mtd: Add an helper to make erase request aligned on ->erasesize

There's currently nothing forcing alignment of einfo->addr and
einfo->len on mtd->erasesize. Since we don't know if automatically
aligning those field in mtd_erase() will hurt some drivers, we add an
helper function to let drivers that need such an alignment explicitly
ask for it.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Miquel Raynal <miquel.raynal@free-electrons.com>

include/linux/mtd/mtd.h

@@ -489,6 +489,34 @@ static inline uint32_t mtd_mod_by_eb(uint64_t sz, struct mtd_info *mtd)
 	return do_div(sz, mtd->erasesize);
 }
 
+/**
+ * mtd_align_erase_req - Adjust an erase request to align things on eraseblock
+ *			 boundaries.
+ * @mtd: the MTD device this erase request applies on
+ * @req: the erase request to adjust
+ *
+ * This function will adjust @req->addr and @req->len to align them on
+ * @mtd->erasesize. Of course we expect @mtd->erasesize to be != 0.
+ */
+static inline void mtd_align_erase_req(struct mtd_info *mtd,
+				       struct erase_info *req)
+{
+	u32 mod;
+
+	if (WARN_ON(!mtd->erasesize))
+		return;
+
+	mod = mtd_mod_by_eb(req->addr, mtd);
+	if (mod) {
+		req->addr -= mod;
+		req->len += mod;
+	}
+
+	mod = mtd_mod_by_eb(req->addr + req->len, mtd);
+	if (mod)
+		req->len += mtd->erasesize - mod;
+}
+
 static inline uint32_t mtd_div_by_ws(uint64_t sz, struct mtd_info *mtd)
 {
 	if (mtd->writesize_shift)