[efi] Add efi_path_guid() utility function

EFI provides no API for determining the partition GUID (if any) for a
specified device handle.  The partition GUID appears to be exposed
only as part of the device path.

Add efi_path_guid() to extract the partition GUID (if any) from a
device path.

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

src/include/ipxe/efi/efi_path.h

@@ -20,6 +20,7 @@ struct aoe_device;
 struct fcp_description;
 struct ib_srp_device;
 struct usb_function;
+union uuid;
 
 /**
  * Terminate device path
@@ -43,6 +44,7 @@ extern EFI_DEVICE_PATH_PROTOCOL *
 efi_path_end ( EFI_DEVICE_PATH_PROTOCOL *path );
 extern size_t efi_path_len ( EFI_DEVICE_PATH_PROTOCOL *path );
 extern unsigned int efi_path_vlan ( EFI_DEVICE_PATH_PROTOCOL *path );
+extern int efi_path_guid ( EFI_DEVICE_PATH_PROTOCOL *path, union uuid *uuid );
 extern EFI_DEVICE_PATH_PROTOCOL * efi_paths ( EFI_DEVICE_PATH_PROTOCOL *first,
 					      ... );
 extern EFI_DEVICE_PATH_PROTOCOL * efi_netdev_path ( struct net_device *netdev );

src/include/ipxe/errfile.h

@@ -80,6 +80,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_acpimac		       ( ERRFILE_CORE | 0x00280000 )
 #define ERRFILE_efi_strings	       ( ERRFILE_CORE | 0x00290000 )
 #define ERRFILE_uuid		       ( ERRFILE_CORE | 0x002a0000 )
+#define ERRFILE_efi_path	       ( ERRFILE_CORE | 0x002b0000 )
 
 #define ERRFILE_eisa		     ( ERRFILE_DRIVER | 0x00000000 )
 #define ERRFILE_isa		     ( ERRFILE_DRIVER | 0x00010000 )

src/interface/efi/efi_path.c

@@ -22,9 +22,11 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <stdlib.h>
 #include <stdarg.h>
 #include <string.h>
+#include <errno.h>
 #include <byteswap.h>
 #include <ipxe/netdevice.h>
 #include <ipxe/vlan.h>
+#include <ipxe/uuid.h>
 #include <ipxe/tcpip.h>
 #include <ipxe/uri.h>
 #include <ipxe/iscsi.h>
@@ -132,6 +134,47 @@ unsigned int efi_path_vlan ( EFI_DEVICE_PATH_PROTOCOL *path ) {
 	return 0;
 }
 
+/**
+ * Get partition GUID from device path
+ *
+ * @v path		Device path
+ * @v guid		Partition GUID to fill in
+ * @ret rc		Return status code
+ */
+int efi_path_guid ( EFI_DEVICE_PATH_PROTOCOL *path, union uuid *guid ) {
+	EFI_DEVICE_PATH_PROTOCOL *next;
+	HARDDRIVE_DEVICE_PATH *hd;
+	int rc;
+
+	/* Search for most specific partition device path */
+	rc = -ENOENT;
+	for ( ; ( next = efi_path_next ( path ) ) ; path = next ) {
+
+		/* Skip non-harddrive device paths */
+		if ( path->Type != MEDIA_DEVICE_PATH )
+			continue;
+		if ( path->SubType != MEDIA_HARDDRIVE_DP )
+			continue;
+
+		/* Skip non-GUID signatures */
+		hd = container_of ( path, HARDDRIVE_DEVICE_PATH, Header );
+		if ( hd->SignatureType != SIGNATURE_TYPE_GUID )
+			continue;
+
+		/* Extract GUID */
+		memcpy ( guid, hd->Signature, sizeof ( *guid ) );
+		uuid_mangle ( guid );
+
+		/* Record success, but continue searching in case
+		 * there exists a more specific GUID (e.g. a partition
+		 * GUID rather than a disk GUID).
+		 */
+		rc = 0;
+	}
+
+	return rc;
+}
+
 /**
  * Concatenate EFI device paths
  *