platform_pci_set_power_state(dev, PCI_D0) : 0;
 		if (!ret)
 			pci_update_current_state(dev, PCI_D0);
 	}