[PATCH] mbuf: clarify comment on adding TX offload flags

[Bruce Richardson <bruce.richardson@intel.com>]

The comment for TX offload flags stated that those flags started at bit
55 and then were added to the right of that, leaving 8 bits reserved for
generic mbuf (i.e. non-offload) use. This comment may not have been
clear as 5 of the 8 flags which were reserved have now been used for TX
offloads.
This patch:
* updates the description so that it now reflects reality that
only three flags are available for generic mbuf use
* reserved the final generic flag so that it can't be taken over for TX
offload in future
* clarifies the comment for TX flags to indicate that they should be
counting downwards not upwards.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 lib/librte_mbuf/rte_mbuf.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 80419df..0c0034b 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -75,9 +75,10 @@ extern "C" {
  *
  * - RX flags start at bit position zero, and get added to the left of previous
  *   flags.
- * - The most-significant 8 bits are reserved for generic mbuf flags
- * - TX flags therefore start at bit position 55 (i.e. 63-8), and new flags get
- *   added to the right of the previously defined flags
+ * - The most-significant 3 bits are reserved for generic mbuf flags
+ * - TX flags therefore start at bit position 60 (i.e. 63-3), and new flags get
+ *   added to the right of the previously defined flags i.e. they should count
+ *   downwards, not upwards.
  *
  * Keep these flags synchronized with rte_get_rx_ol_flag_name() and
  * rte_get_tx_ol_flag_name().
@@ -197,6 +198,8 @@ extern "C" {
  */
 #define PKT_TX_OUTER_IPV6    (1ULL << 60)
 
+#define __RESERVED           (1ULL << 61) /**< reserved for future mbuf use */
+
 #define IND_ATTACHED_MBUF    (1ULL << 62) /**< Indirect attached mbuf */
 
 /* Use final bit of flags to indicate a control mbuf */
-- 
2.4.3