UBI: amend commentaries WRT dtype

Richard removed the "dtype" hint, but few commentaries were left and this patch
removes them. I've also added a better description about the "dtype" field in
the ubi-user.h for people who may ever wonder what was that dtype thing about.

This patch also adds an important note that it is better to use value "3" for
the "dtype" field.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/mtd/ubi-user.h b/include/mtd/ubi-user.h
index 370c750115ce..8787349fbafe 100644
--- a/include/mtd/ubi-user.h
+++ b/include/mtd/ubi-user.h
@@ -358,7 +358,17 @@ struct ubi_rnvol_req {
  *                             requests.
  * @lnum: logical eraseblock number to change
  * @bytes: how many bytes will be written to the logical eraseblock
+ * @dtype: pass "3" for better compatibility with old kernels
  * @padding: reserved for future, not used, has to be zeroed
+ *
+ * The @dtype field used to inform UBI about what kind of data will be written
+ * to the LEB: long term (value 1), short term (value 2), unknown (value 3).
+ * UBI tried to pick a PEB with lower erase counter for short term data and a
+ * PEB with higher erase counter for long term data. But this was not really
+ * used because users usually do not know this and could easily mislead UBI. We
+ * removed this feature in May 2012. UBI currently just ignores the @dtype
+ * field. But for better compatibility with older kernels it is recommended to
+ * set @dtype to 3 (unknown).
  */
 struct ubi_leb_change_req {
         __s32 lnum;
@@ -369,6 +379,7 @@ struct ubi_leb_change_req {
 
 /**
  * struct ubi_map_req - a data structure used in map LEB requests.
+ * @dtype: pass "3" for better compatibility with old kernels
  * @lnum: logical eraseblock number to unmap
  * @padding: reserved for future, not used, has to be zeroed
  */