PCI: Enable runtime PM of the host bridge

The controller driver is the parent device of the PCIe host bridge,
PCI-PCI bridge and PCIe endpoint as shown below.

  PCIe controller (Top level parent & parent of host bridge)
                  |
                  v
  PCIe Host bridge (Parent of PCI-PCI bridge)
                  |
                  v
  PCI-PCI bridge (Parent of endpoint driver)
                  |
                  v
          PCIe endpoint driver

Now, when the controller device goes to runtime suspend, PM framework will
check the runtime PM state of the child device (host bridge) and will find
it to be disabled. So it will allow the parent (controller device) to go to
runtime suspend. Only if the child device's state was 'active' it will
prevent the parent to get suspended.

It is a property of the runtime PM framework that it can only follow
continuous dependency chains.  That is, if there is a device with runtime
PM disabled in a dependency chain, runtime PM cannot be enabled for devices
below it and above it in that chain both at the same time.

Since runtime PM is disabled for host bridge, the state of the child
devices under the host bridge is not taken into account by PM framework for
the top level parent, PCIe controller. So the PM framework allows the
controller driver to enter runtime PM irrespective of the state of the
devices under the host bridge. And this causes the topology breakage and
also possible PM issues like controller driver going to runtime suspend
while the endpoint driver is doing transfers.

Because of the above, in order to enable runtime PM for a PCIe controller
device, one needs to ensure that runtime PM is enabled for all devices in
every dependency chain between it and any PCIe endpoint (as runtime PM is
enabled for PCIe endpoints).

This means that runtime PM needs to be enabled for the host bridge device,
which is present in all of these dependency chains.

After this change, the host bridge device will be runtime-suspended by the
runtime PM framework automatically after suspending its last child and it
will be runtime-resumed automatically before resuming its first child which
will allow the runtime PM framework to track dependencies between the host
bridge device and all of its descendants.

The PM framework expects parent runtime PM to be enabled before enabling
runtime PM of the child. Ensure pm_runtime_enable() is called for the
controller drivers before calling pci_host_probe().

Link: https://lore.kernel.org/r/20241111-runtime_pm-v7-2-9c164eefcd87@quicinc.com
Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Author: Krishna chaitanya chundru

drivers/pci/probe.c

@@ -3106,6 +3106,17 @@ int pci_host_probe(struct pci_host_bridge *bridge)
 		pcie_bus_configure_settings(child);
 
 	pci_bus_add_devices(bus);
+
+	/*
+	 * Ensure pm_runtime_enable() is called for the controller drivers
+	 * before calling pci_host_probe(). The PM framework expects that
+	 * if the parent device supports runtime PM, it will be enabled
+	 * before child runtime PM is enabled.
+	 */
+	pm_runtime_set_active(&bridge->dev);
+	pm_runtime_no_callbacks(&bridge->dev);
+	devm_pm_runtime_enable(&bridge->dev);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(pci_host_probe);