mtd: mtdconcat: map through panic write handler

Allows a mtdconcat's subdevice->_panic_write to be used for
capturing a mtdoops dump.

Note: The ->_panic_write is mapped through from the first chip
      that is part of the concat virtual device.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[miquel.raynal@bootlin.com: return err, not void]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200602143403.13465-1-matthew.weber@rockwellcollins.com

Author: rc-matthew-l-weber

drivers/mtd/mtdconcat.c

@@ -102,6 +102,48 @@ concat_read(struct mtd_info *mtd, loff_t from, size_t len,
 	return -EINVAL;
 }
 
+static int
+concat_panic_write(struct mtd_info *mtd, loff_t to, size_t len,
+	     size_t * retlen, const u_char * buf)
+{
+	struct mtd_concat *concat = CONCAT(mtd);
+	int err = -EINVAL;
+	int i;
+	for (i = 0; i < concat->num_subdev; i++) {
+		struct mtd_info *subdev = concat->subdev[i];
+		size_t size, retsize;
+
+		if (to >= subdev->size) {
+			size = 0;
+			to -= subdev->size;
+			continue;
+		}
+		if (to + len > subdev->size)
+			size = subdev->size - to;
+		else
+			size = len;
+
+		err = mtd_panic_write(subdev, to, size, &retsize, buf);
+		if (err == -EOPNOTSUPP) {
+			printk(KERN_ERR "mtdconcat: Cannot write from panic without panic_write\n");
+			return err;
+		}
+		if (err)
+			break;
+
+		*retlen += retsize;
+		len -= size;
+		if (len == 0)
+			break;
+
+		err = -EINVAL;
+		buf += size;
+		to = 0;
+	}
+	return err;
+}
+
+
 static int
 concat_write(struct mtd_info *mtd, loff_t to, size_t len,
 	     size_t * retlen, const u_char * buf)
@@ -648,6 +690,8 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 		concat->mtd._block_isbad = concat_block_isbad;
 	if (subdev[0]->_block_markbad)
 		concat->mtd._block_markbad = concat_block_markbad;
+	if (subdev[0]->_panic_write)
+		concat->mtd._panic_write = concat_panic_write;
 
 	concat->mtd.ecc_stats.badblocks = subdev[0]->ecc_stats.badblocks;