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, 11 insertions, 1 deletions

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 0a7d77e65335..d201feeb3ca6 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -932,12 +932,22 @@ EXPORT_SYMBOL_GPL(mtd_read_user_prot_reg);
 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);