mtd: Fix the behavior of OTP write if there is not enough room for data

If a write to one time programmable memory (OTP) hits the end of this
memory area, no more data can be written. The count variable in
mtdchar_write() in drivers/mtd/mtdchar.c is not decreased anymore.
We are trapped in the loop forever, mtdchar_write() will never return
in this case.

The desired behavior of a write in such a case is described in [1]:
- Try to write as much data as possible, truncate the write to fit into
the available memory and return the number of bytes that actually
have been written.
- If no data could be written at all, return -ENOSPC.

This patch fixes the behavior of OTP write if there is not enough space
for all data:

1) mtd_write_user_prot_reg() in drivers/mtd/mtdcore.c is modified to
return -ENOSPC if no data could be written at all.
2) mtdchar_write() is modified to handle -ENOSPC correctly. Exit if a
write returned -ENOSPC and yield the correct return value, either
then number of bytes that could be written, or -ENOSPC, if no data
could be written at all.

Furthermore the patch harmonizes the behavior of the OTP memory write
in drivers/mtd/devices/mtd_dataflash.c with the other implementations
and the requirements from [1]. Instead of returning -EINVAL if the data
does not fit into the OTP memory, we try to write as much data as
possible/truncate the write.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html

Signed-off-by: Christian Riesch <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/devices/mtd_dataflash.c

···
 	struct dataflash	*priv = mtd->priv;
 	int			status;
 
-	if (len > 64)
-		return -EINVAL;
+	if (from >= 64) {
+		/*
+		 * Attempting to write beyond the end of OTP memory,
+		 * no data can be written.
+		 */
+		*retlen = 0;
+		return 0;
+	}
 
-	/* Strictly speaking, we *could* truncate the write ... but
-	 * let's not do that for the only write that's ever possible.
-	 */
+	/* Truncate the write to fit into OTP memory. */
 	if ((from + len) > 64)
-		return -EINVAL;
+		len = 64 - from;
 
 	/* OUT: OP_WRITE_SECURITY, 3 zeroes, 64 data-or-zero bytes
 	 * IN:  ignore all

drivers/mtd/mtdchar.c

···
 		default:
 			ret = mtd_write(mtd, *ppos, len, &retlen, kbuf);
 		}
+
+		/*
+		 * Return -ENOSPC only if no data could be written at all.
+		 * Otherwise just return the number of bytes that actually
+		 * have been written.
+		 */
+		if ((ret == -ENOSPC) && (total_retlen))
+			break;
+
 		if (!ret) {
 			*ppos += retlen;
 			total_retlen += retlen;

drivers/mtd/mtdcore.c

···
 int mtd_write_user_prot_reg(struct mtd_info *mtd, loff_t to, size_t len,
 			    size_t *retlen, u_char *buf)
 {
+	int ret;
+
 	*retlen = 0;
 	if (!mtd->_write_user_prot_reg)
 		return -EOPNOTSUPP;
 	if (!len)
 		return 0;
-	return mtd->_write_user_prot_reg(mtd, to, len, retlen, buf);
+	ret = mtd->_write_user_prot_reg(mtd, to, len, retlen, buf);
+	if (ret)
+		return ret;
+
+	/*
+	 * If no data could be written at all, we are out of memory and
+	 * must return -ENOSPC.
+	 */
+	return (*retlen) ? 0 : -ENOSPC;
 }
 EXPORT_SYMBOL_GPL(mtd_write_user_prot_reg);