Improve error message when wrong --update option is given.

ChangeLog

@@ -14,6 +14,7 @@ Changes Prior to this release
     -   When --assemble --scan is run, if all arrays that could be found
 	have already been started, don't report an error.
     -   Fix a couple of bugs related to raid10 and the new 'offset' layout.
+    -   Improve error message when a wrong '--update' option is given.
 
 Changes Prior to 2.5.4 release
     -   When creating devices in /dev/md/ create matching symlinks

ReadMe.c

@@ -272,7 +272,7 @@ char OptionHelp[] =
 "  --config=     -c   : config file\n"
 "  --scan        -s   : scan config file for missing information\n"
 "  --force       -f   : Assemble the array even if some superblocks appear out-of-date\n"
-"  --update=     -U   : Update superblock: one of sparc2.2, super-minor or summaries\n"
+"  --update=     -U   : Update superblock: try '-A --update=?' for list of options.\n"
 "  --auto(=p)    -a   : Automatically allocate new (partitioned) md array if needed.\n"
 "  --no-degraded      : Do not start any degraded arrays - default unless --scan.\n"
 "\n"
@@ -414,7 +414,7 @@ char Help_assemble[] =
 "                       for a full array are present\n"
 "  --force       -f   : Assemble the array even if some superblocks appear\n"
 "                     : out-of-date.  This involves modifying the superblocks.\n"
-"  --update=     -U   : Update superblock: one of sparc2.2, super-minor or summaries\n"
+"  --update=     -U   : Update superblock: try '-A --update=?' for option list.\n"
 "  --no-degraded      : Assemble but do not start degraded arrays.\n"
 ;
 

mdadm.c

@@ -596,7 +596,13 @@ int main(int argc, char *argv[])
 
 				continue;
 			}
-			fprintf(stderr, Name ": '--update %s' invalid.  Only 'sparc2.2', 'super-minor', 'uuid', 'resync' or 'summaries' supported\n",update);
+			if (strcmp(update,"?") == 0 || strcmp(update, "help") == 0)
+				fprintf(stderr, Name ": ");
+			else
+				fprintf(stderr, Name ": '--update=%s' is invalid.  ", update);
+			fprintf(stderr, "Valid --update options are:\n"
+				"     'sparc2.2', 'super-minor', 'uuid', 'name', 'resync',\n"
+				"     'summaries', 'homehost', 'byteorder'.\n");
 			exit(2);
 
 		case O(ASSEMBLE,NoDegraded): /* --no-degraded */