pciexp_device: Introduce pciexp_find_ext_vendor_cap()

Vendors can choose to add non-standard capabilities inside a
Vendor-Specific Extended Capability. These are identified by
the Extended Capability ID 0x0b.

Change-Id: Idd6dd0e98bd53b19077afdd4c402114578bec966
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66454
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/device/pciexp_device.c

@@ -55,6 +55,35 @@ unsigned int pciexp_find_extended_cap(const struct device *dev, unsigned int cap
 	return pciexp_get_ext_cap_offset(dev, cap, next_cap_offset);
 }
 
+/*
+ * Search for a vendor-specific extended capability,
+ * with the vendor-specific ID `cap`.
+ *
+ * Returns the offset of the vendor-specific header,
+ * i.e. the offset of the extended capability + 4,
+ * or 0 if none is found.
+ *
+ * A new search is started with `offset == 0`.
+ * To continue a search, the prior return value
+ * should be passed as `offset`.
+ */
+unsigned int pciexp_find_ext_vendor_cap(const struct device *dev, unsigned int cap,
+					unsigned int offset)
+{
+	/* Reconstruct capability offset from vendor-specific header offset. */
+	if (offset >= 4)
+		offset -= 4;
+
+	for (;;) {
+		offset = pciexp_find_extended_cap(dev, PCI_EXT_CAP_ID_VNDR, offset);
+		if (!offset)
+			return 0;
+
+		const unsigned int vndr_cap = pci_read_config32(dev, offset + 4);
+		if ((vndr_cap & 0xffff) == cap)
+			return offset + 4;
+	}
+}
 
 /*
  * Re-train a PCIe link

src/include/device/pci_def.h

@@ -460,6 +460,7 @@
 #define PCI_EXT_CAP_ID_VC	2
 #define PCI_EXT_CAP_ID_DSN	3
 #define PCI_EXT_CAP_ID_PWR	4
+#define PCI_EXT_CAP_ID_VNDR	0x0b
 
 /* Extended Capability lists*/
 #define PCIE_EXT_CAP_OFFSET		0x100

src/include/device/pciexp.h

@@ -34,6 +34,8 @@ extern struct device_operations default_pciexp_hotplug_ops_bus;
 
 unsigned int pciexp_find_extended_cap(const struct device *dev, unsigned int cap,
 				      unsigned int offset);
+unsigned int pciexp_find_ext_vendor_cap(const struct device *dev, unsigned int cap,
+					unsigned int offset);
 
 static inline bool pciexp_is_downstream_port(int type)
 {