PCI: dwc: Add helper to find the Vendor Specific Extended Capability (VSEC)

The new dw_pcie_find_vsec_capability() helper will be used within
different DWC APIs to find the VSEC capabilities like PTM, RAS, etc.

Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Shradha Todi <shradha.t@samsung.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Tested-by: Hrishikesh Deleep <hrishikesh.d@samsung.com>
Link: https://lore.kernel.org/r/20250221131548.59616-3-shradha.t@samsung.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>

Author: shradhat

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/pcie-dwc.h>
 #include <linux/platform_device.h>
 #include <linux/sizes.h>
 #include <linux/types.h>
@@ -283,6 +284,45 @@ u16 dw_pcie_find_ext_capability(struct dw_pcie *pci, u8 cap)
 }
 EXPORT_SYMBOL_GPL(dw_pcie_find_ext_capability);
 
+static u16 __dw_pcie_find_vsec_capability(struct dw_pcie *pci, u16 vendor_id,
+					  u16 vsec_id)
+{
+	u16 vsec = 0;
+	u32 header;
+
+	if (vendor_id != dw_pcie_readw_dbi(pci, PCI_VENDOR_ID))
+		return 0;
+
+	while ((vsec = dw_pcie_find_next_ext_capability(pci, vsec,
+						       PCI_EXT_CAP_ID_VNDR))) {
+		header = dw_pcie_readl_dbi(pci, vsec + PCI_VNDR_HEADER);
+		if (PCI_VNDR_HEADER_ID(header) == vsec_id)
+			return vsec;
+	}
+
+	return 0;
+}
+
+static u16 dw_pcie_find_vsec_capability(struct dw_pcie *pci,
+					const struct dwc_pcie_vsec_id *vsec_ids)
+{
+	const struct dwc_pcie_vsec_id *vid;
+	u16 vsec;
+	u32 header;
+
+	for (vid = vsec_ids; vid->vendor_id; vid++) {
+		vsec = __dw_pcie_find_vsec_capability(pci, vid->vendor_id,
+						      vid->vsec_id);
+		if (vsec) {
+			header = dw_pcie_readl_dbi(pci, vsec + PCI_VNDR_HEADER);
+			if (PCI_VNDR_HEADER_REV(header) == vid->vsec_rev)
+				return vsec;
+		}
+	}
+
+	return 0;
+}
+
 int dw_pcie_read(void __iomem *addr, int size, u32 *val)
 {
 	if (!IS_ALIGNED((uintptr_t)addr, size)) {