PCI/PM: Return error when changing power state from D3cold

pci_raw_set_power_state() uses the Power Management capability to change a
device's power state.  The capability is in config space, which is
accessible in D0, D1, D2, and D3hot, but not in D3cold.

If we call pci_raw_set_power_state() on a device that's in D3cold, config
reads fail and return ~0 data, which we erroneously interpreted as "the
device is in D3hot", leading to messages like this:

  pcieport 0000:03:00.0: Refused to change power state, currently in D3

The PCI_PM_CTRL has several RsvdP fields, so ~0 is never a valid register
value.  If we get that value, print a more informative message and return
an error.

Changing the power state of a device from D3cold must be done by a platform
power management method or some other non-config space mechanism.

Link: https://lore.kernel.org/r/20190822200551.129039-4-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/pci/pci.c

@@ -853,6 +853,12 @@ static int pci_raw_set_power_state(struct pci_dev *dev, pci_power_t state)
 		return -EIO;
 
 	pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
+	if (pmcsr == (u16) ~0) {
+		pci_err(dev, "can't change power state from %s to %s (config space inaccessible)\n",
+			pci_power_name(dev->current_state),
+			pci_power_name(state));
+		return -EIO;
+	}
 
 	/*
 	 * If we're (effectively) in D3, force entire word to 0.