UBI: power cut emulation for testing

Emulate random power cuts by switching device to ro after a number of
writes to allow simple power cut testing with nand-sim.

Maximum and minimum number of successful writes before power cut and
what kind of writes (EC header, VID header or none) to interrupt
configurable via debugfs.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/drivers/mtd/ubi/io.c b/drivers/mtd/ubi/io.c
index ed0bcb3..5bbd1f0 100644
--- a/drivers/mtd/ubi/io.c
+++ b/drivers/mtd/ubi/io.c
@@ -859,6 +859,9 @@
 	if (err)
 		return err;
 
+	if (ubi_dbg_power_cut(ubi, POWER_CUT_EC_WRITE))
+		return -EROFS;
+
 	err = ubi_io_write(ubi, ec_hdr, pnum, 0, ubi->ec_hdr_alsize);
 	return err;
 }
@@ -1106,6 +1109,9 @@
 	if (err)
 		return err;
 
+	if (ubi_dbg_power_cut(ubi, POWER_CUT_VID_WRITE))
+		return -EROFS;
+
 	p = (char *)vid_hdr - ubi->vid_hdr_shift;
 	err = ubi_io_write(ubi, p, pnum, ubi->vid_hdr_aloffset,
 			   ubi->vid_hdr_alsize);