ethdev: fix xstats by ID API documentation

[ upstream commit bc5112c ]

Document valid combinations of input arguments in accordance with
current implementation in ethdev.

Fixes: 79c913a ("ethdev: retrieve xstats by ID")

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>

lib/librte_ethdev/rte_ethdev.h

@@ -2384,21 +2384,23 @@ int rte_eth_xstats_get(uint16_t port_id, struct rte_eth_xstat *xstats,
  * @param port_id
  *   The port identifier of the Ethernet device.
  * @param xstats_names
- *   An rte_eth_xstat_name array of at least *size* elements to
- *   be filled. If set to NULL, the function returns the required number
- *   of elements.
- * @param ids
- *   IDs array given by app to retrieve specific statistics
+ *   Array to be filled in with names of requested device statistics.
+ *   Must not be NULL if @p ids are specified (not NULL).
  * @param size
- *   The size of the xstats_names array (number of elements).
+ *   Number of elements in @p xstats_names array (if not NULL) and in
+ *   @p ids array (if not NULL). Must be 0 if both array pointers are NULL.
+ * @param ids
+ *   IDs array given by app to retrieve specific statistics. May be NULL to
+ *   retrieve names of all available statistics or, if @p xstats_names is
+ *   NULL as well, just the number of available statistics.
  * @return
  *   - A positive value lower or equal to size: success. The return value
  *     is the number of entries filled in the stats table.
- *   - A positive value higher than size: error, the given statistics table
+ *   - A positive value higher than size: success. The given statistics table
  *     is too small. The return value corresponds to the size that should
  *     be given to succeed. The entries in the table are not valid and
  *     shall not be used by the caller.
- *   - A negative value on error (invalid port id).
+ *   - A negative value on error.
  */
 int
 rte_eth_xstats_get_names_by_id(uint16_t port_id,
@@ -2411,22 +2413,23 @@ rte_eth_xstats_get_names_by_id(uint16_t port_id,
  * @param port_id
  *   The port identifier of the Ethernet device.
  * @param ids
- *   A pointer to an ids array passed by application. This tells which
- *   statistics values function should retrieve. This parameter
- *   can be set to NULL if size is 0. In this case function will retrieve
- *   all available statistics.
+ *   IDs array given by app to retrieve specific statistics. May be NULL to
+ *   retrieve all available statistics or, if @p values is NULL as well,
+ *   just the number of available statistics.
  * @param values
- *   A pointer to a table to be filled with device statistics values.
+ *   Array to be filled in with requested device statistics.
+ *   Must not be NULL if ids are specified (not NULL).
  * @param size
- *   The size of the ids array (number of elements).
+ *   Number of elements in @p values array (if not NULL) and in @p ids
+ *   array (if not NULL). Must be 0 if both array pointers are NULL.
  * @return
  *   - A positive value lower or equal to size: success. The return value
  *     is the number of entries filled in the stats table.
- *   - A positive value higher than size: error, the given statistics table
+ *   - A positive value higher than size: success: The given statistics table
  *     is too small. The return value corresponds to the size that should
  *     be given to succeed. The entries in the table are not valid and
  *     shall not be used by the caller.
- *   - A negative value on error (invalid port id).
+ *   - A negative value on error.
  */
 int rte_eth_xstats_get_by_id(uint16_t port_id, const uint64_t *ids,
 			     uint64_t *values, unsigned int size);