[PATCH v4 1/9] ethdev: specify return value of xstats-get API

[Chengwen Feng <fengchengwen@huawei.com>]

Currently the value returned when xstats is NULL of rte_eth_xstats_get()
is not specified, some PMDs (eg. hns3/ipn3ke/mvpp2/axgbe) return zero
while others return the required number of elements.

In this patch, special parameter combinations are restricted:
1. specify that xstats is NULL if and only if n is 0.
2. specify that if n is lower than the required number of elements, the
function returns the required number of elements.
3. specify that if n is zero, the xstats must be NULL, the function
returns the required number of elements.

This patch also makes sure that xstats is NULL if n is lower or equal to
the number of basic stats when invoke driver's ops.

Fixes: ce757f5c9a4d ("ethdev: new method to retrieve extended statistics")
Cc: stable@dpdk.org

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
---
 lib/ethdev/rte_ethdev.c | 4 +++-
 lib/ethdev/rte_ethdev.h | 6 +++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/ethdev/rte_ethdev.c b/lib/ethdev/rte_ethdev.c
index 29a3d80466..d9661bd64f 100644
--- a/lib/ethdev/rte_ethdev.c
+++ b/lib/ethdev/rte_ethdev.c
@@ -2973,6 +2973,8 @@ rte_eth_xstats_get(uint16_t port_id, struct rte_eth_xstat *xstats,
 	int ret;
 
 	RTE_ETH_VALID_PORTID_OR_ERR_RET(port_id, -ENODEV);
+	if (xstats == NULL && n > 0)
+		return -EINVAL;
 	dev = &rte_eth_devices[port_id];
 
 	nb_rxqs = RTE_MIN(dev->data->nb_rx_queues, RTE_ETHDEV_QUEUE_STAT_CNTRS);
@@ -2989,7 +2991,7 @@ rte_eth_xstats_get(uint16_t port_id, struct rte_eth_xstat *xstats,
 		 * xstats struct.
 		 */
 		xcount = (*dev->dev_ops->xstats_get)(dev,
-				     xstats ? xstats + count : NULL,
+				     (n > count) ? xstats + count : NULL,
 				     (n > count) ? n - count : 0);
 
 		if (xcount < 0)
diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
index 04cff8ee10..04225bba4d 100644
--- a/lib/ethdev/rte_ethdev.h
+++ b/lib/ethdev/rte_ethdev.h
@@ -3174,9 +3174,13 @@ int rte_eth_xstats_get_names(uint16_t port_id,
  * @param xstats
  *   A pointer to a table of structure of type *rte_eth_xstat*
  *   to be filled with device statistics ids and values.
- *   This parameter can be set to NULL if n is 0.
+ *   This parameter can be set to NULL if and only if n is 0.
  * @param n
  *   The size of the xstats array (number of elements).
+ *   If lower than the required number of elements, the function returns
+ *   the required number of elements.
+ *   If equal to zero, the xstats must be NULL, the function returns the
+ *   required number of elements.
  * @return
  *   - A positive value lower or equal to n: success. The return value
  *     is the number of entries filled in the stats table.
-- 
2.33.0