PCI: dwc: Add dw_pcie_parent_bus_offset()

Return the offset from CPU physical address to the parent bus address of
the specified element of the devicetree 'reg' property.

[bhelgaas: cpu_phy_addr -> cpu_phys_addr, return offset, split
.cpu_addr_fixup() checking and debug to separate patch]

Link: https://lore.kernel.org/r/20250315201548.858189-6-helgaas@kernel.org
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Author: nxpfrankli

drivers/pci/controller/dwc/pcie-designware.c

@@ -16,6 +16,7 @@
 #include <linux/gpio/consumer.h>
 #include <linux/ioport.h>
 #include <linux/of.h>
+#include <linux/of_address.h>
 #include <linux/platform_device.h>
 #include <linux/sizes.h>
 #include <linux/types.h>
@@ -1105,3 +1106,25 @@ void dw_pcie_setup(struct dw_pcie *pci)
 
 	dw_pcie_link_set_max_link_width(pci, pci->num_lanes);
 }
+
+resource_size_t dw_pcie_parent_bus_offset(struct dw_pcie *pci,
+					  const char *reg_name,
+					  resource_size_t cpu_phys_addr)
+{
+	struct device *dev = pci->dev;
+	struct device_node *np = dev->of_node;
+	int index;
+	u64 reg_addr;
+
+	/* Look up reg_name address on parent bus */
+	index = of_property_match_string(np, "reg-names", reg_name);
+
+	if (index < 0) {
+		dev_err(dev, "No %s in devicetree \"reg\" property\n", reg_name);
+		return 0;
+	}
+
+	of_property_read_reg(np, index, &reg_addr, NULL);
+
+	return cpu_phys_addr - reg_addr;
+}

drivers/pci/controller/dwc/pcie-designware.h

@@ -500,6 +500,9 @@ void dw_pcie_setup(struct dw_pcie *pci);
 void dw_pcie_iatu_detect(struct dw_pcie *pci);
 int dw_pcie_edma_detect(struct dw_pcie *pci);
 void dw_pcie_edma_remove(struct dw_pcie *pci);
+resource_size_t dw_pcie_parent_bus_offset(struct dw_pcie *pci,
+					  const char *reg_name,
+					  resource_size_t cpu_phy_addr);
 
 static inline void dw_pcie_writel_dbi(struct dw_pcie *pci, u32 reg, u32 val)
 {