[PATCH v4] PCI: Enable runtime pm of the host bridge

[Krishna chaitanya chundru <quic_krichai@quicinc.com>]

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.

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 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 goes to
runtime suspend while endpoint driver is doing some transfers.

So enable runtime PM for the host bridge, so that controller driver
goes to suspend only when all child devices goes to runtime suspend.

Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
---
Changes in v4:
- Changed pm_runtime_enable() to devm_pm_runtime_enable() (suggested by mayank)
- Link to v3: https://lore.kernel.org/lkml/20240609-runtime_pm-v3-1-3d0460b49d60@quicinc.com/
Changes in v3:
- Moved the runtime API call's from the dwc driver to PCI framework
  as it is applicable for all (suggested by mani)
- Updated the commit message.
- Link to v2: https://lore.kernel.org/all/20240305-runtime_pm_enable-v2-1-a849b74091d1@quicinc.com
Changes in v2:
- Updated commit message as suggested by mani.
- Link to v1: https://lore.kernel.org/r/20240219-runtime_pm_enable-v1-1-d39660310504@quicinc.com
---

---
 drivers/pci/probe.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 8e696e547565..fd49563a44d9 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -3096,6 +3096,10 @@ int pci_host_probe(struct pci_host_bridge *bridge)
 	}
 
 	pci_bus_add_devices(bus);
+
+	pm_runtime_set_active(&bridge->dev);
+	devm_pm_runtime_enable(&bridge->dev);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(pci_host_probe);

---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20240708-runtime_pm-978ccbca6130

Best regards,
-- 
Krishna chaitanya chundru <quic_krichai@quicinc.com>