[smbios] Allow access to multiple instances of SMBIOS structures

Extend the syntax for numerical SMBIOS settings from

  smbios/<type>.<offset>.<length>

to

  smbios/[<instance>.]<type>.<offset>.<length>

Where SMBIOS provides multiple structures with the same <type>, this
extended syntax allows for access to structures other than the first.
If <instance> is omitted then it will default to zero, giving access
to the first instance (and so matching existing behaviour).

The 16-bit SMBIOS handle (which is an alternative way to disambiguate
multiple instances of the same type of structure) can be accessed, if
required, using

  smbios/<instance>.<type>.2.2:uint16

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

src/include/ipxe/smbios.h

 #define SMBIOS_VERSION( major, minor ) ( ( (major) << 8 ) | (minor) )
 
 extern int find_smbios ( struct smbios *smbios );
-extern int find_smbios_structure ( unsigned int type,
+extern int find_smbios_structure ( unsigned int type, unsigned int instance,
 				   struct smbios_structure *structure );
 extern int read_smbios_structure ( struct smbios_structure *structure,
 				   void *data, size_t len );

src/interface/smbios/smbios.c

  * Find specific structure type within SMBIOS
  *
  * @v type		Structure type to search for
+ * @v instance		Instance of this type of structure
  * @v structure		SMBIOS structure descriptor to fill in
  * @ret rc		Return status code
  */
-int find_smbios_structure ( unsigned int type,
+int find_smbios_structure ( unsigned int type, unsigned int instance,
 			    struct smbios_structure *structure ) {
 	unsigned int count = 0;
 	size_t offset = 0;
 		      structure->header.len, structure->strings_len );
 
 		/* If this is the structure we want, return */
-		if ( structure->header.type == type ) {
+		if ( ( structure->header.type == type ) &&
+		     ( instance-- == 0 ) ) {
 			structure->offset = offset;
 			return 0;
 		}

src/interface/smbios/smbios_settings.c

 			  struct setting *setting,
 			  void *data, size_t len ) {
 	struct smbios_structure structure;
+	unsigned int tag_instance;
 	unsigned int tag_type;
 	unsigned int tag_offset;
 	unsigned int tag_len;
 	int rc;
 
-	/* Split tag into type, offset and length */
+	/* Split tag into instance, type, offset and length */
+	tag_instance = ( ( setting->tag >> 24 ) & 0xff );
 	tag_type = ( ( setting->tag >> 16 ) & 0xff );
 	tag_offset = ( ( setting->tag >> 8 ) & 0xff );
 	tag_len = ( setting->tag & 0xff );
 
 	/* Find SMBIOS structure */
-	if ( ( rc = find_smbios_structure ( tag_type, &structure ) ) != 0 )
+	if ( ( rc = find_smbios_structure ( tag_type, tag_instance,
+					    &structure ) ) != 0 )
 		return rc;
 
 	{