[usb] Use non-zero language ID to retrieve strings

We currently use a zero language ID to retrieve strings such as the
ECM/NCM MAC address.  This works on most hardware devices, but is
known to fail on some software emulated CDC-NCM devices.

Fix by using the first supported language ID, falling back to English
(0x0409) if any error occurs when fetching the list of supported
languages.  This matches the behaviour of the Linux kernel.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/bus/usb.c

 	return rc;
 }
 
+/**
+ * Get default language ID
+ *
+ * @v usb		USB device
+ * @ret language	Language ID
+ */
+static unsigned int usb_get_default_language ( struct usb_device *usb ) {
+	struct {
+		struct usb_descriptor_header header;
+		uint16_t language[1];
+	} __attribute__ (( packed )) desc;
+	unsigned int language;
+	int rc;
+
+	/* Get descriptor */
+	if ( ( rc = usb_get_descriptor ( usb, 0, USB_STRING_DESCRIPTOR, 0, 0,
+					 &desc.header, sizeof ( desc ) ) ) !=0){
+		DBGC ( usb, "USB %s has no default language: %s\n",
+		       usb->name, strerror ( rc ) );
+		return USB_LANG_ENGLISH;
+	}
+
+	/* Use first language ID */
+	language = le16_to_cpu ( desc.language[0] );
+	DBGC2 ( usb, "USB %s default language %#04x\n", usb->name, language );
+	return language;
+}
+
 /**
  * Get USB string descriptor
  *
  * @v usb		USB device
  * @v index		String index
- * @v language		Language ID
+ * @v language		Language ID, or 0 to use default
  * @v buf		Data buffer
  * @v len		Length of buffer
  * @ret len		String length (excluding NUL), or negative error
 	unsigned int i;
 	int rc;
 
+	/* Use default language ID, if applicable */
+	if ( ( language == 0 ) && ( index != 0 ) ) {
+		if ( ! usb->language )
+			usb->language = usb_get_default_language ( usb );
+		language = usb->language;
+	}
+
 	/* Allocate buffer for string */
 	desc = malloc ( sizeof ( *desc ) );
 	if ( ! desc ) {

src/include/ipxe/usb.h

 /** A USB string descriptor */
 #define USB_STRING_DESCRIPTOR 3
 
+/** Language ID for English */
+#define USB_LANG_ENGLISH 0x0409
+
 /** A USB interface descriptor */
 struct usb_interface_descriptor {
 	/** Descriptor header */
 	struct usb_endpoint control;
 	/** Completed control transfers */
 	struct list_head complete;
+
+	/** Default language ID (if known) */
+	unsigned int language;
 };
 
 /** USB device host controller operations */