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 <christian.riesch@omicron.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/drivers/mtd/devices/mtd_dataflash.c b/drivers/mtd/devices/mtd_dataflash.c
index a6fdbe8..dd22ce2 100644
--- a/drivers/mtd/devices/mtd_dataflash.c
+++ b/drivers/mtd/devices/mtd_dataflash.c
@@ -542,14 +542,18 @@ static int dataflash_write_user_otp(struct mtd_info *mtd,
 	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