mtd: ubi: skip programming unused bits in ubi headers

This patch prevents unnecessary programming of bits in ec_hdr and
vid_hdr that are not used or read during normal UBI operation. These
unused bits are typcially already set to 1 in erased flash and do not
need to be explicitly programmed to 0 if they are not used.

Programming such unused areas offers no functional benefit and may
result in unnecessary flash wear, reducing the overall lifetime of the
device. By skipping these writes, we preserve the flash state as much as
possible and minimize wear caused by redundant operations.

This change ensures that only necessary fields are written when preparing
UBI headers, improving flash efficiency without affecting functionality.

Additionally, the Kioxia TC58NVG1S3HTA00 datasheet (page 63) also notes
that continuous program/erase cycling with a high percentage of '0' bits
in the data pattern can accelerate block endurance degradation.
This further supports avoiding large 0x00 patterns.

Link: https://europe.kioxia.com/content/dam/kioxia/newidr/productinfo/datasheet/201910/DST_TC58NVG1S3HTA00-TDE_EN_31442.pdf

Signed-off-by: Cheng Ming Lin <[email protected]>
Reviewed-by: Miquel Raynal <[email protected]>
Reviewed-by: Zhihao Cheng <[email protected]>
Signed-off-by: Richard Weinberger <[email protected]>

Author: zmlin1998

drivers/mtd/ubi/io.c

@@ -868,6 +868,8 @@ int ubi_io_write_ec_hdr(struct ubi_device *ubi, int pnum,
 		return -EROFS;
 	}
 
+	memset((char *)ec_hdr + UBI_EC_HDR_SIZE, 0xFF, ubi->ec_hdr_alsize - UBI_EC_HDR_SIZE);
+
 	err = ubi_io_write(ubi, ec_hdr, pnum, 0, ubi->ec_hdr_alsize);
 	return err;
 }
@@ -1150,6 +1152,14 @@ int ubi_io_write_vid_hdr(struct ubi_device *ubi, int pnum,
 		return -EROFS;
 	}
 
+	if (ubi->vid_hdr_shift) {
+		memset((char *)p, 0xFF, ubi->vid_hdr_shift);
+		memset((char *)p + ubi->vid_hdr_shift + UBI_VID_HDR_SIZE, 0xFF,
+		       ubi->vid_hdr_alsize - (ubi->vid_hdr_shift + UBI_VID_HDR_SIZE));
+	} else {
+		memset((char *)p + UBI_VID_HDR_SIZE, 0xFF, ubi->vid_hdr_alsize - UBI_VID_HDR_SIZE);
+	}
+
 	err = ubi_io_write(ubi, p, pnum, ubi->vid_hdr_aloffset,
 			   ubi->vid_hdr_alsize);
 	return err;