dm switch: efficiently support repetitive patterns

Add support for quickly loading a repetitive pattern into the
dm-switch target.

In the "set_regions_mappings" message, the user may now use "Rn,m" as
one of the arguments. "n" and "m" are hexadecimal numbers. The "Rn,m"
argument repeats the last "n" arguments in the following "m" slots.

For example:
dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10
is equivalent to
dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \
:1 :2 :1 :2 :1 :2 :1 :2 :1 :2

Requested-by: Jay Wang <[e-mail]>
Tested-by: Jay Wang <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/device-mapper/switch.txt

···
     The path number in the range 0 ... (<num_paths> - 1).
     Expressed in hexadecimal (WITHOUT any prefix like 0x).
 
+R<n>,<m>
+    This parameter allows repetitive patterns to be loaded quickly. <n> and <m>
+    are hexadecimal numbers. The last <n> mappings are repeated in the next <m>
+    slots.
+
 Status
 ======
 
···
 Set mappings for the first 7 entries to point to devices switch0, switch1,
 switch2, switch0, switch1, switch2, switch1:
     dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1
+
+Set repetitive mapping. This command:
+    dmsetup message switch 0 set_region_mappings 1000:1 :2 R2,10
+is equivalent to:
+    dmsetup message switch 0 set_region_mappings 1000:1 :2 :1 :2 :1 :2 :1 :2 \
+	:1 :2 :1 :2 :1 :2 :1 :2 :1 :2
+

drivers/md/dm-switch.c

···
 }
 
 static int process_set_region_mappings(struct switch_ctx *sctx,
-			     unsigned argc, char **argv)
+				       unsigned argc, char **argv)
 {
 	unsigned i;
 	unsigned long region_index = 0;
···
 	for (i = 1; i < argc; i++) {
 		unsigned long path_nr;
 		const char *string = argv[i];
+
+		if ((*string & 0xdf) == 'R') {
+			unsigned long cycle_length, num_write;
+
+			string++;
+			if (unlikely(*string == ',')) {
+				DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+				return -EINVAL;
+			}
+			cycle_length = parse_hex(&string);
+			if (unlikely(*string != ',')) {
+				DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+				return -EINVAL;
+			}
+			string++;
+			if (unlikely(!*string)) {
+				DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+				return -EINVAL;
+			}
+			num_write = parse_hex(&string);
+			if (unlikely(*string)) {
+				DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+				return -EINVAL;
+			}
+
+			if (unlikely(!cycle_length) || unlikely(cycle_length - 1 > region_index)) {
+				DMWARN("invalid set_region_mappings cycle length: %lu > %lu",
+				       cycle_length - 1, region_index);
+				return -EINVAL;
+			}
+			if (unlikely(region_index + num_write < region_index) ||
+			    unlikely(region_index + num_write >= sctx->nr_regions)) {
+				DMWARN("invalid set_region_mappings region number: %lu + %lu >= %lu",
+				       region_index, num_write, sctx->nr_regions);
+				return -EINVAL;
+			}
+
+			while (num_write--) {
+				region_index++;
+				path_nr = switch_region_table_read(sctx, region_index - cycle_length);
+				switch_region_table_write(sctx, region_index, path_nr);
+			}
+
+			continue;
+		}
 
 		if (*string == ':')
 			region_index++;
···
 
 static struct target_type switch_target = {
 	.name = "switch",
-	.version = {1, 0, 0},
+	.version = {1, 1, 0},
 	.module = THIS_MODULE,
 	.ctr = switch_ctr,
 	.dtr = switch_dtr,