[PATCH v7 2/2] 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.

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 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.

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.

PM framework expectes parent runtime pm enabled before enabling runtime
pm of the child. Ensure pm_runtime_enable() is called for the controller
drivers, before calling pci_host_probe() as pm frameworks expects if the
parent device supports runtime pm then it needs to enabled before child
runtime pm.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
---
Changes in v7:
- update commit text & and add a comment in the driver as suggested by bjorn.
Changes in v6:
- no change
Changes in v5:
- call pm_runtime_no_callbacks() as suggested by Rafael.
- include the commit texts as suggested by Rafael.
- Link to v4: https://lore.kernel.org/linux-pci/20240708-runtime_pm-v4-1-c02a3663243b@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 | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 4f68414c3086..ee30e6024f52 100644
--- a/drivers/pci/probe.c
+++ b/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() as pm frameworks expects if the
+	 * parent device supports runtime pm then it needs to enabled before
+	 * child runtime pm.
+	 */
+	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);

-- 
2.34.1