[PATCH v4 1/5] PCI: host-common: Add helper to determine host bridge D3cold eligibility

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

Add a common helper, pci_host_common_d3cold_possible(), to determine
whether PCIe devices under host bridge can safely transition to D3cold.

This helper is intended to be used by PCI host controller drivers to
decide whether they may safely put the host bridge into D3cold based on
the power state and wakeup capabilities of downstream endpoints.

The helper walks all devices on the all bridge buses and only allows
the devices to enter D3cold if all PCIe endpoints are already in
PCI_D3hot. This ensures that we do not power off the host bridge while
any active endpoint still requires the link to remain powered.

For devices that may wake the system, the helper additionally requires
that the device supports PME wake from D3cold (via WAKE#). Devices that
do not have wakeup enabled are not restricted by this check and do not
block the devices under host bridge from entering D3cold.

Devices without a bound driver and with PCI not enabled via sysfs are
treated as inactive and therefore do not prevent the devices under host
bridge from entering D3cold. This allows controllers to power down more
aggressively when there are no actively managed endpoints.

Some devices (e.g. M.2 without auxiliary power) lose PME detection when
main power is removed. Even if such devices advertise PME-from-D3cold
capability, entering D3cold may break wakeup. So, return PME-from-D3cold
capability via an output parameter so PCIe controller drivers can apply
platform-specific handling to preserve wakeup functionality.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
---
 drivers/pci/controller/pci-host-common.c | 63 ++++++++++++++++++++++++++++++++
 drivers/pci/controller/pci-host-common.h |  2 +
 2 files changed, 65 insertions(+)

diff --git a/drivers/pci/controller/pci-host-common.c b/drivers/pci/controller/pci-host-common.c
index d6258c1cffe5ec480fd2a7e50b3af39ef6ac4c8c..34e4c4c1d8c0fdead3e714525a497b722a41392e 100644
--- a/drivers/pci/controller/pci-host-common.c
+++ b/drivers/pci/controller/pci-host-common.c
@@ -17,6 +17,9 @@
 
 #include "pci-host-common.h"
 
+#define PCI_HOST_D3COLD_ALLOWED        BIT(0)
+#define PCI_HOST_PME_D3COLD_CAPABLE    BIT(1)
+
 static void gen_pci_unmap_cfg(void *ptr)
 {
 	pci_ecam_free((struct pci_config_window *)ptr);
@@ -106,5 +109,65 @@ void pci_host_common_remove(struct platform_device *pdev)
 }
 EXPORT_SYMBOL_GPL(pci_host_common_remove);
 
+static int __pci_host_common_d3cold_possible(struct pci_dev *pdev, void *userdata)
+{
+	u32 *flags = userdata;
+
+	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ENDPOINT)
+		return 0;
+
+	if (!pdev->dev.driver && !pci_is_enabled(pdev))
+		return 0;
+
+	if (pdev->current_state != PCI_D3hot)
+		goto exit;
+
+	if (device_may_wakeup(&pdev->dev)) {
+		if (!pci_pme_capable(pdev, PCI_D3cold))
+			goto exit;
+		else
+			*flags |= PCI_HOST_PME_D3COLD_CAPABLE;
+	}
+
+	return 0;
+
+exit:
+	*flags &= ~PCI_HOST_D3COLD_ALLOWED;
+
+	return -EOPNOTSUPP;
+}
+
+/**
+ * pci_host_common_d3cold_possible - Determine whether the host bridge can transition the
+ *				     devices into D3Cold.
+ *
+ * @bridge: PCI host bridge to check
+ * @pme_capable: Pointer to update if there is any device which is capable of generating
+ *		 PME from D3cold.
+ *
+ * Walk downstream PCIe endpoint devices and determine whether the host bridge
+ * is permitted to transition the devices into D3cold.
+ *
+ * Devices under host bridge can enter D3cold only if all active PCIe endpoints are in
+ * PCI_D3hot and any wakeup-enabled endpoint is capable of generating PME from D3cold.
+ * Inactive endpoints are ignored.
+ *
+ * The @pme_capable output allows PCIe controller drivers to apply
+ * platform-specific handling to preserve wakeup functionality.
+ *
+ * Return: %true if the host bridge may enter D3cold, otherwise %false.
+ */
+bool pci_host_common_d3cold_possible(struct pci_host_bridge *bridge, bool *pme_capable)
+{
+	u32 flags = PCI_HOST_D3COLD_ALLOWED;
+
+	pci_walk_bus(bridge->bus, __pci_host_common_d3cold_possible, &flags);
+
+	*pme_capable = !!(flags & PCI_HOST_PME_D3COLD_CAPABLE);
+
+	return !!(flags & PCI_HOST_D3COLD_ALLOWED);
+}
+EXPORT_SYMBOL_GPL(pci_host_common_d3cold_possible);
+
 MODULE_DESCRIPTION("Common library for PCI host controller drivers");
 MODULE_LICENSE("GPL v2");
diff --git a/drivers/pci/controller/pci-host-common.h b/drivers/pci/controller/pci-host-common.h
index b5075d4bd7eb31fbf1dc946ef1a6afd5afb5b3c6..7eb5599b9ce4feb5c8ba2aa1f2e532b0cf3e1c03 100644
--- a/drivers/pci/controller/pci-host-common.h
+++ b/drivers/pci/controller/pci-host-common.h
@@ -20,4 +20,6 @@ void pci_host_common_remove(struct platform_device *pdev);
 
 struct pci_config_window *pci_host_common_ecam_create(struct device *dev,
 	struct pci_host_bridge *bridge, const struct pci_ecam_ops *ops);
+
+bool pci_host_common_d3cold_possible(struct pci_host_bridge *bridge, bool *pme_capable);
 #endif

-- 
2.34.1