mtd: spi-nor: otp: return -EROFS if region is read-only

SPI NOR flashes will just ignore program commands if the OTP region is
locked. Thus, a user might not notice that the intended write didn't end
up in the flash. Return -EROFS to the user in this case. From what I can
tell, chips/cfi_cmdset_0001.c also return this error code.

One could optimize spi_nor_mtd_otp_range_is_locked() to read the status
register only once and not for every OTP region, but for that we would
need some more invasive changes. Given that this is
one-time-programmable memory and the normal access mode is reading, we
just live with the small overhead.

By moving the code around a bit, we can just check the length before
calling spi_nor_mtd_otp_range_is_locked() and avoid an underflow there
if a len is 0. This way we don't need to take the lock either. We also
skip the "*retlen = 0" assignment, mtdcore already takes care of that
for us.

Fixes: 069089ac ("mtd: spi-nor: add OTP support")
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>

drivers/mtd/spi-nor/otp.c

@@ -249,6 +249,29 @@ static int spi_nor_mtd_otp_info(struct mtd_info *mtd, size_t len,
 	return ret;
 }
 
+static int spi_nor_mtd_otp_range_is_locked(struct spi_nor *nor, loff_t ofs,
+					   size_t len)
+{
+	const struct spi_nor_otp_ops *ops = nor->params->otp.ops;
+	unsigned int region;
+	int locked;
+
+	/*
+	 * If any of the affected OTP regions are locked the entire range is
+	 * considered locked.
+	 */
+	for (region = spi_nor_otp_offset_to_region(nor, ofs);
+	     region <= spi_nor_otp_offset_to_region(nor, ofs + len - 1);
+	     region++) {
+		locked = ops->is_locked(nor, region);
+		/* take the branch it is locked or in case of an error */
+		if (locked)
+			return locked;
+	}
+
+	return 0;
+}
+
 static int spi_nor_mtd_otp_read_write(struct mtd_info *mtd, loff_t ofs,
 				      size_t total_len, size_t *retlen,
 				      const u8 *buf, bool is_write)
@@ -264,14 +287,26 @@ static int spi_nor_mtd_otp_read_write(struct mtd_info *mtd, loff_t ofs,
 	if (ofs < 0 || ofs >= spi_nor_otp_size(nor))
 		return 0;
 
+	/* don't access beyond the end */
+	total_len = min_t(size_t, total_len, spi_nor_otp_size(nor) - ofs);
+
+	if (!total_len)
+		return 0;
+
 	ret = spi_nor_lock_and_prep(nor);
 	if (ret)
 		return ret;
 
-	/* don't access beyond the end */
-	total_len = min_t(size_t, total_len, spi_nor_otp_size(nor) - ofs);
+	if (is_write) {
+		ret = spi_nor_mtd_otp_range_is_locked(nor, ofs, total_len);
+		if (ret < 0) {
+			goto out;
+		} else if (ret) {
+			ret = -EROFS;
+			goto out;
+		}
+	}
 
-	*retlen = 0;
 	while (total_len) {
 		/*
 		 * The OTP regions are mapped into a contiguous area starting