[uuid] Abstract UUID mangling code out to a separate uuid_mangle() function

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

src/include/ipxe/uuid.h

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <stdint.h>
+#include <byteswap.h>
 
 /** A universally unique ID */
 union uuid {
 	uint8_t raw[16];
 };
 
+/**
+ * Change UUID endianness
+ *
+ * @v uuid		UUID
+ *
+ * RFC4122 defines UUIDs as being encoded in network byte order, but
+ * leaves some wriggle room for "explicit application or presentation
+ * protocol specification to the contrary".  PXE, EFI and SMBIOS
+ * (versions 2.6 and above) treat the first three fields as being
+ * little-endian.
+ */
+static inline void uuid_mangle ( union uuid *uuid ) {
+
+	__bswap_32s ( &uuid->canonical.a );
+	__bswap_16s ( &uuid->canonical.b );
+	__bswap_16s ( &uuid->canonical.c );
+}
+
 extern char * uuid_ntoa ( const union uuid *uuid );
 
 #endif /* _IPXE_UUID_H */

src/interface/smbios/smbios_settings.c

 #include <stdint.h>
 #include <string.h>
 #include <errno.h>
-#include <byteswap.h>
 #include <ipxe/settings.h>
 #include <ipxe/init.h>
 #include <ipxe/uuid.h>
 		     ( smbios_version() >= SMBIOS_VERSION ( 2, 6 ) ) ) {
 			DBG ( "SMBIOS detected mangled UUID\n" );
 			memcpy ( &uuid, &buf[tag_offset], sizeof ( uuid ) );
-			__bswap_32s ( &uuid.canonical.a );
-			__bswap_16s ( &uuid.canonical.b );
-			__bswap_16s ( &uuid.canonical.c );
+			uuid_mangle ( &uuid );
 			raw = &uuid;
 		}
 

src/net/udp/dhcp.c

 	client_uuid.type = DHCP_CLIENT_UUID_TYPE;
 	if ( ( len = fetch_uuid_setting ( NULL, &uuid_setting,
 					  &client_uuid.uuid ) ) >= 0 ) {
-		__bswap_32s ( &client_uuid.uuid.canonical.a );
-		__bswap_16s ( &client_uuid.uuid.canonical.b );
-		__bswap_16s ( &client_uuid.uuid.canonical.c );
+		uuid_mangle ( &client_uuid.uuid );
 		if ( ( rc = dhcppkt_store ( dhcppkt, DHCP_CLIENT_UUID,
 					    &client_uuid,
 					    sizeof ( client_uuid ) ) ) != 0 ) {