mtd: gpio_flash: handle case where offset + len exceeds the window size

Fix the bug in handling gpio flash read/write when offset + len
from MTD exceeds the window size

Signed-off-by: Aaron Wu <Aaron.wu@analog.com>

[Brian: made some commentary edits. Also note that the BUG_ON() was
provably false for all non-negative inputs (since x % y <= x), so we
dropped it.]

Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/maps/gpio-addr-flash.c

@@ -99,22 +99,28 @@ static map_word gf_read(struct map_info *map, unsigned long ofs)
  *	@from: flash offset to copy from
  *	@len:  how much to copy
  *
- * We rely on the MTD layer to chunk up copies such that a single request here
- * will not cross a window size.  This allows us to only wiggle the GPIOs once
- * before falling back to a normal memcpy.  Reading the higher layer code shows
- * that this is indeed the case, but add a BUG_ON() to future proof.
+ * The "from" region may straddle more than one window, so toggle the GPIOs for
+ * each window region before reading its data.
  */
 static void gf_copy_from(struct map_info *map, void *to, unsigned long from, ssize_t len)
 {
 	struct async_state *state = gf_map_info_to_state(map);
 
+	int this_len;
+
+	while (len) {
+		if ((from % state->win_size) + len > state->win_size)
+			this_len = state->win_size - (from % state->win_size);
+		else
+			this_len = len;
+
 		gf_set_gpios(state, from);
-
-	/* BUG if operation crosses the win_size */
-	BUG_ON(!((from + len) % state->win_size <= (from + len)));
-
-	/* operation does not cross the win_size, so one shot it */
-	memcpy_fromio(to, map->virt + (from % state->win_size), len);
+		memcpy_fromio(to, map->virt + (from % state->win_size),
+			 this_len);
+		len -= this_len;
+		from += this_len;
+		to += this_len;
+	}
 }
 
 /**
@@ -147,13 +153,21 @@ static void gf_copy_to(struct map_info *map, unsigned long to,
 {
 	struct async_state *state = gf_map_info_to_state(map);
 
+	int this_len;
+
+	while (len) {
+		if ((to % state->win_size) + len > state->win_size)
+			this_len = state->win_size - (to % state->win_size);
+		else
+			this_len = len;
+
 		gf_set_gpios(state, to);
-
-	/* BUG if operation crosses the win_size */
-	BUG_ON(!((to + len) % state->win_size <= (to + len)));
-
-	/* operation does not cross the win_size, so one shot it */
 		memcpy_toio(map->virt + (to % state->win_size), from, len);
+
+		len -= this_len;
+		to += this_len;
+		from += this_len;
+	}
 }
 
 static const char * const part_probe_types[] = {